On his popular science blog Pharyngula, PZ Meyers has a regular series of posts called "I get email," (example) wherein he discusses e-mails he gets. I get e-mail, too (as I'm sure, so do other science bloggers of all stripes). Here's one I got the other day from Brian, a recent graduate from one of my many almae matres (oh yeah, I took Latin). I post it here in case anyone else is wondering the same thing:

I have a simple question for you... I was out at the Pimmit Run-Potomac
confluence collecting rock samples with that awesome chlorite/pyrite/garnet
assemblage and I encountered a couple pieces of unakite float. I'm just
wondering about its provenance. Your blogs seem to indicate that unakite is
typically found in situ farther west in the Shenandoah which would be a pretty
long way to travel (and pretty cool too!) although I believe there is Antietam
around Mather Gorge so I guess it's not impossible; unless it was
anthropogenically relocated which would be much less cool. A little insight
would be greatly appreciated so I can wow my friends when describing what is now the piece de resistance in my fish tank.

So I wrote back with this (links are additions, since I'm blogging it):

Yes, you could certainly have found some Blue Ridge unakite as float in the Potomac Gorge. I've seen many other Blue Ridge Formations as float on the bedrock terraces of the Potomac: Catoctin Formation, Harpers, Weverton, Antietam (like you mentioned), and something that looks a hell of a lot like the Old Rag Granite. I've found well-rounded bituminous coal cobbles, too! I've found unakite further out, in the Coastal Plain, as well as blue quartz (which is unique to the Blue Ridge). So I think it's quite likely you could have found some unakite.

Anyone else have any questions? Like PZ, I could make this a regular series. The more local and the more geo-centric, the better.

Labels: blue ridge, coastal plain, i-get-mail, metamorphism, piedmont, rivers, sediment

Following on from Sunday's post showcasing new outcrops seen recently along the Billy Goat Trail, here's a cool ptygmatically-folded quartz vein I saw:



Can't quite make it out? The boulder's kind of weathered, so let me highlight it for you:


...and a close-up of the left side, which is better exposed:


That's all I noticed that was new this time around... but next time I'm sure there will be something else. The Billy Goat Trail is the gift that keeps on giving...

Labels: maryland, metamorphism, piedmont, structure

Went for a hike on the good old Billy Goat Trail last Sunday and saw this beautiful outcrop. I love it how every time I walk that trail, I see something new and blog-worthy. Here you see the metagraywacke of the Mather Gorge Formation getting squished and squeezed under conditions of partial melting. Granitic magma (light-colored rock) is leaking out, while the foliated mafic residue (schist chips) are getting strung out and boudinaged under conditions of mountain-building. This granite yeilds late Ordovician isotopic ages (Taconian Orogeny, ~460 Ma).

Seeing an outcrop like this reminds me of making cheese: squeezing the liquid whey (felsic magma) out from the solid curds (higher-melting-temperature solid minerals like those comprising the 'schist chip' boudins). As orogenic forces squeeze from the sides, granite oozes out the top.

I love that there are outcrops where this process is caught in freeze-frame: not all the granite escaped from its migmatitic source rock here; instead the process stopped before it was complete, and through the luck of uplift and exposure by the probing erosion of the Potomac, we get a glimpse of a fundamental process in making the Earth look the way it does. A single outcrop shows rocks that were oceanic sediments, then became metamorphic schist, and now are were transitioning to igneous granite! That's pretty wild. We have caught the rock cycle red-handed.

Labels: analogies, granite, igneous, maryland, metamorphism, piedmont, primary structures

The fall edition of Walkingtown, DC again features my walking tour of DC geology, "History Before History: the Geologic Saga of Washington, DC." It will be on Sunday, September 20, and is free (but reservations are required; sign up with Cultural Tourism DC, the sponsors of the event). Hope you can join us.

Labels: dc, geology, meetings, piedmont

Unannotated photo:


Photo with quartz veins outlined, highlighting boudinage and parasitic folding:


Photo with vein quartz boudins and folds highlighted in yellow:


Sketch interpreting stresses that produced these structures:


This nice example of ~horizontal shortening and ~vertical stretching is seen in metagraywacke muscovite schist with hydrothermal quartz veins, near Potomac, Maryland. It is located on the C&O Canal, just upstream from the bridge going to Olmstead Island and the Great Falls overlook.

Labels: art, maryland, piedmont, structure

On Saturday's Bedrock Geology of Washington, DC class, my students and I had the good fortune to stumble upon two geologists out doing field work: Tony Fleming, lead author of the geologic map of the Washington West quadrangle, and Steve Self, senior volcanologist with the Nuclear Regulatory Commission. They were out looking at the Sykesville Formation at Chain Bridge Flats, assessing a potential reinterpretation of the unit.

Fortunately, they were willing to take a little time and discuss their findings with the students. Here's a couple shots of Steve talking to the group:




I joined Steve and Tony in the field yesterday (Monday) too, looking at some outcrops on the other side of the river, and trying to make sense of them. Fun stuff! More on that at a later date...

Labels: dc, field trips, geologists, geology, igneous, metamorphism, nova, piedmont, sediment, volcano

Yesterday, I lead a NOVA field studies trip to the Billy Goat Trail, and we were pleased to see that the recent rains had plumped up the big patch of Opuntia (beavertail) cactus near the boulder of Seneca Sandstone. These are native cacti which grow on poor soils in the area. These soils are poor because periodically the Bear Island strath (bedrock terrace), where the Billy Goat Trail is located, is scoured by the Potomac River's floods. These cacti are growing essentially in pine and juniper needles on top of bare rock (metagraywacke). What a beautiful sight!

Labels: maryland, piedmont, plants

As a high school student in Arlington County, Virginia, I used to take regular hikes down a path called Windy Run, and then walk along the south shore of the Potomac River, upstream. It was in the days before I knew anything about rocks, and I was mainly appreciating other aspects of nature, like the plant life, the birds, the bugs, the salamanders, and occasionally something really cool like a raccoon. But I was aware that the scene I observed and enjoyed was not the same scene that had always persisted.

I heard rumors from my uncle about patches of woods inside the DC Beltway that preserved virgin forest -- giant trees that gave a hint of the former majesty of this eastern hardwood forest. I read about an eastern herd of bison that would migrate north and south through the Piedmont and Coastal Plain, crossing the Potomac near Alexandria (before we killed them all). I noticed a gazillion deer, and had it explained to me that the lack of predators like cougars and wolves resulted in the herbivores' population explosion. We used to have elk here, but European colonists had extirpated them. The last of the bison were killed off by 1800, and the final elk met a bullet around 1850. This used to be a pretty wild place!

I observed trash nearly constantly, often mixed obscenely with natural debris, sheathed in mud, or woven into birds' nests. Every few minutes, a jet airplane on its approach to National Airport would thunder overhead. Those of us who lived in the flight path would learn to automatically put conversations on "pause" during the 30 seconds it took for the planes to pass. Visitors didn't know what to do about the noise; it was too pervasive to be ignored. But live here long enough, and you learned to ignore it. You adapted, like the birds adapted by putting aluminum foil and plastic bags into their nests.

And the river itself? It's gross. In the modern day, it's constantly muddy and silty, with a foul-smelling sewage/sediment biofilm all over the rocks and logs in the water. There's scummy flotsam and rumors that you'll get a rash if you swim in it. There's people fishing down by Teddy Roosevelt Island, and you have to wonder why... They're not going to eat the fish they catch out of this polluted stream, are they?

The theme of this month's Accretionary Wedge is "time warp." The Wedge is a geoblog 'carnival,' though it's been inactive for a while, this month sees its return to 'accreting.' For those of you who are new readers to NOVA Geoblog, it's probably a great opportunity to check out some of the dozens of other interesting geoblogs out there. So what does this have to do with my reflections on the local woods, and the Potomac River? This month's Wedge host is Lockwood from Outside the Interzone. He asked geobloggers, "Where and when would you most like to visit to witness and analyze an event in Earth's history?"

I'm going to use my time travel experience to go back in time right here, in Washington, DC. I want to go back to 1491*. I want to see what my home looked like before European settlers showed up and brought their particular brand of industrialization / civilization / land use changes / ecological perturbations to the Potomac River valley. It may surprise readers to learn that I'd opt for this -- a simple experience of pre-colonization North American nature -- over something tectonic and structural, but that's what calls to me on a deep, emotional level. I want to see a vibrant ecosystem with big trees. I want to see the water of the Potomac River look like water; I want to go swimming in it. I want to see what bird migration looked like before it dropped off so precipitously. I want to see a passenger pigeon, a carolina parakeet. I want to see for myself what a healthy amphibian population looks like. And bison fording the Potomac in Alexandria... perhaps emerging from the clear water with the autumn colors ablaze on the far side of the river? That would just be... awesome.

* Note that there's a good book by this same name, on this same theme, 1491. The book makes the case that there was already a lot of landscape/ecological modification playing out before Europeans arrived: that native Americans played a significant role in messing with natural systems and we shouldn't imagine an ecological paradise, just less of an ecological disaster.

Of course, going back to 1491 may have some negative aspects to it: there would be malaria endemic to DC at that time, and the native tribes might not take kindly to a time traveler popping in to ogle their forested homes. But I'll take those risks (they exist today in other places I've visited), since the pay-off would be such a profound deepening of perspective.

If I had the ability to go back in time, I'd use it to gain experience with pre-colonial North America. I'd check out the same river banks I would walk 500+ years later, and see what we've lost.

...And, once I've seen that former world, I can't guarantee that I'd come back.

Labels: amphibians, birdies, blogs, dc, environmental, geologic time, mammals, piedmont, rivers, virginia

A couple of days ago, I asked for someone to tell me the geologic history of this boulder, in correct chronological order. To make it easier, I labeled the relevant rock units with letters. I promised that the first person to post the correct sequence of events in the comments would win a GEOLOGY ROCKS bumper sticker.



From first to last, the correct sequence of events is X, D, R, M, F.

Thomas Donlon got it right! Congratulations, Thomas -- I'll mail you a bumper sticker.

So let's delve into more detail: what actually happened with this rock?

First, a mafic source rock was weathered, generating chunks of rock "X." Then those clasts were mixed in with a bunch of sand and mud to generate the graywacke that makes up most of the boulder. This was later metamorphosed (not shown with a letter) to generate rock "D." Later, rock "D" with inclusions of "X" was split open, and granitic magma intruded into that fracture to make the dike labeled as "R." Later still, another cross-cutting event took place, cutting across everything that had come so far, to generate the vein of milky (hydrothermal) quartz labeled as "M." Finally, these rocks were uplifted and exposed, and various fractures, including "F," liberated this boulder from its source area. Now it is free, adrift on the Chain Bridge Flats, and posing for geologists. The final event was me discovering and gracing it with a quarter before snapping its portrait.

Labels: contest, dc, geologic time, piedmont

Inspired to give those Californians a run for their money with their cool examples of relative dating exercises, I took this photo last week down at Chain Bridge Flats, the westernmost corner of Washington, DC:



Your assignment, should you choose to accept it: tell me the geologic history of this boulder, in correct chronological order. To make it easier for you, I've labeled the relevant rock units with letters here. (The letters were chosen randomly, and do not by their alphabetic nature imply any sort of order. Note that "F" is the fracture surface defining the planar outer edge of the boulder.) First person to post the correct sequence of events in the comments area below wins a GEOLOGY ROCKS bumper sticker.



Answer in a couple of days. Good luck!

Labels: contest, dc, geologic time, piedmont

Here's a few shots from Sunday's "History before history: The geologic saga of Washington, DC" tour for Walkingtown, DC. We had ~forty people show up; I was glad to have the NOVA Geology megaphone system so I could broadcast to a crowd that size.

Thanks to Michelle Arsenault (NSF) for playing caboose to our group (and taking these photos), and to Laura Moore (volunteer for Cultural Tourism DC) for keeping us safely out of the road.







Classic "subduction" arm pose:




Pondering garnets in the Laurel Formation:

(note the cyclist above, running the gauntlet of interested geologists!)

Checking out the Clydesdale Fault:


What's up with this limestone? Why is it gray higher than about seven feet or so?




And the lovely quarry where we can see the various members of the Georgetown Intrusive Suite (arm-waving to indicate boudinage):

Labels: dc, piedmont

And now, a few images from April's Environmental Geology class field trip. We made three stops: (1) a large coal-fired power plant in Maryland, (2) Westmoreland State Park in Virginia to look at coastal erosion, and (3) Prince William Forest Park in Virginia to look at pyrite emplacement and acid mine drainage.

Here's one of the bluffs on the Potomac River at Westmoreland:

Note the recent pile of breakdown in the middle of the bluff where all the water seepage is, and also the orange trail as soil from the uppermost bluff has marked another mass wasting event's passage down to the river.

These are Miocene-aged sedimentary layers known as the Calvert Formation, part of the Coastal Plain. In places, the gray clay has been altered along fracture surfaces, as shown by these orange stripes criss-crossing one another. My toes for scale:


The students spent some time searching for fossils: this is an area where lots of shark teeth are found. We didn't have much luck, but after a long cold winter, it was nice to be standing in the warm sunshine and water:


At Prince William Forest Park, we hiked down to the Cabin Branch Pyrite Mine to look at the massive denudation there due to acid mine drainage, and we also spent some time poking around for treasures, in this case chunks of pyrite:


We had better luck than at Westmoreland...




...But of course we were in a national park at Prince William, so we left the pyrite where we found it. (Westmoreland, in contrast, allows you to keep any fossils you find in loose sediment: that figures, eh?)

I'd like to say that the group of students I had in Environmental Geology this past semester was terrific, one of the best groups I've worked with in a long time. Maybe it was because the class was discussion-focused, or maybe it was the cookies we ate every Tuesday night, but it was a great experience for me, and I'm looking forward to teaching the course again. Thanks, everyone, for making it so much fun!

Labels: coastal plain, environmental, field trips, maryland, miocene, nova, ore, piedmont, virginia

I've been meaning to go check out the Soapstone Valley for years, but finally got around to it on Memorial Day. The park is a valley that shoots off to the east from Rock Creek Park, with an eastern terminus at Connecticut Avenue:



I didn't have far to walk before I found my first cobble of soapstone. It felt soapy in my hand, and was easily scratched by my fingernail. (Fingernail = 2.5 on the Mohs scale of hardness; talc = 1) I found it interesting that the soapstone cobbles had less algae growing on them than the other cobbles in the stream... Hmm. Because they slough off their outer layers more easily? Or because there's something chemical going on that prevents algae growth?


Why does anyone care about soapstone? Well, people who care about prehistory are interested in soapstone because it was easily carved to make various artifacts. As a geologist, I'm more interested in it because it's a metamorphic rock that implies an ultramafic protolith. In other words, as the various rocks that would become DC's bedrock were squished and squeezed and heated during Taconian mountain-building, one of the ingredients in the mix may have been a peridotite. As the graywacke around it metamorphosed to metagraywacke, the putative peridotite metamorphosed into soapstone.

The stuff I found in Soapstone Valley is a talc schist with porphyroblasts or relict phenocrysts of something dark and chunky in it:


Here's a close-up. The big crystals were dark green, like augite, but they had a texture that looked more like hornblende. Not sure as to their identity. I'll put one under the microscope later to try and suss out the relationship between the cleavage planes.


They're definitely mafic though! Here's an example where the large crystals are rusted out:


So there was plenty of soapstone float, but no bedrock outcrops. At first, I was in the highly foliated metagraywacke schist of the Rock Creek Shear Zone...


...but as I headed upstream I found boulders of the Kensington Tonalite, implying exposures of the KT further up the valley...


... and sure enough, that's what I found. This is the Kensington Tonalite, a late Ordovician granitoid.


Where I first crossed the contact, I thought it looked a little odd, and then a later look at the geologic map of the Washington West quadrangle (Fleming, et al., 1995):

Fleming, et al., list it as a sheared biotite tonalite of the Georgetown Intrusive Suite, which I guess explains its appearance as distinct from the Kensington Tonalite.

When I got up to the eastern edge of the park, I saw the source of the stream:


The valley widens out here, almost as if the rock is weaker... And where concrete has been poured (to stabilize the slope??) the underlying rock is etched away: it's the super-soft soapstone...


Here's a boulder of soapstone (my fingernail scratches it to demonstrate that it's soft):


Here's the geologic map of the area. You can see Soapstone Valley cutting an east-west swath across the strike of the structures. ("ss" means "soapstone"...)

My annotations on Tony Fleming's map (reference below).

Reference:
Geologic map of the Washington west quadrangle, District of Columbia, Montgomery and Prince Georges Counties, Maryland, and Arlington and Fairfax Counties, Virginia. Anthony H. Fleming, Lucy McCartan, and Avery Ala Drake. U.S. Geological Survey (Reston, VA), 1995.
_________________________________________________________________

A quick tangent to note a milestone: this is my 700^th post on NOVA Geoblog. Thanks to everyone for reading. Looking forward to 700 more...

Labels: appalachians, dc, granite, igneous, metamorphism, minerals, ordovician, piedmont

These videos were shot by NOVA's videoman extraordinaire Richard Attix, who helped me immensely this morning by splicing together these movies for use in my MSSE capstone presentation at the end of next month. Enjoy!

Teaching on the Billy Goat Trail (a blend of instructor-focused lecture and student-focused exploration):

Hiking on the Billy Goat Trail:


End-of-trip activity - "Ordering Geologic Events":

Labels: field trips, nova, piedmont, teaching

Whew!

Sorry I haven't posted much here in the past week. I've been swamped.

The good news is that my biggest task is now off my plate (just turned in the first draft of my MSSE capstone to my advisor), and that means I've got some spare attention left for the blog.

I thought I would take the opportunity to share some images from this past weekend's NAGT (National Association of Geoscience Teachers) Eastern Section conference, held at the NOVA Loudoun campus. On Saturday, I led a version of my "Bedrock Geology of Washington, DC" trip for a group of eight conference attendees.

All these photos are from Randy Newcomer, Director of Training and Services for Complete Safety Solutions of Lititz, Pennsylvania, and are posted with his permission... and my annotations!















If you're interested in seeing (most of) these rocks, join next Sunday's Walkingtown, DC tour!

Labels: conferences, dc, field trips, meetings, msse, nova, piedmont

The other day I mentioned the Setters Schist.

Here's a couple of cobbles of the same formation, but lower stratigraphically than the stuff we saw on the University of Maryland petrology trip. The basal Setters has beautiful metamorphic tourmalines lying willy-nilly within the plane of foliation:







According to Mindat.org, "the general formula for this group may be written:

• A = Ca, Na, K, or is vacant (large cations);
• D = Al, Fe²⁺, Fe³⁺, Li⁺¹, Mg²⁺, Mn²⁺ (intermediate to small cations - in valence balancing combinations when the A site is vacant);
• G = Al³⁺, Cr³⁺, Fe³⁺, V³⁺ (small cations);
• T = Si (and sometimes minor Al³⁺, B³⁺);
• Y = O and/or OH; and
• Z = F, O and/or OH."

Note the constant there: boron! ...A lot of boron! Three boron atoms per unit cell... These metamorphic rocks have a sedimentary protolith. Where did the pre-metamorphic sediments get all that boron from?

Any ideas?

Labels: geology, maryland, metamorphism, minerals, piedmont

Yesterday, I showed you the Port Deposit Tonalite, stop #1 on the University of Maryland's annual ig/met pet trip. Today I'll share pictures of the next stop. We voyaged to the Hunt Valley Shopping Mall, where a lovely exposure of the Setters Schist can be found.

It's a lovely example of a classic-looking muscovite schist:


It is also chock-full of garnets! Millions and millions of them....

Some are small:


Some are medium:


Some are large:


Some are fresh:


Some are weathered:


Some are weathered-out:


There's also staurolite present:




Here's a nice big chunky staurolite:


In one localized zone, we also see some very big, rather lovely kyanite:




...Awesome! I love this suite of metamorphic minerals!

The Setters Schist is a highly metamorphosed pelitic rock (meaning that its protolith was clay-rich). It was presumably metamorphosed in the late-Ordovician-aged Taconian Orogeny, like everything else in the Mid-Atlantic Piedmont.

Next up, another member of the Glenarm Series, the Cockeysville Marble...

Labels: appalachians, maryland, metamorphism, minerals, ordovician, piedmont

Here's some photos from today's Physical Geology class field trip to the Billy Goat Trail. It actually snowed on us a little bit... cold! My student Luke O'Neil took all of these, hosted on his Facebook page, and this is an experiment to see if I can post Facebook photos on my blog... keeping my fingers crossed...

Migmatite:


Il profesore showing tilted tree trunks (knocked in a downstream direction during floods):


Folded graded bed in metagreywacke:


Students circle around an exotic boulder of the Catoctin Formation greenstone (from the Blue Ridge province); the boulder was transported downstream by the ancestral Potomac River when it was flowing on the Bear Island strath, before incision and abandonment of the former river bottom to become a bedrock terrace:

Labels: field trips, geology, maryland, national parks, nova, piedmont, rivers, teaching

The backlog of photos from my hikes several weeks ago still looms. I've showed you exotic cobbles, migmatites, graded beds, flood debris, and boudins, now for some folds...

As with the others, these are images from the Maryland Piedmont, along the Billy Goat Trail in C&O Canal National Historical Park.

Here's two repeats that fall, Venn-diagram-like, into the overlap area between the "graded beds" theme and the "folds" theme:




Now for some fresh, never-before-seen images:







(that's a fold cut twice oblique to its axis, resulting in an elliptical outcrop pattern).

Tiny folds:


Folds in one direction (top to bottom); boudinage in the perpendicular direction (left to right):


Found this one on the side of a cliff I probably should not have been scaling:


That's all for now... have a good Tuesday!

Labels: igneous, maryland, metamorphism, national parks, piedmont, primary structures, structure

Last week, I updated my field trip photo page with a fresh batch of images from last spring's Field Studies in Geology course to the Billy Goat Trail. Here are the new shots:












All photos are by Kevin Mattingly, NOVA photographer.

Labels: field trips, maryland, nova, piedmont, teaching

I'm returning now to the slew of new images I shot a couple of weekends ago on the Billy Goat Trail (BGT). Previous posts from these back-to-back morning hikes here, here, here, and here.

Today's theme: boudinage, the stretching & breaking of more competent rock units, and the gaps in between the 'chunks' filled in with less competent (more 'flowy') rock units, or by magma or other fluids. It's a behavior that's neither purely brittle nor purely ductile, but somewhere in between.

Boudinage of granite in metagreywacke:


Ditto (although some of this looks closer to hydrothermal quartz than granite, but there is some K-spar present...):


Felsite boudins in amphibolitic gneiss:


Pretty cool here; you can see that fluid magma filled in the gaps between the boudins. When this boudinage happened, the surrounding amphibolite was too viscous to flow into the gap. Furthermore, the asymmetry of these granite-filled tension gashes indicates some shearing: Was it a sense of shear that was concurrent with the boudinage (top to the left)? That was my initial take, but Kim (in the comments) suggested an alternative, which I like more and more: initial boudinage, and then later shearing in the opposite direction (top to the right). See the discussion in the comments section for more insight...



Some of the weirdest rocks on the Billy Goat Trail are these ones near Trail Marker 2. They are coarsely layered by composition, but I'm not able to figure out quite what the heck is going on with them. Is it just a gneiss with compositional banding ~3 inches thick? Regardless, it shows boudinage, both in horizontal cross-section...



...and in vertical cross-section:


When a rock gets boudinaged in two directions, it records flattening strain perpendicular to the plane of foliation, and goes by the colorful moniker "chocolate table boudinage." (Think of a Hershey bar's grid-like segments. If you smashed your hand down on it, the square chunks would separate from another and move apart, perpendicular to the direction in which you're pressing on it.)


Here's a quartz vein (cross-cutting metagreywacke) that's been boudinaged:



Part of this vein is milky quartz (on the left: white & easy-to-see), but part is transparent quartz (looks kind of grey in outcrop; difficult to see against a grey host rock), so I've used the wonders of Photoshop to turn that portion white, too, in this modified image:



Here's a new boudin that I never had seen before, on a diversion trail off the main C&O Canal towpath due to a breach in the Canal after Tropical Storm Hanna last year:


Lastly, here's something new (to me) that I found on my hike. It's a gigantic boudin of amphibolite in the foliated felsic rock showing chocolate-tablet boudinage that I showed up above. Unadulterated photo:


...And with annotations:


This is a big, angular block of amphibolite (about 1.5 m across) that has the foliation of the "gneiss" wrapping around it. Along strike of the foliation, there are two big rusty square holes, where I interpret other big boudins of amphibolite have weathered out. (As I showed the other day, the granite stands up signficantly better to weathering than does the amphibolite.) I was somewhat astonished to recognize this as a big boudin: it has very crisp edges, and is huge in comparison to other boudins that I am familiar with. Neat-O! I'm going to take my structural geology students here in a couple of weeks and have them examine and interpret these structures.

Labels: geology, igneous, metamorphism, piedmont, structure

GSW Spring Field Trip: Sunday, May 17, 2009

The Potomac Gorge: An Extraordinary Meeting Place of Geological and Biological Diversity

Led by: Tony Fleming, Natural Areas Geologist, and Gary Fleming, Vegetation Ecologist, Virginia Department of Conservation and Recreation

The Potomac Gorge between Great Falls and Georgetown is recognized as one of the most biologically diverse sites in the eastern United States, with an unusually large concentration of rare flora, fauna, and natural communities. For more than a century, the gorge has also enjoyed an iconic reputation as the region's premier geological area, both for its exceptional exposures of Piedmont bedrock and the complex tectonic history they reveal, and for the natural fluvial cycle of flood disturbance that still operates on the Potomac, the only Fall Zone river of its size whose flow is not altered by dams. Geology and ecology converge on this field trip, as we visit two sites where geologic processes exert a powerful influence on the distribution of unusual natural communities. At Turkey Run Park, we will hike past steep boulderfield communities and regionally rare sugar maple/mixed mesophytic forests more typical of New England, here growing on soils weathered from basic intrusive rocks in a cool microclimate created by processes driven by Pleistocene glaciation and the ongoing southward migration of the Potomac Valley. Chain Bridge Flats, by contrast, is a unique flood-scoured bedrock terrace hundreds of hectares in size that displays a complete ecotone of communities adapted to progressive changes in the form and intensity of natural flood disturbance as one approaches the river. Among these are disjunct, prairie grasslands containing calcium-loving plants more typical of the Midwest and Great Lakes. This site also is the largest and cleanest exposure anywhere in the Piedmont of the Sykesville Formation, the enigmatic and often inscrutable submarine trench deposit from the Taconic subduction zone that makes up much of the local bedrock. Here, a phenomenal array of textures, exotic inclusions, mega stratification, volcanic detritus, and metamorphic features can be seen together at a clarity and scale unlike anywhere else, providing insights into the origin of this enormous sedimentary melange.

Key Topics: Ecogeology; Georgetown Intrusive Suite; Sykesville Formation; Pleistocene and Holocene history of the Potomac Gorge; weathering, ground water, and nutrient cycling; flood frequency and dynamics

Field Trip Details: Hike departs promptly at 9:30 AM from lot C-1 at Turkey Run Park, and will follow the Potomac Heritage Trail towards Dead Run, returning to the parking lot by around noon. Eat lunch at the picnic area overlooking the old soapstone quarry at Turkey Run, before driving across the river to Chain Bridge Flats. Afternoon hike will depart around 1:30 from the parking area on Clara Barton Parkway immediately north of Chain Bridge, and will follow the towpath and ACE spillway out to the flats. Return by 4 PM. Expect spring wildflowers, poison ivy, some steep grades at Turkey Run, and rough terrain involving scrambling on rocks at Chain Bridge Flats. Sturdy footwear is a must. Bring lunch, snacks, and water. Restrooms are available at Turkey Run Park, but not at Chain Bridge Flats.

Questions? Contact Bill Burton at bburton@usgs.gov

Labels: field trips, gsw, piedmont, plants, rivers

Picking up where I left off last week with cool new pictures of rocks from the Billy Goat Trail, today we examine igneous beasties...

As you may have picked up from previous posts on this blog [e.g. here], the rocks of the Piedmont province are essentially the mangled remains of an ancient ocean basin: deep sea sediments, oceanic crust, volcanic islands, even microcontinents -- and all were crushed between North America and Africa during the mountain-building that closed the Iapetus Ocean and formed the supercontinent Pangea. Along the Billy Goat Trail, Piedmont rocks are exposed that started off as deposits of mud and dirty sand, but then were metamorphosed during mountain-building. From the bottom of the ocean to the center of a mountain belt: that forces rocks to change. In some places, they heated up so much that they began to melt.

When rock partially melts, but then the melt crystallizes in places (i.e., it doesn't completely drain out of the source rock), we call it a migmatite. The Billy Goat Trail has some spectacular exposures of migmatite. Here's three shots from the downstream end of the trail:







If migmatitic rock rips open while it is in this partially-molten state, that generates cavitites that the fluid magma flows into and fills. Here, for instance, you can see a rip in the foliated migmatitic metagraywacke that is filled with granite.


Further away from the source rock, mobilized magma can fill in planar fractures that cut across older rocks of many varieties. These cracks are filled in with magma that cools into igneous rocks, and we call them dikes. Here is a new dike I discovered on my hike last week: a vertical dike of granite about one foot thick, cutting across non-migmatitic metagraywacke:


Here's a granite dike cutting amphibolite; weathered out in high relief:


Same dike, from a slightly different angle (I leaned over to the left), to show how it pokes up above the amphibolite like a little wall:


Metamorphosed (some epidote present) granite dike cutting amphibolite:


These fractures didn't open up wide enough to admit large volumes of fluid (either magma or hydrothermal solutions), but there was some fluid flow along them. How do we know? The rock immediately adjacent to each crack weathers out in high relief, suggesting a higher proportion of stable, tough minerals (like quartz). [We've seen this before.] The base rock here is fine grained amphibolite.


Contact between a small granite pluton (or a large dike?) and neighboring amphibolite:


Tension gash in amphibolite, filled in with a mix of potassium feldspar and quartz:


Xenoliths of foliated biotite-rich rocks which I interpret to be metagraywacke that has had all its felsic melt expressed from it, then ripped off by the growing granitic magma chamber (stoping) and dropped into the magma (relatively low temperature, so the biotite doesn't melt), and rotating around to new orientations which do not match the regional foliation orientation. I'm seeing these as shreds of the 'depleted' migmatitic source rock...


Closer-up of these xenoliths #1:


Closer-up of these xenoliths #2:


Another cool thing I saw on last weekend's hikes was pegmatite. Pegmatites are present where there is a particularly watery magma. Water, the universal solvent, helps act as a courier, ferrying atoms around to where growing crystals can access them and add to their bulk. As a result, pegmatites are characterized by really large crystals. These potassium feldspars are highlighted by lichens which grow at the interface between the feldspars and the surrounding milky quartz:


Those same black-colored lichens can also highlight the cleavage planes of the feldspars:


Another big-ass K-spar:


...and another:


I love this stuff. Hope you enjoy these igneous treats as I much as I enjoy sharing them.

Labels: igneous, maryland, metamorphism, piedmont

It's getting to be springtime... and that means field trips!

My first field trip of the semester is tomorrow: my friend David Dantzler has organized a trip to look at stratigraphy and structure out on a new highway in West Virginia. I'm supplying half a dozen Honors students and a NOVA minivan, but David's handling the content. And of course, I'll be on hand to comment on "teachable moments." Looking forward to it.

Other trips upcoming this semester: Billy Goat Trail (x4!), Massanutten Mountain, Old Rag Mountain, Washington DC walking tour, and a weekend-long structural geology trip to the Blue Ridge and Valley & Ridge provinces. I love field trips; really they were the aspect of majoring in geology that appealed to me the most - the fascination with Earth processes took longer to develop.

See you in the field!

Labels: blue ridge, field trips, maryland, nova, piedmont, teaching, valley and ridge, virginia, west virginia

I mentioned seeing some cool stuff when I went hiking on the Billy Goat Trail last weekend.

One of the things that really caught my eye were multiple new exposures of graded bedding. These rocks began as deposits of sediment offshore from a volcanic island arc: they consist of turbidite deposits that were then squished and squeezed as that volcanic island arc collided with eastern North America during the closure of the Iapetus Ocean. As a result of this, they were metamorphosed and deformed. But in a few places, you can still see the relict graded beds that originated through the settling out of turbidity currents.

Here's some images:

I count four or five here:





A nice central fault zone displaced the central block downward:




This one is a little more subtle...


Here's one that's been turned upside down (by tectonics):


And there were also some folded examples:




A close-up of the hinge of this folded graded bed:


Pretty cool, eh? The only problem is these samples aren't on the Billy Goat Trail itself, which means I'll really never be able to show them to students except in photographs...

Labels: geology, maryland, metamorphism, national parks, piedmont, primary structures, sediment, structure

Ladies and gentlemen, spring has arrived in the Washington, DC region. It is sublime. I'm very grateful that it's my spring break this week because even though I still have a ton of work to do, I've had the opportunity to get outside every day and enjoy a bit of the weather.

This weekend, I got up early both days and headed out the the Billy Goat Trail, a rugged hiking trail along the Potomac River's gorge about 12 miles upstream from DC. I departed from the trail itself both days, which was great because it brought me to places I hadn't seen before. I found a lot of cool new structures and rocks! Over the next few days or weeks, I'll be sharing some of those images with you, but for today, I figured I'd show you some 'soft' imagery, just to celebrate the fun of being outside on a hike on a lovely day. ...and wearing short sleeves, no less!

Here's a shot of typical scenery along the Billy Goat Trail. This is looking upstream:



One of my side-trips off the trail... because the water level was pretty low, I was able to get to some islands that are often inaccessible. This is the channel between the Rocky Islands (downstream of Great Falls, upstream of Mather Gorge):



This land is all part of the C&O Canal National Historical Park. Here's a spot where rains from Tropical Storm Hanna breached the wall of the C&O Canal, allowing its water to drain downward into the Potomac. Because the canal's towpath was located there, the Park Service has constructed a temporary path which detours around the breach:



I saw some good birds on my hikes there. Red-tailed hawks, double-crested cormorants, Canada geese, mallards, belted kingfishers, pileated woodpeckers, red-bellied woodpeckers, tufted titmice, chickadees, robins, blue jays, and great blue herons. Also, both local species of vultures: the turkey vulture and the black vulture. This is a black vulture (note the black, not red, head):



Here's some tracks: theropod dinosaurs? ...or great blue heron? You be the judge:



Here's a cool fish skull I found:



Of course, it wasn't all scenery, birds, and fish. There were rocks, too. I took a lot of rock photos, and you'll get to see them all in due course... But for now, let me start you off with the tame stuff. Here's some cobbles I encountered along the hike...

Cobble of the Seneca Sandstone (Triassic arkose) showing a mudchip rip-up clast:



Tilting it a bit, you can see other mudchips too:



Cobble of cement containing Seneca chunks:



Cobbles of quartzite of the Antietam Formation showing Skolithos 'worm' tube trace fossils:



I love these Skolithos tubes. It's hard not to love them, and they're everywhere around here. Like the Seneca cobbles, they come from source areas to the west (Culpeper Basin & Blue Ridge, respectively), and were transported to the Maryland Piedmont by the ancestral Potomac River.



My favorite Skolithos-bearing quartzite cobble:



...And the same cobble, end-on:



More to come, tomorrow...

Labels: birdies, blue ridge, culpeper basin, fish, fossils, maryland, national parks, piedmont, sediment

Labels: granite, metamorphism, piedmont, quartz, structure, virginia

Just got a batch of images from the NOVA photographer, Kevin Mattingly. I particularly like this image of last spring's Field Studies class at the Billy Goat Trail:

Here, we're overlooking the upstream end of Mather Gorge, checking out some ~360 Ma lamprophyre dikes exposed there -- but offset on either side of the river!

Labels: devonian, field trips, national parks, nova, piedmont, teaching

This weekend, my MSSE advisor John Graves was in town, and I took him out to a couple of field locations that I bring geology students to. We started off on Saturday afternoon on the Billy Goat Trail, where I went through the usual rigamarole, what with the Iapetus Ocean, Taconian Orogeny, migmatites, and what-not.

We also saw some cool fractures involving quartz, in two different situations, each instructive in its own way.

First, here at the base of the legendary "Traverse," is some metagraywacke that has fractured. Quartz-rich fluids flowed along these fractures, and the quartz they precipitated (presumably in interstitial spaces between grains?) made that particular zone on either side of the fracture more resistant to weathering than the non-quartz-infused metagraywacke. This "fortifying" effect falls off with increased distance from the fracture. Note that you can actually see the crack in each of these high-relief ridges; it's not a quartz vein per se, but a separate, related phenomenon. Penny for scale in both photos below -- one zoomed out, one zoomed in...





Second, check out these photos, of a spot near the downstream end of the Billy Goat Trail, where usually I don't have time to take students. The bedrock here is a migmatitic schist/gneiss. Here, you'll see ~vertical foliation cut by a ~horizontal quartz vein. Once again, a penny is for scale (this time held in place with some chewing gum, as the outcrop surface is vertical, striking at a right angle to foliation). These two structures are both representative of the same stress regime. With a dominant (tectonically-induced) stress directed ~horizontally, the various minerals in the original rock rotated (or grew) into new positions perpendicular to that stress (e.g., ~vertical). But that wasn't quite enough to accomodate the ~horizontal shortening. Some additional strain was accomodated by ~vertical extension through fracturing. That fracture was infilled with hydrothermal fluids that precipitated "milky" quartz, at almost a perfect right angle to the foliation:





John was suitably impressed, and we both appreciated the afternoon hike in EXCELLENT weather (55 degrees F; gorgeous!).

Labels: metamorphism, msse, piedmont, quartz, structure

One of my favorite hiking and geologizing destinations, the Billy Goat Trail (in C&O Canal National Historical Park) was the site of a gruesome discovery Saturday: a dead body! More here from MSNBC. Hat tip to Michelle A. for the prompt notification.

UPDATE: Same info, but from the Post.

Labels: field trips, maryland, news, piedmont

Strange Maps has an interesting piece up today about where West Virginia came from (as a state): turns out it was all about the Civil War. The accompanying map shows the original proposed name for West Virginia, "Kanawha," as well as a proposed demarcation between Virginia and Maryland that trended along the western margin of the Blue Ridge physiographic province. If this boundary had come to pass, Virginia would have gotten the Valley & Ridge province, but Maryland would have retained the Blue Ridge, Piedmont and Coastal Plain.

Labels: blogs, blue ridge, coastal plain, history, maps, maryland, piedmont, valley and ridge, virginia, west virginia

Yesterday, I took a three Honors students and a colleague to Difficult Run, Virginia. This is a hiking trail that goes from Georgetown Pike, in the tony neighborhood of McLean, Virginia, down through a deep, steep river valley to the Potomac River.

As noted a couple days ago, the trail is right across the Potomac River from my beloved Billy Goat Trail. In a recap from that post, here's a map of the area... Feel free to switch it to "satellite" view.



Some discussion of the bedrock geology of Difficult Run can be found here, in an excellent field trip guide by Scott Southworth (USGS) and colleagues that's part of Excursions in Geology and History (Frank Pazzaglia, editor).

We began our trip by meeting up with Doug Dupin of the Palisades Museum of Prehistory, who joined us for our exploratory geohike. We walked a short distance down the trail and found a big (abandoned) quarry where it was rumored there was a good fault. This is one of these pieces of information that I heard somewhere, at some point. I couldn't find it in any literature, so maybe I heard it in discussion when I taught at George Mason University for a year between grad school and when I got my position at NOVA. Anyhow, I had never actually checked it out...

...So our first order of business was to review the criteria for identifying a fault: What would we look for? Fault breccia, fault gouge, slickensides, hydrous mineral veins, and of course, offset. However, here in the Virginia Piedmont, it's rare to have a good marker unit to compare on opposite sides of the fault: usually it's just schist on one side, schist on the other. In some places, you could add the presence of a fault scarp to that list, but being as how this was an old quarry, geomorphic features like that didn't seem likely. So our search focused on the search for fault breccia, fault gouge, veins of odd minerals, and slickensides.

A few minutes in, we found some slickensides on this boulder of float:

This is a boulder of migmatitic phyllonite, with a wavy texture due to mylonitic flow at depth. (The picture doesn't show this very well at all, though you can see faint undulations 'cascading' from the top of the photo towards the bottom. It's much clearer in cross-section.) Anyhow, the 'slicks' are a faint upper-left to lower-right lineation seen on this surface, one or two degrees off from the orientation of the ballpoint pen. The surface you're looking at here was a fault plane at some point in its history. Ballpoint pen for scale.

We did eventually locate the fault, uphill from this boulder. It was characterized by a zone of fault gouge (pulverized rock), three inches wide to a foot wide in places, and highly oxidized (presumably by oxygen-rich meteoric waters percolating along this fractured surface)... but there were no good marker units to judge the total offset.

Here's a different section through a similar rock (though I wouldn't apply the "phyllonite" textural description to this one). Instead of looking at the plane of foliation here, we're looking at a surface which is perpendicular to the foliation plane(s)....

Here in this image, you can see two cleavages... One which runs roughly upper-left to lower-right through the photo, defined by gneissic banding including bands of granite (light-colored; late Ordovician in age... Taconian Orogeny). A second cleavage runs roughly left-to-right through this photo. This second cleavage overprints the first. The overall interpretation is that the first cleavage developed due to lower-left-to-upper-right compression, forming the foliation defined by alternating bands of different compositions of minerals in an upper-left to lower-right direction. The second cleavage formed due to compressive stress sub-parallel to the pre-existing foliation, deforming it into a series of tight folds. The limbs of these folds line up parallel to one another, defining the second-generation, overprinting cleavage. Can anyone else add to this interpretation? Dime for scale.

Along Difficult Run itself, the outcrops were all relatively recently scoured (in 1972 by Hurricane Agnes), so there are some good exposures. As I noted earlier this week, the area shows some nice exposures of granite pegmatites (keys, and the edge of the Pazzaglia volume, for scale):


On our field trip yesterday, we took at closer look at these beautiful pegmatites, and the associated amphibolite bodies. Take a look at this close-up... Dime for scale.

What's going on here? You've got a beautiful (euhedral/subhedral) example of an orthoclase feldspar ("potassium feldspar") crystal amid a bunch of quartz. But look closer at the feldspar crystal... this sucker has been fractured in many places, and it's shot through with very small veins of quartz. Somehow, as this pegmatite dike was cooling, the earlier-crystallizing feldspar was broken and intruded by the presumably-still-fluid silica-rich magma. Anybody able to expand on this interpretation and shed some light on how this all played out? Or contradict it and give a different story to explain this relationship?

In the neighboring amphibolite, we checked out these cool ridges of resistant rock which are centered on thin fractures. Here, you see a couple of intersecting joint sets, each of which was the "plumbing system" for silica-rich hydrothermal fluids (my interpretation). These silica-rich hydrothermal fluids impregnated the surrounding amphibolite with quartz, which made the immediately-adjacent areas more silica-rich, and hence more resistant to weathering and erosion: Hence, now that they've made it to the surface, they're weathering out in high-relief. Dime for scale.


A bit further downstream, Doug showed us a 'cave' (central dark area, just to the right of the waterfall) between the bedrock and a big slab of sloughed-off migmatitic metagraywacke:

We each edged into the 'cave' to the end, where Doug has shown that a distinctly-rectangularly shaped hole admits a direct beam of sunlight during the fall and spring equinoxes. From the inside, it's a striking arrangement, enough to make you wonder whether it's anthropogenic. However, from the outside I was unconvinced that the hole's position was anything other than natural. Doug's initial intepretation of the site was strongly influenced by the fact that there are some unambiguous petroglyphs a short distance away from here, and based on this proximity, I think it's acceptable to infer that Native Americans may have visited this cave. However, I interpreted the opening to be completely natural, with no need to invoke anthropogenic modification in any way.

We hiked on along a ridge overlooking Mather Gorge, sighting a fox and an accipiter (Coopers? Sharp-shinned?) and a few vultures, and returned to the parking lot as the sun dipped low in the sky. On the way back to campus, Honors students Ana and Hope fed us Swiss cookies and cheese & crackers. Altogether, it was a pretty great way to spend a November afternoon...

Labels: birdies, faults, field trips, granite, igneous, metamorphism, ordovician, piedmont, rivers, virginia

This week, I'm taking some of my Honors students to Difficult Run, Virginia.

It's right across the Potomac River from my beloved Billy Goat Trail. Here's a map of the area:



Some discussion of the bedrock geology of Difficult Run can be found here, in an excellent field trip guide by Scott Southworth (USGS) and colleagues that's part of Excursions in Geology and History (Frank Pazzaglia, editor).

Here's a look at Difficult Run, looking upstream from below one of the several waterfalls there:



These outcrops were all relatively recently scoured (in 1972 by Hurricane Agnes), so there are some good exposures. We're going to look for a fault reported to be there, as well as the incision geomorphology of Difficult Run itself, and some nice exposures of granite pegmatites (keys for scale):





This field trip is less a guided tour, and more of an exploration, so I hope when we get back, I'll have some photos of new and interesting things to share.

Labels: faults, granite, igneous, metamorphism, ordovician, piedmont, rivers, virginia

The Virginia Community College System (VCCS) organizes conferences occasionally where faculty in different disciplines can get together. This weekend was the "peer conference" for the natural and physical sciences. It was held at the lovely mountain resort called Wintergreen, in central Virginia's Blue Ridge Mountains.

Here's a map of the area:

That's the Shenandoah Valley on the left (part of the Valley & Ridge province), the Blue Ridge in the middle (running from NE to SW), and the Piedmont province on the far right. Wintergreen is a bit SW of Charlottesville.

The conference was fruitful and interesting. I enjoyed getting to meet a bunch of the other VCCS geology faculty and discussing what we want to do in the future in terms of supporting one another and professional development. I gave a talk about new technologies in geology instruction, which included information about the geoblogosphere and other sundry web resources I use. My colleague Erik Burtis at NOVA-Woodbridge led us on a cool "field trip" to Glacial Lake Missoula, via Google Earth.

I spent a lot of time talking with Pete Berquist, from Thomas Nelson Community College, discussing next summer's Regional Field Geology of the Northern Rocky Mountains course. We laid out a series of goals for the students, and created a tentative itinerary. Pete and I took a great hike at the end of the first day, poking around in the rocks and watching the sun set over those gorgeous mountains. Friday evening, there was a cool astronomy session, where Ed Murphy from UVA showed us the Ring Nebula, the Andromeda Galaxy, and assorted other stuff in outer space. He had a great laser pointer that extended a green laser line up about 80 feet into the sky... Very useful for pointing things out. Low light levels in the forested mountains meant excellent stargazing. Saturday morning, Bill Warren of Lord Fairfax Community College gave a good talk about the global energy crisis, and potential solutions. I picked up a few good resources there that I'll use next semester in teaching Environmental Geology. And then when the conference concluded, there was a geology "hike" out to look over the landscape. By driving us to a couple of different overlooks, Doug Coleman of the Wintergreen Nature Foundation showed us spots where we were able to look east into the Piedmont, and west into the Valley & Ridge. Pretty cool, though we didn't look too closely at the actual rocks exposed there. Fortunately, I have an inclination to do that on my own... as you'll see below:

Catoctin Formation greenstone (meta-basalt), showing chlorite-rich portions (left) and epidote-rich portions (right). Quarter for scale.


More Catoctin, the volcanic breccia layer. Lots o' epidote. Quarter for scale.


Is this a quartz vein or a granite dike?
At first glance, it appears to be your standard hydrothermal quartz vein full of milky quartz, but then you'll notice that it's not just quartz. There are also two crystals of orthoclase feldspar in there. (The dark shapes are just empty holes & shadow, not mafic minerals.) I pointed this phenomenon out before, but I'll state it again: I think that hydrothermal quartz veins and granite dikes are not separate phenomena, but points along a spectrum of composition. Quarter for scale.

Looking southeast towards the Piedmont:


Looking northwest towards the Valley & Ridge:

Labels: appalachians, blue ridge, conferences, igneous, metamorphism, piedmont, valley and ridge

Last week was field trip week for me. I led trips to the Billy Goat Trail on Tuesday and Thursday, and to Washington, DC, on Saturday.

On the Physical Geology field trip to the Billy Goat Trail, we saw rocks like amphibolite, metagraywacke, and migmatite:







Hope and Ana checking out the migmatite:


The group poses with the migmatite, to show how close anatexis is to their hearts...


Jane examines lamprophyre in a weathered-out dike:


Noting the characteristics of metagraywacke:




Traversing 'Pothole Alley'... Joel looks chilly...


Our lunch spot... Alex pretends to dive into the Potomac River...


Traversing 'The Traverse':


On the Historical Geology field trip to DC on Saturday, we were amused to find a jack-o-lantern that had facial hair resembling mine...



But that's not all! We also saw some geology. While you can get a more complete picture at my "DC Rocks" webpage, I'll post a few new photos of new outcrops here...

Here's a nice slab of granite (very angular) set in metagraywacke matrix (metamorphosed accretionary wedge complex)...


Here's two members of the Georgetown Intrusive Suite, showing the (earlier) gabbro stoping xenoliths into the (later) granite:


I love field trips. I love seeing my students light up at being outside, at getting a handle on the stuff we talk about all semester in class. I think field trips are super duper important.

Labels: dc, field trips, granite, igneous, maryland, metamorphism, nova, piedmont, teaching, xenoliths

I was reminded this morning to update the list of activities for the coming week, including the Billy Goat Trail hike scheduled for Friday:

Wednesday evening: GSW. Free and open to the public.

Thursday: James Ussher's "birthday of the Earth." How will you be celebrating?

Friday afternoon: I'll be leading a public geology hike along the Billy Goat Trail, starting from the Great Falls Tavern Visitor Center (C&O Canal NHP) at 12:30pm, going til 4:30pm or so. Maybe 5pm. Free and open to the public.

Saturday: GSW Fall Field trip: "Tidewater Geomorphology at George Washington's Birthplace National Monument, Westmoreland County, VA." RSVP.

Labels: coastal plain, field trips, gsw, meetings, piedmont

This weekend, I spent three days on an extended field trip down to southwestern Virginia with NOVA adjunct geology instructor Chris Khourey and four of my Honors students. We left Annandale on Friday morning, and made our first stop at Willis Mountain, Virginia, site of one of the most productive kyanite mines in the world.

Here's a Google Map of the mountain:


The Kyanite Mining Corporation was very gracious in hosting us. I'd particularly like to thank Mike Morris, who took two hours out of his day to show us the site and the mining operation.

Why mine kyanite? It's used as a refractory mineral: that is, one that won't melt under high temperatures. A lot of their kyanite is heated in kilns to produce a second mineral, mullite. The mullite is even more stable than kyanite in high temperature refractory situations. (It won't melt until it hits over 1800 degrees C!) Additionally, they cleverly saw up big blocks into dimensional stone for countertops and the like.

The kyanite mined at Willis Mountain is in a quartzite which also includes a fair amount of pyrite and hematite. We heard about the different procedures used to extract the non-kyanite minerals so that their end product is relatively pure and of constant quality.

Here's Mike showing the overall anticlinal shape of the deposit:

It's a plunging anticline, as you can probably make out from the Google Map terrain view up top.

Some of the dimensional stone, which I think is pretty spectacular:


Close up of the kyanite (light blue, on left) in the dimensional stone.


Nearby Baker Mountain also hosts kyanite deposits, which show a deeper blue color (Mike wasn't sure why, but suggested that chromium may be responsible):


Inside a huge storage building where the mullite (white powder at our feet) is stored:


Atop Willis Mountain itself, showing the weathered kyanite quartzite exposed there:


Honors students ask questions of Mike:


Mike and Chris standing near some fresh boulders of kyanite quartzite:


It wasn't all metamorphism and mining... I also noticed these nice raindrop impressions in a drying mud puddle:


After lunch atop the mountain, we hopped back in the van and hightailed it for southwestern Virginia, on our way to the Virginia Geological Field Conference. More on that tomorrow.

Thanks again to Mike and the good folks at the Kyanite Mining Corporation for hosting our visit!

Labels: conferences, economics, field trips, metamorphism, minerals, nova, piedmont, teaching, virginia

The largest meteorite (or maybe comet?... we don't really know which) impact crater in the United States is in Virginia, underneath the lower Chesapeake Bay. In the Eocene, a large bolide (unidentified space chunk) slammed into the Earth. Dating of microfossils found in the same sedimentary layers as impact ejecta have provided a date of ~35.5 Ma for the event. The impactor hit on the continental shelf offshore of Eocene Virginia, carving through the Atlantic-deposited sediments there and gouging into the crystalline bedrock beneath (igneous and metamorphic rocks like the modern Piedmont province, but buried beneath Coastal Plain layers).

The crater was discovered over a ten-year process that began with offshore sampling near Atlantic City, New Jersey in the mid-1980s. Those drill cores came up with a layer of ejecta (including shocked quartz and little beads of glass called tektites) among the late Eocene layers of sediments. Searching around, eventually the crater was seismically imaged by oil exploration in the Chesapeake Bay in the mid-1990s.

Centered on Cape Charles, Virginia, the crater is about 50 miles across, but appears wider as sedimentary layers adjacent to the hold have slumped inward along listric faults. The James, York, and Rappahannock Rivers all trend into this depression, and ultimately the crater is probably responsible for the Susquehanna River taking on its southerly course. When sea level rose and flooded the valley of the Susquehanna, the Chesapeake Bay was formed.

A similar impact structure offshore of New Jersey, the Toms Canyon Impact Crater, may have formed at the same time as the impactor broke into pieces before impacting.

The lead-off image to this post is by the team at the U-Haul trucking company, which performs a terrific public service by finding out interesting things about the different states (and Canadian provinces) and posting them on the sides of their trucks with eye-catching graphics. A great many of the topics they choose are about geology, from minerals to fossils to impact craters to cartography and canyons. A while ago, I wrote an article for Geotimes looking at their program.

More information on the crater:

Wikipedia's entry on the crater.
W&M Geology Department's page about the crater.
USGS team examining the crater.
National Geographic article (2001).

Labels: art, cenozoic, coastal plain, geology, meteors, new jersey, piedmont, rivers, virginia

Labels: appalachians, geology, piedmont, rivers

Fellow DC metro area residents -- there are a bunch of geology events coming up in the next couple of months that you may be interested in. Everything* listed here is free and open to the public.

Next Sunday, August 24, I'll be leading an event called "Geology Along the C&O Canal," at the Lock 8 River Center from 10am until 11am. My plan is to give an overview of the Appalachian mountain belt, then focus on the Piedmont "chapter" of that story, using local outcrops to illustrate the rock types produced. I'm not sure if you need to reserve a spot or not; Call Bridget Chapin at the Potomac Conservancy (number at link above) to inquire about details.

Friday, September 5: "Geology Along the Billy Goat Trail," I'll lead this hike along the famous Billy Goat Trail, examining its exquisite display of metamorphic geology and geomorphology. 12:30pm-4:30pm. Reserve a spot through the good folks at the Great Falls Tavern Visitor Center.

Wednesday, September 10: first Geological Society of Washington meeting of the fall. Beer served at 7:30pm, and the formal program begins at 8pm. At the Cosmos Club in Dupont Circle.

Saturday, September 20: I'll be leading my "History Before History: the Geologic Saga of Washington, DC" walking tour as part of Walkingtown, DC. The tour runs from 1pm until about 4pm, and involves about 2.5 miles of walking from Adams-Morgan to Georgetown. Limit of 30 people; interested walkers should reserve a spot with Cultural Tourism, DC, the nonprofit group that sponsors Walkingtown, DC each spring and fall.

Sunday, September 21: For those who can't make it Saturday, I'll again be leading my "History Before History: the Geologic Saga of Washington, DC" walking tour as part of Walkingtown, DC. The tour runs from 1pm until about 4pm, and involves about 2.5 miles of walking from Adams-Morgan to Georgetown. Limit of 30 people; interested walkers should reserve a spot with Cultural Tourism, DC, the nonprofit group that sponsors Walkingtown, DC each spring and fall.

Wednesday, September 24: Another Geological Society of Washington meeting, but I'll be delivering a talk at this one. My talk's title is "Rise of the geoblogosphere."

Sunday, October 5: I'll be delivering a talk called "A Geologist's Perspective on Climate Change" at the Chinn Park Regional Library in Woodbridge, Virginia. 2pm-3pm.

Friday & Saturday, October 10-11: The Virginia Geological Field Conference, in Marion, VA. "Geology of the Saltville and Pulaski Fault Blocks" is this year's topic. *This is the one item on the list that is not in the immediate DC metro area, and also the one item on the list that costs money -- registration is $45 for professionals, $20 for students. Transportation, lunch, and guidebook will be provided. See more details on the website. If you're interested in comparing and contrasting two Valley and Ridge fault blocks shoved westward during Alleghenian mountain-building, this might be of interest to you.

Thursday, October 23: the Earth's birthday, according to James Ussher. 4004 BC to 2008 AD; does that make it 6012 years old? Or is it 6011 years old, since there was no year "0"? Tricky... Regardless, I'll be serving lithosphere/asthenosphere cake/pudding to NOVA students in celebration of the day. (I posted on visiting Archbishop Ussher's church here.)

Wednesday, October 22: Another GSW meeting. Same time, same place, but this time I'll be back where I belong: in the audience.

Friday, October 24: "Geology Along the Billy Goat Trail," I'll lead this hike along the infamous Billy Goat Trail, examining its exquisite display of metamorphic geology and geomorphology. 12:30pm-4:30pm. Reserve a spot through the good folks at the Great Falls Tavern Visitor Center.

If you're into geology and you'll be around, I hope you'll join us on one or more of these events.

Labels: blogs, climate change, CO2, dc, geology, global warming, gsw, nova, piedmont

Yesterday, I took my Audubon Society / USDA Grad School "Natural History Field Studies" students on a field trip to examine the bedrock geology of Washington, DC. We had a good time: beautiful weather, great attitudes, and even luck with parking! I guess because it's Memorial Day weekend, a lot of people have left town. One of the great challenges of urban geologizing is finding room for those infernal cars...

Here's a photo of the group at Chain Bridge, DC, on Sunday morning:

That class ends on Monday night, bridging the gap between my NOVA spring and summer semesters. It's been a good run -- thanks, folks!

Labels: dc, field trips, piedmont, teaching

This is how good it is to be a professor on summer break: Yesterday afternoon, after composing yesterday morning's epic account of my Massanutten trip, I toodled on over to the Palisades Museum of Prehistory to (a) drink beer and (b) talk rocks with the museum's curator, Doug Dupin.

The Palisades Museum of Prehistory is in far western Northwest DC, near the Dalecarlia Reservoir and Sibley Hospital. There, you'll find a neighborhood called the Palisades, and in the Palisades, you'll find Doug Dupin's house. In Doug's backyard, you'll find what appears to be a nice shed. Turns out, this is the museum. It's a long story, but basically it boils down to this: Doug was a cartographer, but a contract went sour, and so he was staying at home with a lot of time on his hands. He decided to grow some grapes to make wine, and store that wine in a self-dug wine cellar. He started digging the hole, and encountered arrowheads, pot sherds, and other artifacts. He got intrigued, and decided to showcase the findings atop the wine cellar in a self-made museum.

If you want more details, the Washington DC CityPaper profiled Doug in a 2006 article. A good read; I recommend it.

Doug is a great guy -- pursues what he's interested in, be it homebrew, viniculture, skateboarding (he once rode the length of the C&O Canal on a self-made board -- read about it in this New York Times Magazine article), or archaeology.

Doug attended my "Walkingtown, DC" walking tour of DC's geologic history, and brought along a few odd rocks for me to identify. At the end of the tour, he invited me over to see his museum. Yesterday, I finally got the chance to do that. We cracked open a couple bottles of Dogfish Head 60-minute IPA and started browsing his collection of found prehistoric objects. Doug was very interested in my analysis of rock types (apparently archaeologists use a different set of terminology for describing what rock types projectile points are made out of).

On his own property and in neighboring areas of the Palisades, Doug has found hundreds and hundreds of objects, many of them beautifully worked arrowheads of flint, quartzite, and rhyolite. There are also some oddballs that don't fit with the human prehistory theme: a 1791 coin bearing the image of Louis XVI, crystals of amethyst and gypsum, old glass bottles, rounded river cobbles, and anything else that caught his attention. One of the most astounding things I saw yesterday was a huge woolly mammoth tooth. Doug told me a friend of his found it in the Potomac River while canoing (I think he said near Seneca Creek, but that was a beer and a half in, so maybe I've got that wrong). But there it was, a fully ridged mammoth molar; unmistakable. I hadn't heard of previous mammoth finds in our area, but I guess it's not surprising they were here.

Anyhow, I had a great time, and I recommend that everyone in the DC area make an appointment with Doug to go check out his collection and support his project.

Labels: cenozoic, dc, history, mesozoic, museums, piedmont, tourism

Morning, folks. I awake to a challenge from Chris at GoodSchist, to show where my local bedrock was at the time of Pangea's incipient breakup. (I think Chris chose the late Triassic, 220 Ma, since Ron Blakely's map of that time shows New Zealand clearly in the south.) It's an interesting time for the rock beneath Washington, DC. After have just experienced ~50 million years of crunching between North America and Africa, DC's tortured bedrock is now being stretched as Africa begins to pull away again. A series of rift valleys mark the stretching of the crust, shown clearly in the map as a series of NE-SW oriented lakes along the axis of the Appalachian orogen.

DC's future location is between two of those rift valley lakes: one to the east, one to the west. If I owned DC real estate during the Triassic, I'd be very interested in this process, because one of those rift valleys is going to become a new ocean basin, and one isn't. The one that isn't is destined to stop opening and fill in with dirt. It will be a failed rift valley, an aulacogen of sorts.

The question is: which one is the weakest link? If the one to the west breaks open, that will be the new Atlantic Ocean basin, and DC will stay hitched to Africa. If the one to the east breaks open, that will be the site of the Atlantic, and DC will stay hitched to North America.

As it turned out, the eastern rift was the one that connected up with other rifts to the northeast and southwest, and became the young Atlantic. The western rift, known as the Culpeper Basin, stopped stretching open, and got filled in with sediment. DC stayed attached to North America, and that's the way it is.

Labels: culpeper basin, dc, geology, mesozoic, piedmont, plate tectonics

While toodling along the web on some other business this week, I stumbled across this publication by the Virginia Department of Mines, Minerals, and Energy.

I had no idea that there were any diamond finds in Virginia. But apparently there are, scattered across three different physiographic provinces!

On Thursday's excursion, Chris and I tried to find the "Front Royal Peridotite," one of seven locations mentioned in the DMME publication. It's a single dike which crosses State Road 626 southeast of Waterlick, Virginia. But to no avail! There were no outcrops visible on either side of the road, and there was a dense little cluster of houses bearing manicured lawns. Bummer. That would have been cool.

I'll try and visit a couple other localities mentioned in the report over the next year or so, and hopefully I'll find some of these igneous source rocks, though I don't hold out much hope of actual diamonds.

Labels: blue ridge, diamonds, minerals, piedmont, valley and ridge, virginia

Here's another transcript of one of my field trips, again from uber-dedicated student Jill.

--- T R A N S C R I P T --- B E G I N S ---

Prince William Forest Park - Field Notes
Hike April 6, 2008
Paper due: Sunday, April 20th

Transcription Note - I apologize for the lack of words, at times. We were so near the river, the background noise of the river made it difficult to transcribe. Also, it rained during the morning. It was not the best day for a hike, but the weather outsmarted all of us that weekend.

Valley - Quantico Creek - Main Creek
South Fork - Main Tributary

(Tape #2?)

Field Notes - Weather: Raining...Callan cautions us about being nice to one another in the rain...

On Trail Observation - Top of high hill - rounded river gravel mixed in with sand & clay

On top of another hill - we will talk about it.

Volcano erupting signage. - Hawaiian volcano example. What it would have looked like 500 million years ago. Chain of volcanic offshore islands spewing out lava. As time goes by subduction is bringing those to the edge of North America until they eventually collide. Then all igneous rock - basalt rock gets slathered onto the edge of North America. Ex. - A snowball. New basalt rock is packed onto the edges - younger material added onto the edges.

Continents (low in density 2.7) never subduct. Oceanic crust subducts (2.9 vs. 2.7). Whenever the two butt heads, oceanic crust loses. Result is isotopically dated rocks which make up the continents and ocean floor. Ages are wildly different - 4000 million years (rocks which make up the continents) vs. 200 million year old (ocean floor). There is a huge difference. Continents are 40 times as old as the oceans.

Oceanic crust is constantly getting destroyed vs. continental crust which is constantly getting preserved.

Oldest rock is in the Northwest Territory of Canada.

Old Rag mountain. Rock from Old Rag is 1 Billion years old. 1 Page handouts. 1st event - Grenville Orogeny intruded granites into the crust. Granites are a symptom of mountain belts/building. And the granites cooled and became Old Rag. The Iapetus Ocean Basin was opened up. Cracks opened up into that granite into which mafic igneous rock (basalt) squirted into those cracks. Hiking up Old Rag - narrow little slots - which are those cracks. Floor is made up of dark colored, fine grained rock whereas the sides are coarse grained, light colored granite.

All that transpired before the stuff we are talking about today.

Volcanic rock is underneath us. We eventually will see it. First we will see sediments that accumulated at the bottom of the Iapetus Ocean right next to the volcanoes. Then at lunch today we will see some of this volcanic rock, itself.

We will get to see the rocks that made up volcanic islands.

We are at the boundary of sediments and the island arc further east.

Stop - signage. Small lake, dammed valley Sign about the Fall Line. 2 Virginia physiographic provinces meet. Region of the 1) Piedmont 2) Coastal Plain (as day goes by)

Boundary is the Fall Line. Coastal Plain is made out of very loose sediments, not stuck together in to a rock; loose gravel, loose sand, loose mud. Therefore, it is very easy to erode away. It is easy for water to strip it away.

Whereas the Piedmont is made out of hard rocks. (Like the ones we are gong to spend most of the day looking at). As water is draining from the Appalachian Mountains in the Atlantic Ocean, it is easy to strip away coastal rock and hard to strip away the Piedmont Rock. And, as a consequence, the boundary between the two you tend to get rapids and waterfalls. Those rapids and waterfalls line up on a line from Southwest towards Northeast - we call that line the Fall Line.

Here is the fall zone. The image on the left is a Geologic Map. You can see the difference in colors along the water. Coastal Plain layers vs Piedmont Plain layers. The Coastal Plain is draped on top of the Piedmont like a blanket. (see diagram). The deepest, therefore the oldest part of the Coastal Plain is a series of rounded river gravel. OK? Ring any bells? That is what we are looking at back there...these gravels. So, that is actually part of the Coastal Plain draped on top of the Piedmont. Right here we only find it on tops of the highest hills, but as we head east, it is found at lower and lower elevations. So, what we are going to see as we get down at the bottom of the hill - we are going to see our first little waterfall. Where the water of the South Fork of Quantico Creek which is the creek that carves this valley here where that is falling over some of these hard, difficult to erode rocks. And, then as we get down into Dumfries this afternoon, we are going to move out on the Coastal Plain itself where it is going to be very flat and the rock layers are very easy to erode. It is going to look very different - and it is a major theme we are going to discuss more over the course of the day. Since the sign is here, I figure I had might as well say something about it.

Stop. Our first actual outcrop of rock. I wanted to give you guys a chance to check it out. Outcrops are where we actually get to see some of the Earth is rock formations at the surface - and - you would think they would be more common, but they really are not. One of the reasons for that is we are trying to do Geology in the East. Out West, there are no plants, so all the beautiful rocks are exposed with these ugly forests that cover up the beautiful rocks. So, you only get to see the rocks where the forests have been stripped away. Like here, where a stream has cut down to reveal the rocks or sometimes when we build a road underneath. So, on the East Coast we are largely limited to the rock exposures that are in creek bottoms or in road cuts. We are at the creek bottom of the South Fork Quantico Creek coming out of that little lake from the dam upstream there. I want you guys to take 2 minutes. What you are going to do is examine them. I want you to make observations about this. I do not expect you to come up with the whole geologic history...

Little patches on the surface. Those are not rocks. Those are a symbiotic relationship between a fungus and an alga, right here where it is wet. Go.

So, yes. Yes, what is causing that? Something? The water. When these outcrops are first revealed they have nice angular edges like over here next to David. As the river flows over them with time they get worn down so they get nice and smooth. We will see some really cool smoothing out features downstream like the one, Kathy, you were noticing on that sign. Yeah, we will see some of that later on. Uh, Tee, what did you notice? OK, little sharp edges...OK...breaking...everything is parallel. We see this parallelism in the rock and that extends out to the stream making these little ridges of rock that extend out to the stream... (We will get to that - hold that thought). First, I want to talk about this parallelism that we are looking at here. Jill, what are we looking at here? Foliation. Right. Folio comes from the Greek word for plant or leaf - leaves on trees...(two choppers overhead go by...)

Foliation - originally folios to describe leaves and then eventually used to describe paper or books (a foliated structure...pages in a book parallel to one another is what we are seeing here in the rock). Something layered. What is it that is layered in the rock? Sediments from the breaking down of those mountains? Let us back up. You are right, but I want to make it a little bit simpler. What are rocks made out of? Minerals, right. What we are seeing here is the minerals all lined up in a certain direction. What force could line those minerals up? Pressure from what? A tectonic collision. The North American Continent pushing in this direction and the volcanic islands pushing in this direction. Let us go back to this idea of these two plates colliding - one is a volcanic island arc and one is the North American continent squishing together - this stuff got caught between - a sick analogy...a cute, little fuzzy kitten chasing a butterfly out in the meadow. Then it runs out into traffic on Route 66 and a big Mack truck comes along from one direction and a cement mixer comes from another direction and they collide head on and the kitten gets caught I the middle. The kitten changes its shape as a result of that collision - the kitten starts off with little kitten bones and fur all oriented in different directions, and as the two trucks collide, the bones get aligned perpendicular to the two trucks' collision direction. This is the mangled corpse of these sediments of /in the Iapetus Ocean. They have been crushed. They start off in all types of different directions. I will use my hands to represent two different directions - and then they rotate to the only stable configuration possible. Picture a bunch of papers on your desk. Push your hands toward each other and the papers end up lining upright the closer you put your hands together. The minerals here are all upright - all standing up essentially vertical that is..............See how straight this thing is? (A rock used as an example).

Break...then...

...bubble. OK. Did you guys see that in the back? OK, say again, speak up... "...crack in the rock and water filled in it, warm water and it crystallized into quartz." Right, and how do you know it was deposited by water? What is the clue that is telling you that? Hydrothermally deposited quartz. This white color is due to little tiny bubbles of water in that quartz mineral crystal. That is a sure indicator that that was originally deposited in a crack. Now, however, it is only the little chunk, just like that; then there is another little chunk there and another one there and there and there... They are a little bit pink because they have been stained by rust. Notice how they all line up in the same direction, too. Like this thing here is 7 cm wide by 40 cm long. Compression coming in East to West making everything line up North to South. Now, let us talk about the original sediments that got crushed here. (The so-called kitten). There are three rocks here in my hand; three different sandstones. 1) White quartz sandstone, 2) pinkish arkose (potassium feldspar) 3) greywacke - gray - all made of sand. This one is exclusively quartz. This one is a mix of minerals and has lots of potassium feldspar. Greywacke is a mix of sand and mud. It is a dark, gray color (clay as well). If I asked you to pick one of these three as the original source of these rocks before they got squished, which would you pick? Greywacke. See the color? If I put these three down, the one that blends in the most is the gray one, right? Now there are some other things influencing the color here, but yeah, if you zoom in on these rocks you will notice little grains which were originally sand and mud in this ancient, dead ocean - The Iapetus Ocean. When the tectonic collision happened, a lot of the mud reacted, becoming mica. Mica is another mineral, and a defining characteristic of mica is sheets; flaky sheets lining up one way once again indicating again that squeezing direction.

Zeta (sp?) was asking about some fractures cutting across, like here. See the fractures? There are a bunch of brittle features cutting across this rock. These rocks were squished in a flowing way and then later they were broken by brittle fractures. So, these brittle fractures may be related to the uplift of these rocks, over time. I do not know, or cannot say specifically if it is related to some tectonic cause.

But we know (with certainty) that that happened after the rocks had cooled down again. During mountain building, these rocks were hot and flowing. After mountain building, they cooled down and that is when they broke. To get these rocks to flow, they would have had to have been heated up to 350 degrees Celsius or so. Maybe higher. We will see evidence down the way of partial melting of these rocks. Partial melting yields granite magma. So, that is another symptom of mountain building. So, you have to get the rock up to 450 degrees Fahrenheit (when wet) in order to get it to melt. Using the Principle of Uniformitarianism, you can say they once were molten based upon how we know such rocks form today.

Greywacke. David - greywacke is from Old German for grey rock.

Making the distinction between a sedimentary rock and metamorphosed sedimentary rock...layers of sediment. You do not end up finding these big, long flaky bands of quartz...presence of all this mica. We do not get big deposits out there in the actual world where mica accumulates in big layers. We get layers of mud. So, the mica itself is a metamorphic mineral. Also, we do not see any continuity. As we look along the way here, we do not see that we can detect one layer of coarse grains or one layer of sand, mud, or anything like that. Instead, we see a sort of smeared-out looking feature. These rocks appear smooshed. What would you add to that, David? (The mica here is the metamorphic mineral, not a sedimentary mineral...B word....boudinage...)

Boudinage. French for sausage. Describe a rock layer that has been broken into sausage like segments. Right here, look at this you guys. 1,2,3,4,5 sausage links. This quartz vein is broken (as a brittle phenomenon). Also, the flow was pinched out along the breaks. There are pinched out ends, like a string of sausages all in a row. This occurs at 10-15km depth from crust...right at the transition between brittle behavior in the upper crust where rocks break, and more flowing behavior in the lower crust. Brittle means breaking. Flowing is like silly putty or bread dough.

So, I wanted to elaborate on something. See picture...4 parts. Something we can deduce about these rocks. A preserved sedimentary structure called graded bedding: layers of rocks that are coarse at the bottom and fine at the top. No graded beds here today, but at the Billy Goat Trail, you will see graded beds. That tells you how these sediments initially accumulated. I am correlating these rocks here with the rocks at the Billy Goat Trail, based on similarities in their mineral content, and my knowledge of the area. I'm saying these rocks exhibit all characteristics of Billy Goat Trail rocks except we do not see any graded beds preserved here. In the Billy Goat Trail, there are a few lucky areas where we see graded bedding preserved. Why do I care about graded beds? Graded beds are deposited by currents flowing along at the bottom of the ocean. (Picture of turbidity currents). These currents are very dense, sediment rich flows, that go along the bottoms and they slow down. As they slow down, all the sediments caught up in the rolled up water settled out. The stuff that settles out first is the big stuff. The stuff that settles out last is the light weight, really fine-grained stuff. So, you end up getting these graded beds forming. Those formed down in a location like this, down in the deep sea in what we call an abyssal fan or a submarine fan, where sediments are coming off some land mass piling up in the deep sea making graded beds of greywacke. Again, greywacke means nothing more than a mix of sand and dark mud. So, that is what this used to be. Then it got crushed up. When did it get crushed up? When the Iapetus was closing, good. As the Iapetus was closing (let me pull out another graphic here) it was a scraping up all that sediment. OK, the subduction zone was going down the hatch, but the sediment on top of the oceanic crust was getting scraped off. It was building up into a big pile; a big, jumbled pile of sediment. That is analogous to a bulldozer moving over the ground scraping up a big pile of dirt in front of it, OK? Where the bulldozer is like the island arc, and then in front of the continents are the sediments it is scraping up. OK? So, that is what we are on here. Really, these rocks here are the big pile of muck that got scraped off the subducted plate and then later it got squeezed between the islands and North America. So, we call this big pile an accretionary wedge. Now, Dean, you were talking earlier about California and San Francisco. San Francisco is still on an accretionary wedge. The difference between Prince William Forest Park and San Francisco is, Prince William Forest Park then had that accretionary wedge caught between two continents; Africa and North America, and it squished. Whereas San Francisco, it just filled out. It has not been squished between two continents, yet. Give it another 50-70 million years, something like that. OK. Questions? (Why the silt...? C. - What silt are you observing? Does that just look like it is going into the hills?) What was stable in the middle of the mountain belt is no longer stable. The mica is rotting away. In fact that is why over here, when I was running my fingernail through these little grooves, there is a groove there to run my fingernail through. The mica is soft and it rots away really easily, so it ends up etching out. And, the quartz is very stable and so it does not erode away easily and that is why it makes these little ridges. So, guys if you have not felt this for yourself, come put your finger on the rock and feel this yourself. That is why, like, on the drive down I was noticing these big white boulders on the drive in. Those big, white boulders are made of quartz - it is stable. It does not break down over time. That is why when you go to the beach; the beach is made out of quartz sand. It is not made out of feldspar sand, mica sand or anything like that. It is quartz. Quartz is the stuff that lasts. (Student questions Callan. C. -- Yeah, right. Black beaches are where you are really very, very close to a basalt source. And, there are not...there is not ....adequate time to break down all those unstable minerals, so the beaches built up making those black minerals there. So, we are finding that there are some black minerals even on beaches here on the East Coast, but, the majority of it, when it is a nice mature beach, is quartz sand.)

One of the exercises I had Jill do this semester, and I had Dave do last semester, as well as the rest of my Physical Geology class is that I give them a little, bite-sized Snickers bar and they have to suck on this Snickers bar. The chocolate dissolves away very readily in their mouths followed by the caramel. The nougat lasts about 10 minutes or so. But even after you suck on this thing for about half an hour, the peanuts are still there. Peanuts do not dissolve, right. Peanuts are like quartz whereas all the other ingredients in the Snickers are more like other less stable minerals.

(Student talking about some observation). C.- Actually that is a great observation. Let us take that one step further. Imagine, now this creek here is not a creek but a road and you are driving down it. You take this turn and you take it a little too fast. Which way does your body get pulled? Right, towards the outside of the curve. So, basically, during flood times the creek comes in and slams into that wall right there and strips away the plants and strips away the leaves and strips away the dirt and it exposes rock there. Whereas the rock that is underneath the hill here is not getting hit head-on by the force of that creek. So, it fills up with sand, dirt, and leaves over time. I did hold this rock down here and I just wanted to point something out, it is a little bit difficult to see because the stupid thing is all wet, alright, just like us. But, this is a foliated metamorphic rock. Does everyone see the plane of foliation? So, if I were to line it up here with the regional foliation, it would look like that. Alright, but this one is loose so we can pick it up and examine the plane of foliation itself. And, there are little black needles there on the surface. Do you see those little needle-like minerals? They are needle-like mineral growths. Do you guys see them there? It is almost like if somebody took a bunch of chopsticks and dropped them on a desktop. The chopsticks were in this random orientation because they would be parallel to the surface of the desktop. OK? If I took all your pens and dropped them on top of someone's notebook, they would all splay out on top of that notebook but some might be pointing this way and some might be pointing that way. That is what you are seeing here in this plane of foliation. These are amphibole minerals. And the amphiboles are randomly oriented within the plane of foliation. Pass it around. I know David wants to get a good look at that with his lens. Make sure he gets that. Great. Good. (David and Callan have discussion about amphibole, spelling, etc...David asks, does the random orientation of the amphibole suggest that was it done when the pressure has been released. C. - No, I think basically that what we have here is we have flattening. Remember what we talked about was the different types of deformation; folding, faulting, and I think I said squeezing or flattening. These rocks have been flattened. And what we see is that the dominant pressure direction was coming this way and then the rocks, in order to accommodate that (here we go – here is my little Nerf ball. Yeah, I left my kitten at home...) Um, they are getting squished in this direction. Right now the Nerf ball ends up basically elongating outward this direction and growing in this direction, as well. So, like, you think about three dimensions, these right here are growing and this one...whhish....gets squashed. So, I think what is happening here is that flattening stress is causing the amphibole as they are growing...they ca not grow in this direction. It is like, try growing if the building collapsed on top of you. But, you can grow in this direction.)...we will see a couple of them here today. ...Dave, unfortunately it is not...it takes certain elements to make the amphibole, so unless those elements are present in the original sediments, you do not make it. Dave - OK. Student - Do you want to leave this here? C. Yeah, that is why I brought it down. C. - No, I would not break it, I would just kind of stash it over there underneath a tree or something so maybe in a year from now I can find it.

Another segment...

...you take away the fact that we have determined that these used to be sediments in the Iapetus Ocean basin, and then they got squeezed due to mountain building. Due to that Taconian Orogeny...this episode of mountain building that we mentioned back in the shelter. Taconian stands for the Taconic Mountains of New York, alright. (see the one page handout that I gave you). Dave - What are the compass directions here? C. - Essentially, North/South and then, East/West - squishing. Now is it actually that? Well, no its North/Northeast, but close enough. Dave - it is a good observation to make in the paper the direction in which the squishing seemed to happen. C. - Yeah, I would say that would be a great observation to make in the paper. Um - it may not have been originally in that direction though, because remember a later collision happened. That later collision also squished. So, it is like remember the kitten, Mac truck, cement mixer pile-up we had earlier? Then along comes a tank and crashes into it, OK? And then that ends up rearranging everything again. Dave - could the magnetic poles have changed here? C. - Uh, yeah, but we have no magnetic signature in these rocks. We are just trying to get in our head how - Dave - You did say North and South...C. - Yeah, so we are using modern day directions but then again North America would have been rotated around in a different position, so it is a complicated question. Sounds like it is a simple question but it is really not! Another student asks a question. C. - You do not have to understand about magnetic poles to understand what we are talking about...Dave - ...unless it is present day...(Callan brings discussion back around to)...yeah, that is all we have to work with and that is what is going to be most readily available to you guys. So, again, the one thing to think is this foliation essentially lines up on the same line from the Appalachians, basically from Georgia...Dave - OK, yeah, that is a better way to go, yeah. C. -....so, that, that is due to this first collision...Dave - ...Appalachians...to the ocean, like that? The Appalachians have...to the Ocean, like that? C. - Well, the Appalachians would be parallel to this. So, essentially we are looking at Maine up that way, Georgia down that way...ok, West Virginia, that way, and then the Atlantic out this way. Uh, I was in the middle of making a statement there and I got derailed.

(Dave - If these clay minerals had all been aligned and micas formed in the first compression, and they were tilted up in whatever orientation, and a second collision took place and it came from a different angle, would all this seem likely to get reoriented or would it preserve some of the old orientation? C. - Probably you would have some preservation and some would get reoriented, depending on where you have little bits that poked out, those being more susceptible to being re-rotated.) Um, that being said the overall structural grain of the Appalachians is this North/Northeast to South/Southwest direction, so, I mean I think we can just sort of simplify things - well, it may be an oversimplification - we can simplify things by saying these collisions all essentially came in one after another from the same direction. First, these volcanic islands during the Taconian Orogeny. There was a second collision that happened, later on, we are not really going to talk about that today - called the Acadian Orogeny. And then finally, Africa hit and that was the Alleghanian Orogeny. So, all these different orogenies were episodes of mountain building.

Oh, I know what it was that I was going to ask! When does the Taconian Orogeny happen? Well, Dana, that is just great. How did you know that!? She says 460 million years ago is when this actual collision took place. And, she is right, but, she is just pulling that number out of thin air. You can not see it. Student - she got it out of the papers C. - OK? Yeah, you are on the right track. OK, radioactive decay. So, certain minerals when they form, they take in radioactive isotopes, and then if you can go and you can say that that mineral is a mineral that only formed during an orogenies, then you can say, "Ah ha!" All you have to do is look at the radioactive isotopes that remain in that mineral and compare it to what they decay into. So, in this case the mica here has been isotopically dated. What they did was they looked at radioactive potassium 40 that is present in that mica and they compared it to the daughter product - the stable daughter which is called argon 40 And, the mica as we said earlier formed during metamorphism ...was a good state for the orogeny. The date is 460 million years ago. We are going to see a granite today, and the granite has a date of 464 million years ago. OK, so it is basically the same age. And it tells us about the same event, and granites, you remember, are another symptom of mountain building. So, we have got a really good view on the orogenies then by dating these two independent, isotopic systems. The metamorphic mica here, and then the granite that resulted from partial melting. I will remind you of that again when we get to the granite, OK? Alright, that is a good point to keep in mind. Student - So what overall type of rock is......C. - No, this is not granite, granite is much lighter color and granite does not have this foliation. Um - so this is a metagraywacke. All right. Meta, the prefix meaning change, and, greywacke telling you what it originally was.

OK., I know everybody is hungry, so we are going to hoof it. We are going to walk down to the Cabin Branch Pyrite Mine. We are going to be walking across and along the South Branch of...

...stop and maybe point out this point bar......do not feel like you have to take notes. OK, I’m not going to go over anything really important...


(Tape #3) After lunch...

OK, so granite you remember is produced by the partial melting of other rocks - remember I showed you that other diagram where you had a bunch of starting minerals and then the light colored ones sweated out - they melted? Whereas the dark colored ones stayed behind. So, here, it is a metagraywacke that is being partially melted. And the minerals that are easiest to melt those metagraywacke are quartz, feldspar, and mica - some of those are basically melting out and they are leaving behind the darker colored minerals. Ok, so we are producing these granite blobs and these blobs of magma are moving up to the crust and eventually they are settling down and crystallizing into granite. Remember we call these blobs plutons. So, here in this diagram which is part of your handout, I have got a diagram showing you some igneous plutons, OK? So that bodies that were magma and have crystallized into solid igneous rock, like a granite - OK, here is one pluton, here is another pluton - they cooled underground. Now, basically these plutons are these wet batches of magma and they are moving up through the crust. What happens to the pressure that is on them the higher up they go? Yeah, it is released and as a result they erupt - basically the granite separates out and the stuff that is most readily removed leaves the granite. So, think about a bottle of soda - did anyone bring a carbonated soda? David did. David brought a ginger ale. So, when he popped the top on that, OK, it started off as just liquid, but when he popped the top he released the pressure on the liquid inside. And as a consequence, a gas that was dissolved in the soda came bubbling out. Carbon dioxide came bubbling out of the solution. The same thing is happening to this granite which, remember, originally was magma. As it gets up to shallower depths in the crust, gases and things start coming out of it. It is leaking fluids into the surrounding rock. OK, so it is intruding into this rock and so these fluids and gases are penetrating the surrounding rock. Some of those fluids and gases would probably be water vapor, another one would be carbon dioxide, another one would be hydrogen sulfide, um a bunch of different fluids, OK? And one of the things that these fluids are taking - with - them, (I think I have got my mouth.../sandwich repeat) - OK, the fluids are carrying with them metals. Alright, metals readily dissolve in those fluids that are carried out of the granite by the fluids. And as those fluids penetrate the surrounding rock, they cool down and the metals are deposited there. Frequently, the metals are - they glom onto sulfur. Sulfur joins up with lots of different metals and then it settles out in this big sulfide deposit which surrounds the granite pluton. OK? So, it is kind of like a halo or an aura surrounding the pluton is this big aura of sulfides deposits - sulfur mixed with metals. OK? So, some sulfide minerals contain galena, some of you are familiar with galena, it is beautiful, it is got this silver luster, and these little cubes. Um, another really important one is pyrite. Pyrite is nothing more than iron sulfide. The chemical formula for iron is Fe - the chemical for pyrite is Fe. OK, it is nothing more than iron mixed up with 2 sulfurs. For every one atom of iron, there are two sulfurs bonded to it. And that makes this mineral called pyrite. Then, what color is pyrite? Golden. Yeah it is sort of this golden color and it looks a lot like gold, if you do not know what gold looks like. Um, it is got that same golden luster. Well, they were mining this pyrite here. We have already said it is not gold so why are they mining it? What on earth is the point of pulling up fool's gold from the ground? Gunpowder. C. - Gunpowder. So, basically, they are not interested in iron, they are interested in the sulfur. The main thing they are using the sulfur for is gunpowder. It is also used in many other industrial applications like making soap and refining glass and other things like that. But, not nearly as exciting as warfare. Jill - Civil War? C. - The pyrite mine right here actually started in the aftermath of the civil war, and then it actually hit a fever pitch during WWI, when there was a really big demand for gunpowder. So, they were pulling lots and lots of pyrite out of this mine, processing it to extract the sulfur and then using it to make gunpowder. So, geologically why there is a mine here is that the granite is essentially sweating out all these fluids. The fluids are rich in dissolved metals. It is just like when you sweat, there is salt in your sweat. And if it dries out on your shirt, you get a little white crust left behind on your shirt, right? So it is the same thing her except for instead of salt crustiness left behind, you will see a metal crust. So, the same granites that were produced during the Taconian Orogeny were sweating out these deposits of pyrite into the crust. Later on, people came along and said, "Hey, we can make use of that, let's make a mine here. We will dig into the hills and dig out as much pyrite as we can." So where we are right now is we are sort of geologically on this dome surrounding one of theses plutons, OK, we are in this sweaty armpit region. So, what I want to do now, is I want to go and find some pyrite and look at it. So, what we are going to do is walk back over here to this area where there was nothing growing. And, we are going to go look for some pyrite. Does that sound workable? Eventually we will come back to this place, so if there is something heavy you do not want to carry, you can leave it here and then come back and pick it up again. (Student question - inaudible. C. - The granite pluton was. Yeah, so the granite - the body of magma which would eventually cool into a granite. We will visit that granite this afternoon). (Student - So it is coming out because it...C. - the granite was, yeah, and as it is getting to shallower depths in the crust, it is devolatilizing, so the various gases that are dissolved in it are coming out. So, it is starting off here during partial melting, then the granite is organizing itself into these blobs, they rise through the crust, as it rises it is sweating out into the surrounding rock all these mineral deposits. Alright, sometimes as it moves into the crust, the crust cracks open and you end up getting veins of pyrite or veins of hydrothermal quartz. Some of those veins of hydrothermal quartz have gold in them, actual real gold. Including in the northwestern corner of this park, and by the Billy Goat Trail in the Great Falls area. So, there are gold bearing quartz veins in this area and they are coming from the same source. They are essentially being sweated out of this granitic magma. (Student - ...keep that in mind if the dollar keeps going down. C. - That is right, we will start mining our National Parks.) OK, other questions? Let's go.

(New Spot.) Now, I'm going to start talking now about some of the environmental damage that the mine caused. In this area, where the ground was near the mine operations, they were filtering the lower grade ore, you know the stuff that did not have enough pyrite in it, and they just kind of dumped it, alright? And, that is what we are sorting away here, right? And that pyrite is then soaking out here at the surface of the Earth in water, and that water is oxygenated water. And, those two ingredients end of completing a reaction of water, oxygen, and pyrite. Water, of course, is hydrogen and oxygen. Oxygen is just oxygen. And, pyrite is iron and sulfide. So, basically, after that reaction you end up getting iron mixed with oxygen and hydrogen which is rust (FeOOH) and sulfuric acid (H2So4). So, two things are being produced here due to the weathering of the pyrite; rust and sulfuric acid. Rust is what is making the soil so darn red right here. It is staining everything red. Look at David's boots right now - they are getting all soaked in this red mud. Alright, the other thing is sulfuric acid. What is the effect of sulfuric acid that you see right in this area? Basically, most plants cannot grow in super acidic soils; soils that are essentially drenched in sulfuric acid. As a consequence, nothing grew here for a really long period of time. It was basically a day of awakening - completely environmentally degraded. So, the ground was basically an empty wasteland and then the Parks said, "OK, we have got to clean up this mess." So, they took a series of steps to essentially reclaim the land. OK, this is something that frequently has to happen where they do mining - reclamation - basically making it look like a decent landscape again. And, what they did was they brought in a lot of limestone. Limestone is made out of calcite, and that reacts with acid. So, basically it is a buffer through the sulfuric acid. And, when they laid down all these limestones in the area, some of them are dissolving away as soon as they get set down. They are taking away some of that acid. It is kind of like Tums in the landscape - that is a great way to think about it? Did you have some Tums this morning? David - no. So, it is like Tums through the landscape and it is working better in some areas and not as well in other areas. It is not working so well right here. There is still a lot of sulfuric acid right here in this area which is why you can actually go and pick up rocks there. There are no plants growing out. That is thanks to the acid. Same as that little patch there at the end of the trail - there is nothing growing there. That is so weird for the East Coast to have an area where there are no plants growing. That tells you there is something seriously messed up with the soil underneath.

One of the things I would like you to do, is I would like to have you test the pH of the waterways. You might want to clean off your hands first, because the way you are going to be able to read this thing is to check the color of the paper. Alright, this is a little pH paper here. It is going to change different colors depending on the pH of the water you put it in. Now, you want to make sure you are putting it in relatively clean water otherwise you will get sediment on this which is going to give you a false reading. OK? So, I'm going to give everybody a little strip here. You can choose to test this water here, somebody should go test the water of the South Fork, and somebody should retain their strip so we can test the water of the Main Fork of Quantico Creek. OK, so we want to collect data at several different points, to several different iterations at each point, so we have reliance on the data and then we will share everything we get.

Everybody tests. OK, what do you have? 6! What do you got? (Everyone testing.)

Results 5 or 6 - somewhere between a 6 and a 5....slightly acidic. He just put it on his tongue and it is a 7. So, your tongue is pH neutral, which is a pretty good thing for your body. So, he has a reading of between 5-6 so that is just slightly below neutral, so this area is slightly acidic. That is not as acid as it once was, but it is more acid than just what regular old water would be which is 7. So, as the number gets lower in pH level, the more acidic. Higher is more alkaline. End of segment.

...sweating out of a granite pluton. Then the fact that this was mined for awhile for the purposes mainly of making gunpowder, and one of the elements that make up pyrite is sulfur, and then the breakdown of that sulfur at the conditions found at the surface of the Earth here; mixing it with water and oxygen, making rust and sulfuric acid, and then the Park surface had to treat the area by putting down essentially the geologic equivalent of Tums with limestone chips in order to get rid of the acidity. Oh, and another point that we could make here that the plants that actually are growing here are pine trees. These are acid tolerant plants. Their needles, themselves, are acidic. So, their needles are dropping - so look underneath those pine trees. You see the carpet of needles underneath, right? Those needles are essentially making that soil more acidic and that makes it harder for other plants to grow there, OK? They make a special kind of acid called a tannic acid. It is the same thing if you brew your tea for too long - it is a sort of bitter thing and it makes your stomach hurt. That is essentially what is going on over there with those trees. OK, let's move.

Testing pH at new location. (Dave - So are these the rocks that they dumped? C. - Yeah, so this is the ........that basically have water coming through them. The water is...)

OK, so I imagine we all do not want to stand here for too much longer. Did everyone see that vein of pyrite that Topher found? Time to work our way back to the path.

New location.

...C. - I'm going to turn upstream on Quantico Creek. (Jill - Confluence of Quantico Creek). We are going to look for a place to cross Quantico Creek. We want to be on the other side of it.

New location.

C. - I want to point out we are at the confluence here. So, here is the South Fork of Quantico Creek, which flows under the bridge we just walked over. Right, here coming into the main stream of Quantico Creek, the two merge right here (Jill - the confluence) and they flow downstream. Where we actually want to go is where there are a bunch of branches across the creek down there. It is probably only 200 yards from where we are right now. All right, but, unfortunately there is no great way to cross the creek right here. So, we have got to go upstream a ways, until we get to a good creek crossing. As far as a bridge and cross over. David, if you want you can wade across, but, I do not want to make every...

New location.

...flow. Where were these lava flows accumulating? Jill - Volcanoes. C. - Volcanoes, where were the volcanoes. Jill - the island arc. C. - Yeah, the chain of islands offshore, you remember, ancestral North America about 500 million years ago. Then, subduction narrowed the ocean basin between them, and eventually they got added on to the edge of North America. Now, how do I know these are lava flows? Great. Color, texture, and maybe the mineral content. Yeah, so there are some good color indicators here. What color is this greenstone? It is a very descriptive name - greenstone. It is probably in the Old German for - just kidding. So, greenstone is metamorphosed basalt. Remember basalt is what is coming out of the volcanoes today in Hawaii. (Student - asks question. C. - Well, we call it magma if it is below the surface and we call it lava if it is above the surface. Once the lava cools we have to give it a rock name. The typical name we use for the dark colored rock is basalt). Basalt is what made up the oceanic crust and what made up these volcanic islands. So, when the basalt gets metamorphosed it undergoes chemical reactions and those chemical reactions turn it green. The main green mineral here is called chlorite - it is basically a green mica. There is also epidote. Right here there is a pistachio colored mineral. You guys see that one? It is sort of a bright green? Filling in little veins over here? It is epidote. Alright, so basically, I know Topher is going to ask about this - the "take home message" is that these were once lava flows that got metamorphosed. How do we know that? The Principle of Uniformity. When you see lava flow that gets metamorphosed in the world today they change color into a green color. The reason is that they grow chlorite minerals and epidote. ....yeah, well there is some other stuff. Remember these have gotten squished. So, a lot of the original layering is lost. They are foliated. They are foliated and again it is that squishing effect. OK, I want you guys to come and look at these rocks here. There are little white circles in the rocks. Oh, see these white blobs here? OK, what is going on here is the same thing we were talking about earlier. When you release pressure on a lava, it causes gases to come out of solution. Just like when David opened his Canada Dry the bubbles formed. When lava erupts at the surface bubbles form in the lava and gases come out, right? If those bubbles do not pop before the lava sets up into rock they are preserved as little holes in the rock, like Swiss cheese. We call those vesicles. The vesicles have gotten filled in with mineral deposits. Those mineral deposits are known as amygdules. Amygdule is for the Latin for "almond." So, these were originally decided to look something like almonds - set in a piece of bread, like that. There are some really nice ones over here with mineral deposits. Generally the minerals that are filling them in are quartz, well Let us just say, quartz. David - something about popping. C. - ...if they do not pop, it leaves hole. Later on that hole could get filled in with a mineral deposit. This is important. The bubbles form as gases are coming out of lava, then some of those bubbles pop, we do not have any evidence of it. But, some of the bubbles do not pop and those bubbles get filled in later on with mineral deposits which make these little white blobs in the greenstone. And, notice that these little white blobs are not perfect spheres. Up here they appear kind of like this. Why is that? They got squished - like a little kitten's eyeball. That is due to that tectonic squishing. You can see that they are all basically squished like this line up at the plane of foliation. Alright, they lay exactly parallel to the plane of foliation. That is why the sphere became a pancake. So, they are little pancake shaped fossil gas bubbles from a lava flow. We call them amygdule because they got filled in with mineral deposits. If they were still empty holes, we would call them vesicles. ( ...you can see epidote down here in veins... come down here with David and you can see them.)

Barely audible due to noise from river. C - ...ancient volcano island rock... ...Chopawamsic....Dave and Callan in a lot of discussion about the volcanoes, flows, etc...evidence of it all...Callan sticks to history of Virginia....at one time you could have walked from Ohio to Morocco. Now those rocks were once sediments in the ocean deposited way down...

...discussion about potholes....barely audible. The role in carving it out. When water come flowing through here, there is a vortex of water. Filled with sand, silt, and it acts as liquid drill bits. Layers of quartz and mica, quartz and mica. Quartz stands out in high relief. Sand gets in there and preferentially eats away the mica. So, that is a pothole and potholes are one of the ways that streams are cutting down in areas of waterfall where they are adjusting from one level to another level. Alright, Let us go ahead and work our way back to the trail and we are going to start climbing uphill, towards the bathroom...

OK, so here we have another tree that is tipped over, and it has brought up a nice, fresh sample for us. We can see here more of that gravel that we saw when we first started on the trail today. This is not a rock. This has not been stuck together into a rock - it is just loose gravel...David - it is a root ball, right? C. - Yeah, it is a root ball of a tree. You can pick up the loose grains of sediment, and let it run through your fingers. Actually, I encourage everyone to do this. You will feel that this is a mix of sand and clay, and the clay will feel very sticky on your hand, and then these nice, rounded pebbles and cobbles of mainly quartz, OK? Most of which is present here is quartz. Student - Sand and all that is what is left over after the quartz was left. C. - Well, basically, you said it yourself. This is a very poorly sorted sand pile. This is a mix of different grain sizes, which indicates that it was dropped very rapidly. Now, what does the rounding tell you? Jill - it was well sorted. C. - ...well traveled. It is not well sorted, it is poorly sorted. Yeah, the rounding tells you that this quartz cobble started off somewhere far away and then it traveled a long distance to get here and as it traveled it got more and more rounded. This one must have started off a little bit closer. Alright, because this one is a little bit more angular. And that is typical of river systems because river systems end up draining a whole area. Rocks are dropping into them from far away and nearby and they are both tumbling downstream together. And, Jill, that is how you interpreted this, right? Jill - yeah, well no, sorry, I was off in a zone. C. - Jill, how would we interpret this deposit, how did this form? Jill - Well, basically, it came off of an uplift, and it traveled downward, and it was deposited into a system of water...C. - OK, what kind of water? Jill - Probably very fast. C. - OK, good, why do you say that? Jill - Because, it looks like they are rounded, I mean...obviously they passed through...Student - ...it is the size of the rock...C. Yeah, it is the size. Student - it is a well-rounded big rock. C. - Yeah, it is obviously a well-rounded, nice big cobble of quartz. And, it takes a lot of water energy to move something this size. Jill - Yeah, definitely. C. - OK, good, so what kind of body of water has the energy to move big cobbles like this? Jill - River, a river. C. - Yeah. Rivers, OK? Because we just saw, in fact, I just destroyed my vocal chords trying to shout over - rivers have a lot of energy. Whereas a lake does not have so much, a swamp - less. The ocean has a fair amount of energy, but you generally do not get big cobbles like this in an ocean because as soon as rivers flow into the ocean, they slow down, and then they drop all these things, and then they carry maybe the sand and the mud further out into the ocean. Those are all that really make it into the ocean. So, this is a river deposit. And, when you think about it you might think it is a little bit weird, because we were just down at the river and we just hiked uphill, and now at the top of the hill we see these river deposits? What is going on here, Cathy? Cathy - The river was once up here? C. The river was once up here, she says. These were not recent flood deposits. So, these are ancient, and the reason I know that is I can come up with a rough date for these deposits based on fossils that we find within this same gravelly unit. Now, this gravelly unit here, do you think it is a particularly good setting for preserving fossil remains? Callan hits the “unit” with large cobble, again and again. C. - Do you think that is good for a fossil? All right. Think about the river here. As these things are moving along, all these boulders are smacking into one another and grinding around. This is a lousy environment for preserving fossils. It is a miracle that we have any fossils at all from this unit. The fossils that we do have from this unit are very poor and they have been broken up a bit but we can still identify them and they are dinosaur bones. Alright, we found 3 or 4 different dinosaur bones from this one unit. There are some sauropod fossils, sauropods are the big, sort of lumbering, vegetarian dinosaurs with the long necks. We found some of their teeth and some of their leg bones. OK, there is a species called Astrodon johnstoni, it is the state fossil of Maryland. Basically, Astrodon means star-tooth. Think about your molars in the back, and you have these little points for grinding. There is a series of five radiating ridges for grinding up vegetation. So, Astrodon johnstoni. Also, we found some raptor fossils in here. At any rate, these dinosaur fossils date back to the Cretaceous. Cretaceous is a period of geologic time that ended at about 65 million years ago. It started at about 120 million years ago. The best estimate for an age on this unit is about 100 million years old. One hundred million years ago a river was flowing along this area, and that river was meandering. It was cutting in at a cut bank and it was depositing materials on a point bar. This is an old point bar deposit. OK, remember we saw piles of gravels that look a lot like this being deposited down at Quantico Creek today. So, this river was no longer cutting down, it was simply meandering back and forth on the landscape. Now, something must have happened between what we were just talking about at the base of the hill, and the deposition of these river gravels. We have this great big mountain range that had gotten built up, right? The size of the Himalayas - what happened to that? It was eroded down to essentially a flat level, and over that flat level, this river was meandering back and forth depositing gravel. OK? Then at some point after that what happened? Sea-level dropped and what did the river do in response? It started cutting down again and carving new valleys like the valley that we spent most of the day hiking through, OK? So we have evidence here of a higher sea-level at some point where these rivers were meandering along depositing gravels as they flowed eastward from the west out toward the young Atlantic Ocean. The Atlantic Ocean, by the way at this point, was 100 million years old. The Atlantic Ocean was born 200 million years ago and these gravels were deposited 100 million years ago, therefore, that is about 100 million years into the history of the Atlantic. One of the reasons that I’m able to say that these rivers were flowing from the west to the east, is we find signatures of rocks that we know only come from the west. Like we find pieces of granite that we find from the Blue Ridge Province. And these Blue Ridge granites have blue-quartz in them. Which is an indication that these are from the Blue Ridge Province. Yeah, there is some nice blue-quartz in this sample to right up there by my thumbnail. It has sort of a purplish sheen to it. You guys see that? The other thing that sometimes we find here is quartz cobbles that have a trace fossil right in them. That tells us that that came from the west, the river brought it this direction deposited it here, therefore that river was flowing from west to east. The same direction the rivers are flowing today. Now when the Appalachians were real, real young, it was the opposite. The rivers were starting here in the highlands above our heads and draining off to the west. Student - that is why you find Appalachian rocks way out in the west - west of us...C. - Yeah, there is some fairly compelling evidence in fact that the Petrified Forest of Arizona was buried underneath Appalachian sand and mud. In Arizona - so we are talking Mississippi sized rivers draining these young Appalachian Mountains, transporting the sediments to the west, and then basically they snuffed out a forest out there in Arizona. We can go and pick up a certain mineral from those sands, and that zircon has a chemical signature that is more analogous to the Appalachians than it is to any local source out there like in the Rockies. So, it indicates again at that time the mountains were here and the lowland was there. Who has got their handout handy, the one with the colored map on the back? Callan explains map....Kp designation which stands for Cretaceous. The sub p there indicates the Potomac and this is called the Potomac group. They are exposed up and down the length of the Potomac and you find them on tops of the highest hills. Same unit atop Tyson's Corner. You find it on top of Mt. St. Alban where the National Cathedral is. You find it on top of Mount Pleasant in D.C. - river gravels, river gravels, river gravels....Say that three times fast. So, this surface on top of which the river gravels are deposited is a period of missing time. The last geologic evidence we have in this area is the intrusion of granites. That happened around 460 million years ago, and then the next thing that is recorded in this area is the deposition of this gravel on top of it, which happened 100 million years ago. So, 360 million years of geologic time are missing in this area. We can say nothing about them from Prince William Forest Park. You have to read the geologic record to find out what is missing from those 360 million years. OK? Student - How come no one could think about water or erosion? C. To erode away a Himalayan sized mountain range takes a fair amount of time, and so it took a long time to grind down those mountains to that level. Plus, during the Cretaceous, the world was quite warm and sea-level was quite high. There was very little glacial ice, if any, and at that point then you have this combine effect of having ground down the mountains plus sea-level being high and that is when it deposited gravel all over this area. Wow, so this is like a little mud stone, right. Student - I get extra credit. C. - I would not call this a schist, it does not have nice physical minerals, but I would not mind calling it a mudstone. This is a little clast of mudstone and this is very typical of some mudrock layers that are typical out in the Valley and Ridge Province and that would be consistent with the story of basically transporting eastward. That is rose quartz there...OK, the first thing we are going to do when we get back to the visitors center is use the restroom, and then we are going to go and visit a fossil tree and that fossil tree is growing during the same period and it was probably growing along the banks of the river.

This was deposited by a river after the Appalachian Mountains were ground down. It is Cretaceous in age. It never experienced mountain building. If this had gotten caught up in that collision in would not be a loose pile of gravel, mud, and sand, it would be a rock. What would we call this if this had gotten cemented together into a rock? A conglomerate. Nature's version of cement with big chunks set in a little fine-grained matrix.

And, I do not really know what that means, I mean....

...on the trail today you are not leaving behind your skeleton, but you are leaving behind traces of yourself. Right? So, the thing that this Skolithos worm tube tells us is that it is a piece of the Antietam Formation. Antietam Formation is a big sandstone unit that is out on the western slopes of the Blue Ridge. And you can find them near Antietam National Battlefield, where the bloodiest battle of the Civil War was fought. You can also find this same rock along portions of Skyline Drive and there is an area where David and I have hiked near 66. There is a nice big exposure of it near Front Royal. So, basically it is telling us that the river transport direction was westerly - consistent with the blue-quartz. Sedimentary transport from west to east.

...Fossilized Bald Cypress Tree. It is really not in any danger of being degraded or anything like that, but it is in danger of having stuff grow on it. It is the mineral itself that will break down. All quartz. The quartz was derived from Cretaceous river gravels...And as it met this wood, it percolated through the wood, soaked into it, a chemical reaction took place, and it precipitated silica in place of the wood. This similar process has occurred in the Potomac Formation. In Washington DC, when they were digging out the foundation for the Ronald Reagan Building, they found more of these there. Also, at the base of the Mayflower Hotel, they found these fossil tree trunks down there. And, what do all these areas have in common? They basically have these cretaceous, Potomac Group river gravel deposits. So, you can imagine growing along the banks of this ancient river, Cypress Trees. How would you recognize a Cypress Tree if you saw one today? Yeah, Bald Cypress, and they have got these weird structures that rise up out of the waters. They call them knees - Cypress knees. They poke up above. I noticed in a place up here there are little tension gashes and they are filled in with silica, too. You see that - these little gray, blobs cutting across up there? So, it is like the tree itself is being deformed. It is fracturing and then the silica is depositing in those cracks. Yeah, so David is asking a question I do not have the answer to which is where is all this silica coming from in the groundwater? The groundwater has various things dissolved in it at various places at various sources, you know, that is about as specific as I can get. David - Silica does not dissolve easily in rainwater. C. - Right. David - It has to be warm to dissolve.

C. -...when things die, they just tend to rot. Fossilization is a very rare circumstance, when an organism gets preserved over time. Yeah, so wood tends to rot, so things eat it, beetles eat it. Well, one of the interesting things that is contrasted in this specimen as opposed to the one they found underneath the Reagan Building, or underneath the future site of the Reagan Building, is that one is jet black. This one is very, very light colored, it has got rusty. It basically suggests more oxidized conditions in terms of its preservation. Student - Where is the other one? C. - The other one is in Rock Creek Park. If you take my Bedrock of DC Geology Trip, then I will show it to you. It is jet black and it is also got pyrite preserved in it. Pyrite is again something that breaks down in the presence of oxygen as we have seen today at the Cabin Branch Pyrite mine...where it broke down into rust and sulfuric acid...so, it is more of a typical preservational environment. Low oxygen is more likely to be preserved. So, this is somewhat anomalous, but it is a beautiful specimen. What I just love about it is the grain of the wood here.

(Last segment - Granite outcrop in creek)

...water to get in there. What happens to water in the winter? It freezes. The water freezes and expands in volume by 9%. So, that opens up that crack a little bit wider. That means it is more than likely going to break down pieces, and when those pieces get broken out you carve out a little valley in the rock, right here. The quartz itself is stable, right? But, the neighboring area is not necessarily as stable. So, if you look around this area, you can actually see this quartz vein has actually become this little valley here and it narrows down...Do not take my word for it, come see. Now I want somebody who has never taken a class with me ever before, what is this? A sausage! What is the word for sausage? A boudinage! Oui! A boudinage! Alright, there is a beautiful boudinage in the last quartz vein, there. Now in order for boudinage to happen, it has got to be hot. It has got to be under lots of pressure, right? That is something that I said happened about 10-15km depth in the crust. So, that boudinage must have taken place during mountain building when this rock was still deep and hot, after the granite had already solidified because you ca not break a granite. It went through the quartz vein until the granite was already solid, right? So sometime after 460 million years ago, but before 100 million years ago. (Dave - Does the orientation of that, uh, boudinage tell you anything about the forces? C. - It well could. I have not measured its orientation myself, so I have not even thought about trying to put that into a regional context. But, yes, orientation of different rock structures like foliation or dikes, um - joints which are what we call these little cracks in the rock - those all often tell us something interesting about the forces that went to work on the rock. Now where Adrianna (student) is standing, we see a really interesting feature. Alright, here this granite dike has been faulted. You see here? This crack is not just a crack, it is a crack on which the two sides have moved relative to one another. There is an offset here about 1 inch in this granite dike. And, look here, there is another segment and it is offset about a centimeter. And, then another one, and another one, and another one, and another one. Do you guys see that? It has basically been broken and the rocks over there where Adrianna is standing have moved probably about, you know, just judging from this alone, maybe about by 5 inches that way relative to the rocks where I’m standing. That is a brittle behavior, OK? That is strictly breaking the rock. Alright, you do not see any real evidence of flow, there, OK, unlike the boudinage. (Background discussion/question...C - No, because this water does not have a lot of quartz in it. In order to get quartz to dissolve in water as Davis was pointing out earlier, it has generally got to be kind of warm to dissolve quartz better than cold water like this.) Student - What is the difference between a dike and a vein? C. A vein is just one mineral - like we saw epidote vein in the greenstone and we see quartz veins here. But, a dike is many minerals because it is an igneous rock. It is a tabular mass of igneous rock. And this is - this crack opened up, magma squirted into that crack, solidified into rock, and later it was broken and faulted. David - Wherever you see lines and stuff, you have faults. C. - We can only call it a fault if we have clear evidence of offset on either side. Still, like this one here maybe a fault as well, but I do not have anything good that tells me there is an offset on either side. Jill - So, instead of displacement in a rock, it is just a brittle behavior? C. - You might note that it is a brittle behavior that accommodates a displacement. The displacement can happen by flow or it can happen by breakage. In this case, it is breakage. Jill - It is a displacement? C. - Yeah, it is displaced. (...a 2 second bit of talking over each other/discussion back and forth between David and Callan.) C. - Cathy had a good question like did this happen during the collision? Alright, it is a great question. I would say that this is such a strictly brittle behavior (we can even see like little shards of the dike in there) right there along that zone, that I would say that this happened sometime after these rocks cooled down. And, earlier, I evoke this tremendous mass of rock overlying this location, right? 10-15 km of overlying rock that has been removed. Now, so does that mean that this rock, right here, has been exactly at this point three dimensionally in space through all of time and that there were mountains 15 kilometers tall on top us and have been beveled down to exactly this point? Or, did this rock start off 15 kilometers down and then basically uplift as erosion went to work on the landscape? Or, was it both? So, maybe it started off 7 kilometers down with 7 kilometer tall mountains. The mountains were eroded away, that means the crust is lighter and it pops up. Then more erosion goes to work on it. So, that means the crust is lighter and it pops up. More erosion goes to work - finally exposing the granite at the surface. David - are joints in granite often a loading and unloading feature anyway? C. - Well, a lot of joints in granite are unloading features. A joint is what you call a crack in the rock along which no movement has occurred. But, oftentimes we see that they are parallel to the topography. And these are distinctly not parallel to the topography. They are vertical. If we were to see a similar joint set running through the rock like this way, you know these rocks were under lots of pressure - now on the surface they are under no pressure. Sometimes we see granite actually expand, and then that thing fractures as it expands out, and those fractures then run like this, like an onion skin. That is stuff you see up in the high Sierras in California. Topher (sp?) was just saying he would been out there, up in Lake Tahoe - and then up in Yosemite to see these big granite domes, like Half-Dome, which are sheeting off layer after layer because the granite is actually unloading, and the sheets are just popping off. Are you making a movie of me? Jill - yeah. C. Just do not put it on You-tube or anything, OK? Alright, David noticed something cool, Let us turn our attention over this way.

Look at this. There are two intersecting joint sets here. Now, a joint set is basically more than one joint that is oriented the same way. So, we have got one joint that is basically going like this. OK, very regular. And, you have got another joint set that is going like this through the rock, also very regular. Their intersection produces these little columns like square columns of granite that go downwards, right. So, like we might actually see some up movement - no, none of them are loose. Well, anyhow, these things may be related to the unloading but if they were related to unloading, I would expect to see a third joint going like this through it basically divvying it up into tubes. We do not see that, but I expect it has something to do with tectonic readjustment during the uplift process. Alright, and after the rocks are cooling down and getting uplifted as the overlying rock gets stripped away by erosion, they crack, you know that is a stressful experience for a rock. It happens vertically, sometimes like 7 kilometers or 10 kilometers, something like that. Great observation. Now, what is colonizing those cracks there? Student -What? C. - Colonizing the cracks? Student - Lichen. C. Yes, and moss. Yeah, this is lichen. This is a crustose lichen, and this is a foliose lichen. Foliose lichen has leaves, right, like folios. And, the crustose is like crust on the surface of the rock. Um, yeah, because those cracks end up holding water, that is a good place for moss. Moss likes that - it grows along those cracks. You know, if I was to take a picture of just the moss here, to show you where the moss wants to cover up the joints...... (river is too loud).

How old is this granite? 464 million years old, + or - 5. That means somewhere in the range of 459-469 million years. OK, what else have you guys noticed? There are plenty of other cool things to talk about here? I'm going to climb back over there. C. - Good. I heard nickpoint, I heard waterfall, I heard pothole. OK, the river here is adjusting from a higher level towards a lower level. You can see that it is carving out a nice, deep valley downstream, whereas upstream we do not see that big of a valley. There is a valley, certainly, but its not a serious a size as it is downstream. There is a series of waterfalls downstream from here, and the river is adjusting to a ....level, over and over and over again. Somebody brought up the term nickpoint. I think you did. The profile of a river is like where it starts off at a higher elevation and then it is descending towards a lower elevation. Those little nickpoints are where the waterfalls are. Um, this is a nickpoint right here. This nickpoint is retreating in an upstream direction. Over time, the river will adjust, and basically will be cutting down in an upstream direction, and then the downstream area, if it gets too steep will try to get flat, by mass wasting which will widen the stream valley. Oftentimes we talk about the Grand Canyon haven been cut by the Colorado River. It is only partly true. The Grand Canyon was cut deeper by the Colorado River, and the Grand Canyon got wider over time due to a landslide, not wasting events. So, gravity does not like having a whole bunch of rock supported by neighboring rock. It is more likely to collapse. So, that is the same thing here. The Quantico creek cuts downward, and over time the valley has been widened and widened and widened. Sometime on your own come back to downtown Dumfries and see how wide the valley is, because it is quite wide. Alright, questions? Jill - Did you say earlier that was an example of columnar jointing? C. - No, I was just saying that the intersection of these two joints just end up making these kind of vertical four sided columns of granite. Like you take a block of cheese you go chop, chop, chop and chop, chop, chop. You end up having these little columns of cheese. It is not columnar jointing. It is a completely different process.

Where the moss was the first one to colonize this joint, so you can see that this joint is filled in with moss, and here there is frost wedging to get this expansion of the dike, and various other process have made a little deposit of dirt here in the crack. And, a seed landed in that dirt, and the seed took root. Is this an ideal place for a tree? Probably not. But, it is growing where it ended up. In time of really high floods, you know it is probably more than likely to be stripped away. Notice that the majority of the trunks coming off of this thing are all tilted in a downstream direction. OK, that probably happened during flooding. Um, yeah so, probably not ideal...

Jill - I have a question about nickpoints. Is it like the vertical cut in the waterfall? C. - A nickpoint is basically something we describe on a river profile. The way we recognize nickpoints is we look for water dropping from one level to another. Jill - it is not like an event...C. - it is a feature. Jill - ...it is a feature. C. - And, I would only be comfortable here saying this is one nickpoint here, and there is another waterfall downstream where you get another sort of 10 foot adjustment. I would say that is a second nickpoint. And, one of the neat things at Great Falls is that as you hike along the Billy Goat Trail is, when you go up to Great Falls you can clearly see one, two, three jumps in elevation. There are three nickpoints bunched together at Great Falls. Here, they are more spread out. Jill - but there was an event that created it, right? Callan - Basically, as sea-level drops a new water fall develops there and then that starts working its way upstream. So long as it outpaces sea-level rise it is going to keep propagating upstream. Dave - ...sea levels are rising. C. - Well yeah, I mean sea levels are rising, so lower nickpoints could get drowned and then there is not going to be any more erosion going on. Jill - so basically they are faults...they are faults? C. - No, a fault is a break in the rock in which movement has occurred. These are simply levels in the rock - the river has eroded down to this level, then as base level drops, OK, the river is now eroding down to this level. So, a waterfall develops here and then it moves upstream over time. But, the actual rock underneath is not necessarily faulting. Jill - OK.

Yeah, this is an interesting blob here. I'm noticing this shape. It juts inward here and it juts outward there. It appears to be faulted where this side has moved over to the right relative to that side. That side moved to the left. The way we typically describe these things is when you have a fault - say there is a fault running through here about like that, OK? You look across to the other side, and you use the direction that side tends to move. In this case, the other side looks like it went to the left. It's a left-lateral fault.

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As before, I would be pleased to hear any comments / insights / suggestions you might have.

Labels: field trips, nova, piedmont, teaching

It started raining in DC on Sunday, and it basically hasn't quit since then. Rock Creek is running high and frothy, and the Potomac has about seven times as much water in it today as it did 36 hours ago. The USGS has only one gauging station on the Potomac in the Piedmont -- at Little Falls, approximately on the DC/Maryland border. Here's what that gage's data (available free online from the Survey) tells us (as of last evening) about the river's recent discharge trend:

Labels: dc, nova, piedmont, rivers, sediment, water resources

One of my most dedicated students (a) recorded our field trip last week to the Billy Goat Trail, and (b) transcribed it. Because I'm pretty much overworked at this point, I haven't been very blogophilic over the past couple of days. My apologies. So I'm going to offer you Jill's transcription of the Billy Goat Trail geology field trip instead. I've made a few small edits to clarify, but otherwise it's her transcription of our discussion/my lecturing at the various stops. I find it an interesting document... who would have ever thought anyone would pay this much attention to what I have to say?

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Billy Goat Trail Field Notes - April 8, 2008

Canal - C and O Canal stands for Chesapeake and Ohio. It's the canal that was originally intended to link the Chesapeake Bay watershed with the Ohio River. The Ohio River drains into the Mississippi River. So it's going to basically provide a watery link across the Appalachians. This specific structure right here as part of the canal, is what? It's a lock. The reason they went through all the trouble of building the canal where there's a river right there is you can't sail a river up a waterfall. Right here, there's a major waterfall [Great Falls] that prohibits navigation upstream and downstream. They built this canal where boats could sail upstream in a series of steps. These boats were actually pulled upstream. Technically, they weren't sailing. They were pulled along by mules. The mules were attached to the boat by a rope. The mules would pull the boat through these narrow, little chambers. Then these gates would swing shut at the downstream end and they would open up these gates at the upstream end. And, water would fill it up until the lock was filled with water at the upstream level, there. And then the boat would be pulled on out and do the same thing making steps uphill. Only when you do something like that, allowing a boat to float to a higher level, can you actually move a boat in the uphill direction. And they would do the same thing going downhill. Again, you can''t sail a boat down the waterfall very easily either. It's a little bit easier than sailing it up, but it's still not very safe. So, the C and O Canal was built for that purpose. It was originally the brainchild of none other than George Washington who originally tried to build the canal to get around Great Falls on the other side - on the Virginia side; called the Patowmack Canal. It's a very small canal. You can see its remains today. But, this (the C and O Canal) was a more successful canal - ultimately it was not completely successful. Ground was broken on it by John Quincy Adams about 5 miles downstream from here. He took that first shovel, and couldn't get that shovel into the ground. He tried again and again and he broke a sweat - it was very embarrassing and then he was a very staid individual and he rolled up his sleeves and played the part of like... "I'm going to get this!"...and so eventually that's when the canal construction began. The canal never made it to the Ohio River. It made it as far west as Cumberland, Maryland. In fact, the canal is now over 184.5 miles long. It's almost 185 miles long; it's quite long. At some point, the Baltimore/Ohio Railroad was started. The Baltimore/Ohio Railroad ultimately proved to be a more efficient means of extracting the natural wealth from the Appalachians; timber, and etc... And, the C and O Canal fell into disuse and eventually it was abandoned. At some point, developers were talking about taking this area and turning it into a highway that ran East/West along the Potomac River. Then there was this Supreme Court Justice who stepped in and said, "That's a lousy idea. It's a beautiful area. We should preserve it as a National Park." That Supreme Court Justice's name was William O. Douglas. He challenged a bunch of senators and editors of various Washington newspapers to join him on a walk. They went up to Cumberland, Maryland and they walked down the length of the canal to Georgetown where it ends. At the end, all of them were convinced that this was a place worth saving. And so, it became a National Park thanks to that one man saying, "we need to preserve this place." The reason we're able to come here today and actually look at rocks and experience the landscape is thanks to those efforts made by him and others inspired by him. So, that's why this area is still in a reasonably natural state.

The Billy Goat Trail starts about a quarter mile downstream. We're going to walk the Towpath where the mules once towed the barges up and down the canal. Until we get to the start of the trail.

It's a bridge. However, that's a really big abutment for a teeny bridge like that. The only thing going over that bridge is people. Yet they've got these massive abutments that are 40-50 feet thick. (You can see this one goes off into the woods that way: the abutment). Why would they go through the trouble of making a massive, racking structure just for the sake of a little footbridge? Because of flooding - yeah. This is not a structure for a bridge. This has to do with flooding. Explain yourself. If you're a canal engineer, and you spend years of your life and blood, sweat, and tears making that canal, you don't want the canal destroyed by a flood, right?...shutting down commerce from east to west. You want some kind of a fail/safe that you can activate in times of flooding. That's what this is. This is a flood control structure. You can see that there are grooves there, and in those grooves are slotted wooden structures, kind of like this one that I'm standing on. They're thin at the edge, thick in the middle, and that allows them to resist water slamming into them. And, think about that for a second. If you just walk, ...walk past this amazing view over there to the right, and you saw the Potomac River down 45 feet below you, -- during times of flooding the Potomac River level is up here - during times of flooding the discharge increases and the depth increases, so that the river is actually where you're up here standing, now, during times of highest flooding. And that could be totally destructive to your canal. So, this thing is put here so that in times of flooding they could act quickly and move these things in and make a big wall there. The flood waters slam into that wall and they could divert it off into the woods here and dump back into the Potomac River's main gorge. That main gorge is what we're going to be hiking along today and as we begin walking along the Billy Goat Trail, which I see is officially closed... as we begin walking on the Billy Goat Trail, keep your eyes peeled for evidence of flooding. You're going to see some evidence almost immediately after we start down the trail. Some of you are going to recognize that evidence. Some of you are going to walk right by it and not notice it. Our goal today is to turn up our observation meter so we are observing more. So anybody once you see some evidence that indicates flooding, call it to my attention and we'll stop and we'll discuss. (Question, John.) OK, there is sort of a wetland area right here - a little sag area where the water table is intersecting the surface - we're going to talk about groundwater in lecture next week. Basically, the groundwater is right on the surface so you get standing water there - a small little wetland. Good observation but that doesn't indicate flooding. (Jill - how about all those trees and brush just kind of pushed to the side?) Good, all right, it's a good observation - it's got something to do with trees...

OK. I'm expecting you guys to pay attention today. Probably you're going to want to take notes because ultimately what I'm expecting you to produce for me as a result of this field trip is a summary paper. This paper is going to be about 3 pages long - something like that and the paper is going to describe the geology of the Billy Goat Trail based on what we observe today. So this paper is going to be broken down, essentially, into an observation and then a geologist's interpretation of that observation. And then another observation and how a geologist interprets that. And so by talking about the physical evidence, and then separating it from the story that geologists tell based on that physical evidence, you're going to get an overall history of what happened to these rocks over time. (The paper is due in two weeks).

Head of the Trail - walking from the beginning of BGT.

Right, so during flood times, the water is coming from upriver, it's slamming into that flood diversion structure, and it ...over the landscape in this direction. So you'll see that the trees here are preferentially tilted in that direction. Do you see this one? How it's tilted in that way? This one, in fact, used to have this as its main trunk. That main trunk was killed and a little branch became the new trunk. Or look at this one over here. See how this one is pushed out in the same direction? Both of the original trunks broke off. Here was one, here was the second. Then branches became the new main trunk of that tree. Do you guys see that? -- tilted in a downstream direction. And, if you look around, you'll see plenty... now just tilted trees doesn't necessarily imply flooding. Trees that are all tilted in a common direction imply that they were all knocked down by a similar force. Knocked down but not killed. See how many tilted trees you can count.

Not creep - creep is on a slope. This was not creep. Tilting trees.

Knocked down by a flood, then it continued growing. Those do happen to be knocked down in the same direction but I'm not sure they were knocked down by a flood. Basically because those weren't knocked down last time I was here, and we haven't had a flood since then. I'm extrapolating that they were not knocked down by a flood. Furthermore, there's still dirt in the root. If you had a flood up here that was strong enough to knock down a tree it would likely have stripped away all that dirt.

We do have a couple of people coming through. We do want to clear - just step aside and make a path - part the Red Sea here.

John has made an observation that there's a round boulder up there. And that round boulder looks really different than most of these angular boulders that we see up here. John, is it also the same sort of stuff - does it look the same in terms of its composition? No, so maybe that could have come from somewhere else and the rounding suggests what, Elizabeth? It traveled a long distance (very good, Sal), OK. Remember the farther a sedimentary grain travels the more rounded it gets. So, flood waters may have deposited that, or maybe the Potomac River used to be flowing up here at this level. We'll talk more about that possibility later on when we see more of these boulders. It's a little premature to get into that, but it's - uh - a pretty big boulder. It's the sort of thing that wouldn't be picked up by the current and carried in a suspended load. It's more likely to be bed load along the bottom. So that indicates that that may be evidence that this used to be the bottom of the Potomac River before it incised to a deeper level.

What I'm stopping here for is where we're starting to see some more rocks. We're getting down closer to the river and because this area is more frequently subjected to flooding, that means there's less vegetation here. There's less dirt here. And, we can see more rock here. Your assignment over the next two minutes is to figure out what kind of rock this is. I'll give you two minutes - you're welcome to roam all around this area. What you want to do is you want to find nice, clean surfaces and try and identify the minerals, the texture, and ultimately the kind of rock that this is. Keep in mind that there is junk growing on the rock surface like this. What is this thing? It's a lichen, right. Lichen is a mix of algae and fungus that grows on rock surfaces. So, don't look at the lichens; they will deceive you. There are many different colors; these grey blobs are lichens, there are black ones, there are orange ones. You want to look for nice clean rock surfaces that don't have any lichens growing on them. OK, two minutes!

OK, what I would recommend everybody do is find yourself a nice, hunky seat. We're going to be here about 10-15 minutes, discussing. Somebody start us off with an observation about some of the different minerals that you've seen, or some of the textures that you've seen. Quartz. Big blobs of quartz here (she's got acid she's been dropping and the rocks aren't fizzing - not calcite). Some of those are very striking and obvious - very creamy looking - big blobs like right here, right here on that knob, etc. Good. What other minerals do we see here? Mica - muscovite micas, the silvery micas. Sometimes it's really obvious like, look at this, look at the shine on that, great. Nice and shiny mica. What can you tell me about all those flakes of mica? Are they oriented in random directions? Or are they all aligned in a common direction (Jill - they're in sheets). They're in sheets, says Jill. Would you agree with that John? How about you Elizabeth? OK? Yes, all the micas are aligned in sheets. And, obviously some of these are boulders broken off. Some are bedrock where the sheets are still in their original position. Like the one Jorge is sitting on - this one here - the one Elizabeth is sitting on. What is the orientation of those sheets in space? If you took your hand and made your hand a flake of mica how would you orient it in space? OK, good. Doug is showing us with his hand the orientation of all those flakes of mica in space. So what is that? When you get these layers of quartz and mica all basically strung out in these vertically oriented sheets? It's metamorphic foliation. When we studied metamorphic rocks, there are foliated metamorphic rocks and non-foliated metamorphic rocks. These are foliated. What does it take to produce foliated metamorphic rocks? Pressure, very good, Vivian. What kind of pressure? (Confining pressure is what happens to you when you're at the bottom of a new swimming pool: it may cause your ears to pop, but it doesn't realign your head in a new direction). The answer is differential pressure. So, what's happening here is that these rocks have been compressed, OK? Force is pushing on them this way, and then another force is pushing on them this way. So, all those original minerals got squished together, and they ended up lining up straight up and down as they were squeezed from the sides. So, this is a metamorphic rock. You guys have just figured out something really important about these rocks. What tectonic event creates regional metamorphic rocks that have foliation? Orogeny. So, these rocks have experienced orogeny. They've been squished from the sides due to that tectonic collision. Whoa! That's a pretty big insight to come to about these rocks. I'm sure this raises all kinds of questions in your head. Go ahead and ask some of those questions.

(Vivian - no, talus is often great big blocks like this - talus usually accumulates at the base of a cliff. You might be able to call some of this talus - like this could be a block of talus. This is not - this is bedrock. It's still attached to the solid earth. It's not that it's broken off and made into a piece of sediment like this. You'll see some areas today where you'll see some large accumulations of boulder piles, and I guess you could call that talus. Remember talus is specifically when it's falling into place.)

Jill - we're in the Coastal Plain? No, we're in the Piedmont. Is this a part of the Taconian Orogeny? Well, one way we can answer that question - Jill's bringing up the Taconian Orogeny. I want you guys to think back to when we talked about the geologic history of Virginia in lecture. We talked about this mountain building event that happened in the early Paleozoic/late Ordovician Period, we call it the Taconian Orogeny because it built up the Taconic Mountains in New York - um - what caused that Taconian Orogeny? (Jill - we've discussed this two days ago - maybe I'll put you on hold there, maybe somebody else can remember what caused the Taconian Orogeny?) A volcanic chain of islands bumping into us? Exactly.

Awhile ago, there was an ocean off the East coast of the United States. If you were able to go back in time 500 million years, and hover over North America, you would have seen something that looked roughly like this. Here you've got a smaller North American continent and it's missing some pieces. Notice that Florida's not there, California's not there, Alaska's not there. OK, those have all been added on more recently. 500 million years ago California, Florida, and Alaska were not yet part of North America. And, our location is marked right here. Now actually at that time what we'd really see is this (see map rotated) - North America was in a different position at that time. And, since 500 million years ago North America has rotated and moved north. OK, but at that time it was on the equator and it was rotated in a different position. So today what we call the East coast was really the southwest coast. Let's just call it the East coast and keep it simple. Does that work for you guys? OK. Notice what's offshore there. There's a subduction zone marked on the oceanic crust by a deep trench, and then next to that, paralleling the trench, is a chain of volcanic islands; a volcanic island arc. Subduction is bringing that volcanic island arc closer and closer to North America. It collides with North America. Jill is fortunate because she took my Prince William Forest trip on Sunday. We actually got to go and visit some of the rocks from those islands. They're preserved down by Quantico, Virginia. In between those islands and North America, a bunch of sediments got squished out. I want to remind you guys about the concept of an accretionary wedge. Accretionary wedge. What is an accretionary wedge? Right. Sediments that get scraped off the ocean floor at the sight of a subduction zone. OK, so remember in class I offered you the analogy of my arm covered in peanut butter, and my other arm scraping that peanut butter that went there? There's another analogy at the bottom of a bulldozer. So there's this big pile of oceanic sediments building up at this trench at the sight of subduction. And, those sediments then begin to squash between the volcanic islands and North America. I gave you guys the awful kitten analogy, right? So, this is the crushed-up kitten. These are these poor little oceanic sediments that are getting squashed between a Mac truck, North America, and a mini-Cooper, these volcanic islands. So, the kitten's little bones started off in many different orientations when they rotate to newer orientation which defines the foliation of the kitten. So, that's what you're looking at here, guys. You're looking at rocks that used to be sediments on the floor of an ancient ocean, and got crushed up and metamorphosed into the rocks that you're standing on now. So, if they're now metamorphic rocks and they used to be sedimentary rocks, what kind of sedimentary rocks were they? (Basalt? No, basalt is not a sedimentary rock - basalt is an igneous rock.) Did anyone see any grains when they were looking at these rocks - any grain size? Grains? Check this out, OK? What does this look like? You can see sand grains in there. There are sand grains in here, and sand grains are made out of what mineral? Quartz. Good. What is reacting to make the mica? What is reacting under elevated conditions of heat and pressure to make mica? It used to be greywacke. Greywacke is a mixture of sand and mud. Yeah, mud is made out of clay minerals. These clay minerals are not stable at high temperatures and pressures. So when they experience it, they turn into mica. Muscovite mica that's all lined up in the same direction. So these rocks used to be layers of sand and mud at the bottom of this ancient ocean basin (so-it's metagreyacke - Laura). Right, good. So, for the rest of the day, I'm going to call them metagreywacke And, I'm going to use that term over a more traditional metamorphic rock name like schist because I feel like it tells us more. All that "schist" tells you is it's a metamorphic rock. The term metagreywacke tells us a metamorphic rock and it used to be greywacke. So, it's got a double meaning there. Now, what can you guys tell me about how greywacke accumulates or where it accumulates? (Jill - an accretionary wedge. C. - An accretionary wedge just takes whatever is there and jumbles it into a big pile). Greywacke accumulates from submarine fans at the bottom of the sea. What is bringing sediment down to that deep location? What depositional force? Turbidity flow. You guys remember turbidity currents? Turbidity currents are these big, sediment rich flows that flow down across the bottom of the sea floor. When they slow down, what gets dropped first? Big grains. What gets dropped next? The finer grain stuff. And you end up with this overall sedimentary structure known as graded bedding. Anybody notice any graded bedding here today? (Here's an example...)

OK, so there may have been some preserved but then the river eroded out those boulders and transported them away, that's one reason. But we saw lots of rock left. OK - maybe there wasn't that much there to begin with? That's a possibility. They changed too much - they've been metamorphosed? Yeah! Metamorphosis tends to destroy those original sedimentary features, right. I mean, even though the mud isn't mud anymore, it's now mica. Yeah, metamorphosis has destroyed most of the graded bedding. If you go up and down the Piedmont, back and forth across the Piedmont, it's very, very rare to find graded bedding still preserved in the metagreywacke of the Piedmont. The only place that I'm aware of that you can still see it - no I take that back - there are two places that I know of where you can still find it. One place is here, and the other place is out near Sugarloaf Mountain. But everywhere else it's been destroyed. Like, Jill, did we see any at Prince William Forest Park? (Jill - uh, no). No, right, it was basically too intensely metamorphosed and the graded bedding is gone.

OK, let's try and bring this around full circle now at this point. If these sediments were originally accumulating as graded beds of greywacke, mixed of sand and mud, in an ocean basin, what ocean was that? The Iapetus Ocean - what the heck is that? Before the Atlantic. How does it relate to its name? The father of Atlas... The Atlantic Ocean is named for Atlas - the guy who held the world on his back. The ocean that came in the same place as the Atlantic but earlier is named for the Titan who fathered Atlas, and that was Atlas's dad, and that was Iapetus. So we call this ancient ocean basin the Iapetus Ocean. The Iapetus Ocean no longer exists. It's dead. The Iapetus Ocean was killed in a series of tectonic collisions. First, was a collision between these aforementioned volcanic islands and North America. Second, there was a microcontinent out there in the Iapetus Ocean - that crashed into North America. That microcontinent is now preserved as most of New England. Right, you can go up there and visit that ancient microcontinent. And then, finally, a much bigger land mass crashed into North America, finally killing off the Iapetus Ocean. What land mass is that? Yeah, Africa. Are you feeding them answers over there, John? OK, Africa crashed into North America, and that made a certain supercontinent that I'm certain that everybody knows, without John giving them a hint, -- Pangea. The moment when the Iapetus Ocean died was the moment Pangea was born. As soon as those continents butted up against one another, the Iapetus Ocean was gone.

We're talking about a geologic history here... we're talking about a collision. Exactly, very good, you've got the journalistic instinct. Who, what, when, where, why, when...so when did this happen? How can we answer that question? (By isotopic dating...) C - of what? What isotopic minerals would you choose to date here? The muscovite. That's right. The muscovite is a metamorphic mineral formed during the orogeny. So if you get an isotopic date on that it tells you when the orogeny happened. Well it turns out people have done exactly that. They've taken this muscovite mica and they've analyzed it, looking at the isotopes potassium 40 and argon 40 in that mica. And that gives you a date of 460 million years ago. That's the date of the Taconian Orogeny, Jill.

OK, so the Taconian Orogeny just to sum up here. The Taconian Orogeny was an episode of mountain building that occurred 460 million years ago. (Radioactive parent isotope is potassium 40 and argon 40 is the stable daughter product - question...)

We already noted back there that the rocks had been metamorphosed. Remember that metamorphism is one of the characteristic signatures of mountain building. You can identify a mountain belt even when the mountains themselves have eroded away by the presence of metamorphic rocks.

There were two other characteristics of mountain belts that we discussed in class. Vivian - what's one of them? Folding. And that's exactly what Doug noticed over here. He noticed that the metamorphic foliation has been folded up here. You guys see those sweeping folds going through these quartz layers here? Right here, you can see another one here. Down up, up and down again. Along the trail today, you will see dozens upon dozens examples of layers of folding. Sometimes it's a little hard to spot with the lichens growing all over them. You can see some here - you can see the layers go up and down and then up again. There's plenty - you guys are going to see some real nice, sexy examples of folding as we go along the trail. This isn't the most amazing spot, but since Doug noticed it, I wanted to point it out.

While we're on the topic, what's the third characteristic of mountain belts? Metamorphic rocks, deformed rocks (including folded or faulted rocks), and then the third characteristic is...? Come on guys, you can't take this for granted! Granite! Right. Granite. Remember granites are produced by partial melting when rocks get really hot. So, you want to keep your eyes peeled for granites along the trail today, as well. OK. What we're going to do...

Find yourself a spot where you've got a good, unobstructed view across the river to the other side. Remember, we're in Maryland, and we're looking across the river at Virginia. So, Virginia' on the other side. There's a feature I want to call your attention to here. Can everyone see there's a series of vertical gashes? Four of these gashes all in a row? All oriented in the same direction? If you look for the tallest tree over there, and then go down to the base of that tallest tree you'll see these deep gashes in the cliff face. Those are a series of igneous dikes. Dikes are what happens when a rock cracks open, magma squirts into a crack, then the magma solidifies into an igneous rock. Tell me something about the igneous rock that is inside these dikes. Is it more stable or less stable than the metagreywacke? Less stable. How do you know that Michael? More mafic - how do you know that from here? The color? You can see it looks a little bit darker. It's a mafic igneous rock? Ding! You're right. I'll give you a closer look at it here in a few minutes. But you can also see that these igneous dikes don't project out from the face of the cliff, they're sunk into the face of the cliff. Which means, that that rock 'rots' away more easily - more easily weathered. It's more easily broken down. Remember the Snickers bar that I made you suck on? Whatever is making up those dikes is more like the chocolate and less like the peanuts. It's easily etched away. Everybody with me on this? So, that supports the idea of it being mafic because mafic igneous rock has lots of iron and magnesium. Iron and magnesium like to oxidize. Now tell me this. How old are those dikes? Younger than 460 million years old. How do you know that? They're cutting through the metagreywacke. And, you can't have the dikes cut across the metagreywacke unless the metagreywacke already exists. Therefore the dikes must be younger than 460 million years old. Well it turns out their igneous dikes, so what can you do to them? You can date them isotopically. They've done isotopic dating on biotite that's present in those dikes, and biotite gives a crystallization age of 360 million years ago. Only 100 million years after the greywacke got metamorphosed to metagreywacke. Again, that number is 360 million years. Those dikes are 360 million years old - 100 years younger than the metagreywacke they cut across.

I want to point out that the second Appalachian mountain building event occurred 360 million years ago. This is the collision of that microcontinent with North America. So, as we said earlier, North American experienced a collision first with a mini-Cooper sized land mass of volcanic islands. Now, it's colliding with a good-sized sedan - the microcontinent. Eventually, it's going to collide with a Greyhound bus which, would be Africa. North America gets to collide with larger and larger land masses through time. This series of dikes over here occurred at the same time at that second episode of mountain building, sometimes called the Acadian Orogeny. You can see it well up in Acadia National Park, in Maine. (The highest point on the East Coast, still, is Klingman's Dome in Great Smoky Mountain National Park; 2nd highest is Mount Washington up in New Hampshire) (John - Avalonia up North and Carolinia in the Smoky Mountains?) (C.- That could well be true but...) We tend to divvy up these parts of the Piedmont and call them different terranes - I know there's a terrane called Carolina/Carolinina... but, I don't know if that's necessarily a microcontinent. I would only call it a microcontinent if it's made distinctly out of continental crust before it hit. Avalonia is the name of the microcontinent.

Doug did a great job earlier with his hand showing me the orientation of the foliation of these rocks. Again, we can all see the orientation down at our feet right now. It's oriented something like this. Now what I want you to do with your hands is show me the orientation of these dikes. I specifically chose this spot to view the dikes because we are looking directly down the barrel of these dikes. We're looking down that crack in the earth - it's coming straight towards us here. If we're looking down at our feet, we should expect to see the dikes right here. Where are they? What gives? There's a shift. It turns out the dikes are on our side, they're about 30 feet downstream. Let's go see them.

All right, look at this. Here's some almost vertical gashes in the rock. They have that same orientation. But, if you look (and this is actually a great time to be running this field trip because there's not leaves on the trees yet) if you look over there on the opposite side you can watch these go down and you would expect them to run into the middle of that cliff over there. But, that's not where you see the dikes on that side. Instead they're offset in an upstream direction on the Virginia side by about 30 feet. You guys see that? Pretty cool! What gives? Maybe, a fault? Let's discuss the evidence for faulting here. Oh, by the way. Here's an example right here - this boulder that my foot is on here. That is the igneous rock that makes up the dike. It's a kind of basalt - you remember basalt from lab, right - mafic and fine grained? And, what you see here, and I want everyone to come take a look at this after I move away, is that this basalt has visible flakes of biotite mica in it. Not muscovite mica, that silvery mica that we saw at the first stop, but instead biotite mica, which is jet black. You'll see these little shiny flakes of black biotite mica here in this special basalt. This basalt has a special name. It’s called a lamprophyre, because of those flakes of biotite mica in it, but, it's just a fancy name for a particular kind of basalt. Alright, again that name is lamprophyre. You'll see that in your handout that I gave you earlier. So, these are lamprophyre dikes. How old are the lamprophyre dikes again? 360 million years ago, which is the same age as the Acadian Orogeny. (That's coming from you John- one thing at a time, one thing at a time...) So, Laura please share with everybody your hypothesis on why the dikes do not line up from Virginia to Maryland. All right. Because there was a fault, and that fault offset the dikes on opposite sides of the Potomac River. Here's two diagrams. If you can't see these, move closer. Basically, here I have two different explanations for the offset of the dikes on either side of the Potomac River. The first explanation is that the dikes were originally straight and they were broken by a fault. What kind of fault would this be? Left-lateral or right-lateral? Right? Yes. Right because if you're looking across it looks like the other side has shifted to the right. Very good. The other explanation is that in fact, the dikes were not straight dikes. There's no rule that says if you crack open metagreywacke it must be a straight crack. The crack may have been jagged. Maybe that explains the offset well. Unfortunately the critical area we need to examine to answer this question is underneath the Potomac River. So, if we're going to answer this question, we're going to need to look around for additional lines of evidence. One piece of evidence has to do with the shape of the river. This is an aerial photograph of the Potomac River. We started off our hike today up here at the Great Falls Visitor Center. This white line going across the Potomac River is a dam where they divert water for D.C. Great Falls itself is this great, white blob here. And, then, we are right about here following the Billy Goat Trail along a very, very, very straight section of the Potomac River called Mather Gorge. Mather Gorge is what we're going to be hiking along for the rest of the trip today. Mather Gorge is named for Steven P. Mather, the original superintendent of the National Park Service. You'll find that the National Park Service has honored this guy endlessly. I think I've slept in four Steven P. Mather Memorial Campgrounds in National Parks around the country. They really love this guy. Anyway, Mather Gorge is named for him. Now, look at how straight Mather Gorge is. It is incredibly straight: It's as straight as an arrow. It's as straight as you would expect if there was a fault underneath the river there that had ground up the rock. Remember faults tend to break up rock into fault breccia. And that would be really easy for a river eroding into the landscape to erode away fault breccia opposed to solid bedrock. So, the actual shape of the river is suggestive of the fact that there may be a fault underlying the river at that location. Unfortunately, the only thing that we can use as a marker is these dikes. So, we don't have any other evidence of offset here because basically everything else is just smooshed up metagreyewacke. And, the place is the end of Mather Gorge which is here and here, where you might expect to see the fault exposed up out of the river, you can't really see any good evidence of it. Some geologists claim they've seen it up on the Rocky Islands that we walked by just before the fault diversion structure. I've been there and I've looked at the same outcrop and I don't see indisputable evidence of faulting there. I see a crack, but a crack doesn't mean a fault. ("Can you put divers in the river?" Sure you could put divers there at great expense and risk to the diver. Problem is at the bottom of the river there's all kinds of boulders covering up the bottom. And, there's silt and mud and big catfish and you're not really going to be able to get a good look at what's going on. The one thing you could do is you could back up the Potomac River for a couple of days, excavate away, and arrive at an answer to this question, but its not really that critical a question to arrive at an answer at.) Let me share another piece of evidence for you. Remember there's another way of explaining the offset in the dikes. It may be a fault but it could also be that the dikes were not originally straight. Here are two pictures of outcrops of the dikes. One is on the Virginia side and the other is on the Maryland side. Let's discuss the Maryland side, first. This is a photograph taken from Virginia looking at Maryland. You can see coming up from the river, one, two, three dikes. And up here, one, two, three, four dikes. One, two three. One, two, three, four. Three does not equal four. What's going on? Well, it looks like this middle one is actually branching. It splits into two arms there. When the rock cracks it was a jagged crack and the crack had two little fractures – two little arms that went on and those filled with magma. The other photo is over on the Virginia side. Again you see the lamprophyre dike here the metagreywacke host rock here. And you can see another one of those branching arms coming off the dike. The dikes are not in fact straight. Does that mean that there is no fault? No. A fault could break crooked dikes just as well as a fault could break a straight dike. So, do we have an answer to the question? No. We do not know which of these two hypotheses is correct. We have not been able to prove either one of them false, therefore, they both stand as possible explanations for the offset of these lamprophyre dikes. What questions do you have? Jill - what questions should we have? How about: "Sir, can we look at the lamprophyre, please?" Jill - can, I? OK, yes you may! Come here, stick your head in that hole and check out the lamprophyre up close, and see how it looks different than the metagreywacke. It's dark, fine-grained igneous rock and it has little flakes of biotite in it. It's going to be difficult to see from far away, you actually have to get about a foot away to see that. C I'm not lying to you, you can trust me. So, don't take my word for anything. Trust your own eyes and your own mind.

OK, why have I brought you over to look at this rock? It's fancy. Take it further. Jill - it's been fractured. There's some fracture. What do you see, Vivian? There's a lot of different joints in these rocks - remember joints are fractures along which no movement has occurred. Those are visible all over here making this very blocky landscape. Look at the other side. You can really see the joints. Good observation. But it's not why I brought you here. There's a nice big blob of quartz there. What kind of quartz is that? There's a lot of different kinds of quartz that we saw in our minerals lab. Smoky, rose, citrine, milky quartz - milky quartz, good. Milky quartz is generally whitish. Why is it whitish? Yes, it's got little tiny bubbles of water in it. That indicates how that quartz got there. It got there by hydrothermal fluids. OK, basically hot water in the earth had quartz dissolved in it and it precipitated out these big blobs of quartz. Very cool, this probably happened during the Taconian Orogeny, as well, when these rocks where nice and hot. Again, not why I brought you here. There's a beautiful fold here in graded bedding. Alright, do you see that really coarse-grained layer there that's been folded up? That used to be horizontal on the floor of the Iapetus Ocean deposited by a turbidity current and then during mountain building it got squished up and folded. Squished up and folded and it looks like the hydrothermal quartz was then placed as well. You can see another bed here below it pulling the same trick. Pretty cool, huh? Symptoms of mountain building.

Jill asks questions about source of sediments. Thank-you for being persistent with that.

John, pass me some clam shells. Did you guys notice all these clam shells all over the place all over here in these big sand piles? All right? What's up with that? So you're saying that these clams are the same age as the rocks? Turns out that these rocks have no fossils, whatsoever. For several reasons. One is, deposits in the deep ocean, there's not a whole lot alive down there. Second, these rocks may be older than multi-cellular life. So, they may not have any fossils in them for that reason. Third, they were metamorphosed so any fossils present would have been destroyed like most of the graded bedding. So, these are actually Recent clams. It's actually an Asian species of clam that's an invasive species colonizing North American waterways. It's a freshwater clam. So, these clams have come downstream from higher in the Potomac which means that they were deposited during floods. Just like the tilted over trees are evidence of flooding, so too are all these clam shells and sand deposits way up here above the level of the river. Unlike that big round boulder we saw earlier, this is the stuff that usually gets picked up by flood waters. This is like a little parachute very easily picked up by the waters and wafted around. Good. Let's go.

When does the Billy Goat Trail actually going to get tough? It's about to get Billy-Goaty. So what we're going to do is walk across an area called pothole alley. And as you walk across pothole alley you'll see why it got its name. And there's going to be lots of potholes there, you've got to be really careful. You want to use your hands and your feet. It's a good time to be putting away anything you've got in your hands and you've got your hands free to navigate the landscape. Jill - are you going to stop and talk a lot? C. - No I'm not going to talk at all. We're going to walk across it and then we're going to get to the other side and sit down on a nice broad plateau and have lunch overlooking Mather Gorge. OK?

(right after lunch) Maybe sand, maybe silt. Um, one of the things that you learn about these potholes is that if you take your hand and you reach inside and you run your finger around the inside you'll feel differences - that there are little ridges in there. There are different layers of quartz and mica. Quartz stays up in high relief because quartz is very resistant to erosion: it's hard. Mica on the other hand is soft and chemically unstable - it breaks down into clay at Earth's surface temperatures. So, what this is telling us is that something is preferentially etching away at the mica and leaving the quartz behind. Something really small has to get in there to do that job. Something like a grain of sand or like a grain of silt. So, pebbles may be part of the process, but, definitely sand or silt are part of the process. They're etching away at the mica and then maybe a pebble comes along and slams into these unsupported ridges of quartz and snaps them off. That would be one hypothesis, but the original etching is done by sand and silt. Based on these little ridges.

There's something else you may have noticed, and that is if you look across at the Virginia side, there's this very flat surface, basically parallel with the surface that we're on. Do you see that? Because if you look back up river, there's this sort of flat plateau, maybe not really flat, - it's etched into with all these potholes and stuff, but it basically continues across the Virginia side. That is one of those bedrock terraces ("straths"). They're older levels of the river that used to be the river bottom and then the river cut into a newer, deeper level. Some of the evidence that we have for this being the bottom of the river are these giant potholes. This sort of thing is not going to be scoured out in a flood. It's something where you've got the river working on it for centuries - maybe millennia. ...potholes... also, there are these great big boulders that we find up here. Boulders that were probably once bedload at the bottom of the Potomac River tumbling along, rolling downstream and then eventually when the Potomac cut into a deeper level, they were left high and dry up here on the surface. The next thing that we're going to stop and look at along the trail is one of those boulders that tells us about where the river was flowing from. On the other side (of the river) you see a hill. There is a hill on the other side that rises above this bedrock of terrace steps. That hill is called Glade Hill. On the top of Glade Hill you also find rounded boulders that have been transported downstream by the Potomac River. So the top of Glade Hill used to be the bottom of the Potomac River. So, the bottom of the Potomac River was above our heads and where we're standing now was still solid rock. Then the Potomac cut down to a deeper level. It carved out this bedrock terrace, made these potholes, deposited the boulders here, then it dropped again and cut down to a deeper level. The Potomac is incising over time. (This is not an entrenched meander because the Potomace does not meander here. There are areas where the Potomac does meander, like at the Paw Paw Bands. But, here of course the river is quite straight.)

OK, I want everyone to come and take a look at something. Wow. Alright. What I want you guys to do is I want you to stick your head in the cave. Tell me what you see! Stick your head in there and look at the ceiling. What are you seeing? Folds! You're seeing folds and what's being folded? Alternating layers of quartz and mica. The quartz is light colored milky quartz the mica is dark-colored biotite mixed in with muscovite. And as you look up there you can see that they're strung out in parallel layers. Light minerals - dark minerals. Light minerals, dark minerals, light minerals, dark minerals. It's a very coarse texture. We learned a name for that metamorphic texture - you got coarse alternating bands of light and dark minerals –gneiss. Gneissic banding. So, you got this foliation and remember that the foliating is formed due to differential pressure during mountain building. But what happens to the foliation, here, Laura? The foliation was folded. So you see that these alternating layers of quartz and mica that have been all folded up. And, that's an interesting thing because when you think about it, those layers themselves formed due to pressure in one direction. In order to get them to fold, you have to apply pressure from another direction. This is important stuff here. This tells us that these rocks have experienced more than one generation of deformation. They've been squeezed once. They got to sit still awhile, and they got squeezed again. OK? Questions on this outcrop?

OK., we're going to go down the path. We're not going to go far because we're going to see a stange, green boulder in the middle of the trail.

A Martian! This is an interesting rock, this is a greenstone, clever name. And, a greenstone is metamorphosed basalt. Where does basalt come from? Mafic lava. It's what basically happens when a volcano erupts mafic magma we call that basalt. If you want to see basalt forming today go to the Big Island of Hawaii or Iceland. If that basalt gets caught up in an orogeny, it gets metamorphosed and it becomes a greenstone. Basically, two metamorphic minerals grow - both of them colored green. And you met both of these metamorphic green minerals during our metamorphic rocks lab. Olivine is not metamorphic-that's igneous. I introduced you guys to 5 metamorphic minerals in that lab - garnet, kyanite, staurolite, and then these two. Chlorite - deep forest green, and pistachio colored green - epidote. Epidote indicates hot water in metamorphism. So what happened is this basalt flow got metamorphosed and it produced this greenstone. Now that brings us to the question of what are these little white blobs that are popping through here in different places? They are little round or ellipsoidal blobs of quartz.

Think of what this lava would be doing when it was first erupting. "Kitty eyes." Ignore her! Aren't they crystallizing. Sure, they're crystallizing and they're fine grained texture which makes them a basalt. What does a basalt do when it gets up to the surface and suddenly it's depressurized? Air bubbles... Remember lava often degasses at the surface causing little bubbles that we call vesicles and then those vesicles, those little swiss cheese like holes in the rock they can later get filled in with mineral deposits. In this case, quartz rich ground water flowing through this deposited quartz filling in these vesicles preserving the vesicles as... amydgules. Amydgules are these preserved gas bubbles. Now, I'm going into a lot of detail about this one boulder, even though this boulder is not from this area. This is like I mentioned, a visitor. It is one of these boulders that was deposited on the bottom of the Potomac River, before the Potomac River cut down to a deeper level. This is a piece of a very distinctive greenstone that is present out in the Blue Ridge Province. It's called the Catoctin Formation. I mentioned the Catoctin Formation when we talked about our Geologic history of Virginia, when I said that when Rodinia broke apart there were these big lava flows all over the landscape - flood basalts; that's the Catoctin Formation. Later on of course those flood basalts got metamorphosed during Appalachian Mountain building which made it green. Jill - so these are actually far deeper - have been layered deeper into the landscape, right? C. And the mountain building they got shoved up and erosion exposed them to the surface. The reason I go into all this detail about the identity of this boulder is I know where this boulder came from. I know that outcrops of amygular greenstone - the Blue Ridge province is west of here. So that indicates that when the Potomac River was flowing at this level, it was carrying sediments from the west to the east. Now that may seem obvious to you because of the Potomac today - it flows from west to east. But, we can say with some certainty based on the presence of this boulder right here that that boulder was doing the same thing in the past (Principle of Uniformity). It's a confirmation that the flow direction of the Potomac has been relatively constant at least since it was at this level. OK, I'm going to show you some other evidence if that. I'm going to point out some other boulders along the trail as we go along and they’re all going to have Blue Ridge identities. But, first I've got something even more spectacular to show you.

So I stopped here to show you this outcrop which might not look something too spectacular in the beginning, but once you understand what this thing is your eyes are going to pop out of your head and your jaw's going to drop. Get in close, take a look at this. What do we have here? "Rocks." (Eyes rolling) Sure... there's some nice folding. There's some potassium feldspar in there. See these peachy little potassium feldspars, here? They're opaque relative to the grey quartz here, no longer milky quartz, but grayish. Potassium feldspar, grey quartz - what is that? What are we looking at here? What are these little blobs of a mixture of coarse grained quartz and potassium feldspar? Granite! What's the third characteristic of mountain belts? Granite! You're looking at metagreywacke that's gotten heated up so much that part of the metagreywacke has melted. Not all of it, but some of it. Remember the idea of partial melting where you start off with a rock with a bunch of different minerals. Then if you heat it up, some of those minerals basically dissolve into liquid magma and some of them stay as a solid residue. So, ones that are more likely to melt are the felsic ones. Those that are less likely to melt are the mafic ones. So, essentially what you're seeing here is granite magma being sweated out from super-hot metagreywacke. This rock was originally deposited as sediment at the bottom of the Iapetus Ocean. Then it became metamorphosed and now part of it is becoming igneous. It's all three parts of the rock cycle right here in one outcrop. Jill - it's coming back to itself. C. - right, its coming back to itself. Right now its being weathered off and producing new sediments, so the cycle runs full circle, right? This is a granite being born. You've just got these little blobs of granite magma leaking out of this rock. You've got the midwife's perspective here watching this granite in the act of being born. This granite magma is liquid it's going to go upward in the crust like the blobs in a lava lamp and eventually it will join with other blobs and its going to cool together into a big granite pluton somewhere else. But, here it never made it that far. It just started to sweat out of the rock and then it stopped. So we are lucky enough here to have this snapshot moment of the rock cycle caught in the act: caught red-handed where metasedimentary rock is actually converted into igneous rock. It would have had to be really hot for this to happen. Probably around 400 degrees or 450 degrees; something like that. But because it's coarse grained it cooled slowly underground. Now it's up at the surface today, but originally it cooled down slowly deep underground. We some evidence in this area where we see boudinage. Little sausage shapes squeezed out. Remember we see that at about 10-15 kilometers depth. So this is a rock that formed about 15 kilometers beneath the surface when it was about 450 degrees. Now are there 15 kilometers or rock above us now? No, they've been removed. What removed them? Erosion, yeah. Erosion ground down these ancestral mountains and exposed their roots. The rocks that we're looking at here were once at the roots of the Appalachian Mountains. (Laura - So this surface here could be an unconformity surface?) If something else were deposited on top of it - right here we don't have anything else deposited on top of it - we see these occasional little boulders on top and if you go look on the top of Glade Hill there's a nice layer of boulders over there.) Let these people through and we'll continue our discussion.

Migmatite - partially molten - Jill. Um - migmatite. What can we do with igneous rocks, like we did with the lamprophyre? Isotopic dating. So we can isotopically date this granite. It turns out this granite gives us an age of 460 million years ago. Same age as the metamorphism – same orogeny. The Taconian Orogeny heated up these rocks and squeezed them. What was the cause of the Taconian Orogeny? The collision of a volcanic island arc with North America. So, good work guys! Isn't this a spectacular rock?!!

I have traipsed across this old planet a fair amount and I've seen migmatites in only two places. I've seen them up in Maine, and I've seen them here. So, you're really lucky that you're taking a Geology class where you're really close to a place where you can go and see a migmatite. Most students are not that lucky. OK!

He's showing us the difference between fresh and weathered metagreywacke - Jill - he's showing us... we just looked at the migmatite.

By the way the smell you smell right here is sulfur. This is a creek here that evidently ...some sort of pyrite deposit. There's iron in the creek which is rust - iron oxide. And, it smells sulfurous. Remember that pyrite is iron and sulfur. Here, it's being broken down here by the water.

Is that why they have the ridges here in general? - student. Ridges - Jill Ridges of quartz extruding out of mica. - example/observation

Remember I showed you that image of the scuba diver and he's standing in the river and the sea level is rising over him. OK, this is the river gravel that was deposited in that river. It's part of the Weverton Formation - it's early Cambrian. It's about 540 million years old or so. And again, I wouldn't expect you to know that by just looking at it. I only know that because I walk around thinking about geology a fair amount and I recognize it here. So, I'm correlating this boulder with outcrops to the west. There's some other boulders here as well. As well as this reddish stuff. There's these reddish sandstones. We're going to talk more about those just over the hill here. Here is a nice example of diabase which is going to be related to this red sandstone. This is a mafic igneous rock because I don't want to give away what I'm about to reveal down the trail. But, there is a variety of boulders here. All of these boulders can be sourced to outcrops in the west. Again, more evidence that the Potomac River is being pulled from the west to the east over time, carrying sediments along to prove it. OK? Like little passport stamps telling you where it's been. OK, a little bit further and we've got two more boulders to look at.

Three different sandstones. I've got samples from all three of them here. Somebody tell me the name of one of these sandstones. Quartz sandstone - the white one, almost pure quartz. The other is greywacke - dark grey - that's what the local bedrock was originally. This rock (sample) is a greywacke, not a metagreywacke - these (bedrock) are metagreywackes. The pinkish one is arkose. It's a mixture of sand, mud, and potassium feldspar. Big angular pieces of potassium feldspar. The kind we saw in that granite. In terms of maturity, the arkose and the greywacke are immature sandstones, and the quartz is mature sandstone. We've already learned that the greywacke is deposited in deep sea fans, sometimes called abyssal fans or submarine fans. Where is quartz sandstone deposited? Beaches, good. And where is arkose deposited? Very immature, it still has all its feldspars it hasn't broken down to clays which means it hasn't come very far. Rift valleys. Arkose is a characteristic of rift valleys.

We have two boulders here which can help complete our sandstone triumvirate. We've already got the greywacke down - check that off the list. This one here is a metamorphosed quartz sandstone, so its made out of quartzite. It's a really interesting one. Do you see those little circles on top of it? Those are the tops of fossilized worm burrows. These fossil worm burrows project down into the rock like this. They're cylindrical; called Skolithos. Some of you have noticed Skolithos trace fossils in lab. I've got a few samples out on the countertop. They look like little soda straws running through the rock. Again, I know where that came from. It's from the Antietam Formation. That's a barrier island beach sand that's found in the – the river has traveled from the west to the east - all the others boulders I've stopped to talk about have been located in the Blue Ridge. This is also a Blue Ridge rock. The Antietam National Battlefield is what its named for - the Antietam Formation. Characterized by these little fossil worm tubes.

This red beauty right here, and I encourage you to do so, - these little pink specks are potassium feldspar. This is an arkose. A big beautiful arkose from 10 miles away. This naturally outcrops 10 miles from here. 10 miles upstream at a place called Seneca Creek. Because it outcrops at Seneca Creek we call this the Seneca Sandstone. Seneca sandstone is an arkose. So, Michael found a piece of this early on. We've been walking over various boulders of it all along. Its source is so close by we actually have a lot of it here. What else have we seen that's red sandstone today? The first stop we made - along the canal. The locks were made out of this red Seneca sandstone. It turns out to be a great building stone. This Seneca sandstone is quite young. Because its young, its actually younger than Appalachian Mountain building. Which means it hasn't been metamorphosed. So, it's essentially, it's wet, poorly-lithified sand. So that means that when you cut it out into blocks, it cuts like butter. But, then once you take it out, it dries out, and once it dries out it becomes much harder. That's the ideal building stone. Easy to extract from the ground but hard once you make something out of it. The Smithsonian castle is made out of red, Seneca Sandstone. The "brown"stones in Dupont circle, too...

This is a very interesting chapter in Geologic History because, as Laura pointed out, arkoses get deposited in rift valleys. We've talked about putting Pangea together, killing the Iapetus Ocean through continental collisions, but we all know that Pangea didn't last. Pangea broke apart, and when it broke apart, what opened up? Rift valleys. Some of those rift valleys filled in with sediment and they didn't keep opening. Some of the rift valleys connected together and became a new ocean basin called the Atlantic Ocean. This is from the site of a failed rift. The rift began to open, but it didn't keep opening. It is located to the west of here, called the Culpeper Basin. The Culpeper Basin is a Triassic aged rift valley. When Pangea was breaking apart this big gaping hole opened up in the crust filling in with immature sediments like this arkose and then it stopped. That's where you find Dulles Airport today - it's in the middle of the Culpeper Basin. But, some other rift valleys, over in that direction (east) connected together and they were the weakest link. That's where the crust kept ripping over there. And it ripped and it opened wider, and wider, and wider and eventually sea water came in and it became a little ocean basin and then it widened and widened and widened and its still widening today. And, that's the Atlantic Ocean. So, basically that process began around 200 million years ago in the Triassic, and this is a Triassic sandstone. What organisms were alive during the Triassic? Dinosaurs.

--- T R A N S C R I P T --- E N D S ---

If you've made it to the end of this post, congratulations! I'm sorry, but I won't be able to refund the hour you just spent reading it... But since you're here, I'm interested in your feedback about this -- what elements you read about here caught your attention? Why? Thanks...

Labels: field trips, nova, piedmont, teaching

I've got a nice tough A.W.o.G.E. for you today. Hint: it's somewhere in the Virginia Piedmont. The presence of an airplane over the photographed site may help confirm the location, once you think you've found it.

In the comments section below, be the first to name the location and why the treeless area suffers so much sulfuric acid, and you will win a "GEOLOGY ROCKS" bumper sticker.

Labels: contest, google, maps, nova, piedmont

Cool folds (in metamorphic foliation) in this sample:

Here's the real prize: a big chunk of peridotite (upper right) that's partly surrounded in a crinkly foliated matrix of chlorite schist (lower left):

I'm off to Buffalo, NY today with four Honors students to attend the northeastern section meeting of the Geological Society of America. If anyone from the geoblogosphere happens to be up there, I hope you'll say "howdy." Posting may be sporadic over the next few days... we'll see what the Internet connectivity issue is like up there.

Labels: appalachians, iapetus, meetings, nova, piedmont

I've already introduced you to two of my Honors students' field projects. Now for the last of the three -- Jason's project on the strained metaconglomerate of Klingle Road. Klingle Road is a "road" in D.C. that was damaged by a storm some years back, and never repaired. Some people have started using it as a park, while others clamor for the road to be fixed. Geologically, it's interesting because it exposes a rock unlike any other nearby: a distinctly foliated metaconglomerate. Because I am so clever, I call it the Klingle Road Metaconglomerate. It's part of the "Laurel Formation," which is one of many flavors of metagraywacke / accretionary wedge complex that make up the bulk of the Piedmont in this area. Here's some of the squished clasts that Jason is interested in:

We know these rocks got heated up a fair bit. How do we know this? Well, they flowed out into elongated shapes all oriented in the same direction for one (see the additional photos here). The outcrop is peppered with clusters of little plus-shaped protuberances: they are clusters of sericite (cryptocrystalline muscovite) in the shape of staurolite porphyroblasts. Staurolite is a reasonably high grade metamorphic mineral, and when we see the three-dimensional shape of staurolite, but it's been turned into relatively-low-grade sericite, it's an indication of "retrograde metamorphism." Basically, after hitting the peak of its particular metamorphic conditions (high temperature and pressure, growing staurolite), the rock is readjusting to lower temperatures and pressures, and those staurolite crystals are reacting to a mineral that's more stable at those lower temperatures and pressures: sericite.

But anyhow -- back to the metaconglomerate. It's made of clasts, and those clasts have been stretched. The question is: how much have they been stretched. Sometimes when strain estimates are made, we assume an initial sphere shape, and then measure the lengths of the various axes of the resulting ellipsoidal shape (the "strain ellipsoid"). But is the assumption of initial sphericity valid? Jason is testing this issue by measuring the axes of cobbles and pebbles from the metaconglomerate as well as loose cobbles and pebbles found in nearby Rock Creek. We want to get a sense of how ellipsoidal cobbles are before they experience orogenic shortening/stretching. Here's a shot of Jason, Spencer, and Victoria measuring cobble axis lengths on a gravel bar near the National Zoo:

And a shot of the crew close-up:

And, just for fun, here's one more shot from Victoria's field area on Broad Branch. We hiked up to the contact with the Kensington Tonalite (a ~464 Ma felsic intrusive rock -- essentially a granite) and found a series of small waterfalls over this resistant rock unit. In the sequence of cascades were a series of deep pools. I submerged myself in one of them:

Labels: appalachians, dc, field trips, nova, piedmont, sediment, structure, teaching

Lastly, in this final picture, you can see (on the left and in the foreground) what a lot of the large bodies of migmatite looks like: mostly granite with wisps of mafic residue strung out as thoroughly-foliated xenoliths. Their common alignment is oriented in the same direction as regional foliation. This granite yields U/Pb ages of ~460 Ma, which is Taconian in age.

Labels: appalachians, migmatite, piedmont, virginia

Last Friday, I had a fun local field trip, in search of ultramafic rocks included in the Piedmont's metamorphosed accretionary wedge complex. My companions on the trip were David and John, both of whom are retired gentlemen pursuing geology as a hobby. Because they're doing geology for fun, they are among the most dedicated and interested students I've met at NOVA. Friday's trip was something I've been meaning to do for a while, and both of them thought it sounded like an eye-opener, so they came along too.

Our goal was to find some new outcrops that we hadn't seen before. Of primary interest were several mafic and ultramafic bodies included in the larger metasedimentary complex of rocks that we know today as the Piedmont. As I've mentioned before, these Piedmont rocks are interpreted as being the rocks of an ancient (Neoproterozoic - Paleozoic) ocean basin. When the ocean basin closed during Appalachian mountain-building, the sediments of the ocean got squished and squeezed between North America and Africa. Mixed in with them were chunks of the ancient Iapetus Ocean crust, which would probably be recognizable as ophiolites if it weren't for that pesky regional metamorphism they endured as a result of the collision. Up and down the east coast, there are outcrops of these mafics and ultramafics along the presumed "suture" zone between ancestral North America and terranes (blocks of crust) that were once a volcanic island arc in the Iapetus Ocean. As with most geology field trips, we also found some other stuff worth noting, even though it wasn't our primary objective.

Our first stop (located thanks to Diecchio & Gottfried (2004) in USGS Circular 1264) was in Clifton, Virginia, where we went to see the unconformity between the Piedmont metamorphic rocks and the Triassic sedimentary rocks which overlie them in an ancient rift valley called the Culpeper Basin. Tragically, instead of a beautiful outcrop, we found freshly graded surfaces and several new McMansions. There was only a small strip of undeveloped land, about 20 feet wide and 50 feet long which had any rock left. But in that area, we found an outcrop of soapstone. Here, John scratches the soapstone (talc) with his fingernail. It's soft!

In this case, the soapstone is interpreted as being metamorphosed ultramafic rock. Close to it, we found this piece of conglomerate:

The conglomerate is the base of the sedimentary sequence in the Culpeper Basin: it's the Reston Member of the Manassas Sandstone Formation. Notice that it contains clasts of foliated metamorphic rocks -- these were derived from the older Piedmont rocks it unconformably overlies. The Piedmont rocks got metamorphosed during Appalachian mountain-building, and then when Pangea broke up, the Culpeper Basin (one of the Newark Supergroup basins) opened up and got filled in. The source for the infilling sediment was the neighboring area, not surprisingly including pieces of the Piedmont. Up-sequence, the conglomerate is overlain by the regular Manassas Sandstone, which is a rich brick red in color (classic Triassic red beds), and contains a wealth of primary sedimentary structures. I found this one piece, which unfortunately broke into chunks when I picked it up:

It displays ripple marks, raindrop impressions, and a few horizontal branching trace fossils. Anyhow, that was about it for the Clifton stop. We were bummed about the development destroying the outcrop. On to the next location, Indian Run, on the east side of Annandale. There, using the geologic map that accompanied Drake & Lyttle (1981), we walked along the creek bed looking for exposures of rock. We didn't have to go far before seeing some heavily-rusted green rocks:

The above photo is dominantly chlorite, but check this out:

Pyroxene-rich inclusions (xenoliths? olistoliths?) were observable in the heavily-weathered exposures. The outcrops here were saprolitic, meaning they were essentially "rotten rock." David was struck by how soft they were. He said "It feels like velvet!" We turned our attention to the more coherent specimens which were weathered out and deposited as cobbles in the streambed. I got a watermelon-sized specimen that's about 40% massive peridotite and 60% greenschist. (I showcased this leprechaun-colored specimen last night in Historical Geology lecture, when we were discussing the Taconian Orogeny.) We also found intriguing hints of mountain-building in clasts like this:

That's a couple of beautiful folds in gneissic metamorphic foliation. As above, the bright green minerals are chlorite. We also found some cobbles of sedimentary rocks mixed in with the locally-derived metamorphic rocks. For instance, here's a nice semispherical cobble of flint, likely derived from the flint-bearing limestones of the Shenandoah Valley:

How did this flint nodule travel ~50 miles from its source area to its current resting place in Indian Run? Likely, it was transported by an ancestral version of the Potomac River, which brought many westward-derived cobbles eastward during the Cretaceous. About 100 million years ago, this river deposited a layer of cobbles all over our local area, preserved today as the Potomac Formation. It unconformably overlies the Piedmont rocks, and can be found today as the basal layer of the Coastal Plain. It's even found as a layer topping our highest local hills. The exposures in Indian Run actually offered a nice view of the unconformity surface, with foliated metamorphic rocks below, and unlithified Cretaceous gravel deposits on top:

Just to close out this post, I'll show a few other cobbles found in the streams. Here's a gneiss containing big, beautiful porphyroblasts of garnet:

And here's a Skolithos-bearing boulder of the Antietam Formation (quartz sandstone / quartzite), which I originally posted a few days ago, but is so gorgeous it should be shown again if I'm talking about boulders.

Finally, as a preview of tomorrow's post, I'll show a boulder which hints at the complex relationship between the foliated metamorphic rocks (gneisses) of the Piedmont and felsic igneous rocks (granites) which were derived from the partial melting of the gneiss. In other words, this is a boulder of migmatite: rock that has experienced partial melting. We'll explore this in more depth with some in situ photographs tomorrow.

Labels: appalachians, culpeper basin, field trips, flint, geology, nova, piedmont, unconformities, virginia

Walking around the mid-Atlantic Piedmont (my home territory), we find a lot of these fellows lying around. They are cobbles of the Antietam Formation (a Cambrian quartzite from the Blue Ridge) which were weathered out and transported eastwards (~60 miles or so, as you can probably deduce from their rounding). They were then deposited as part of the Potomac Group (Cretaceous river gravels draped over the metamorphic rocks of the Piedmont; preserved today on Piedmont hilltops and as the basal layer of the Coastal Plain). The cobbles display the vertical trace fossil "Skolithos" (sometimes spelled "Skolithus"), usually interpreted as a worm burrow. Each burrow is 2-3 mm in diameter. Here I've got a few photos: a cross-sectional view, a "plan" view, and a shot of one of the boulders in a stream in Arlington, VA.

Labels: blue ridge, coastal plain, fossils, piedmont, sediment

DC Metro area residents, you're hereby invited to join me (NOVA) or Phil Justus (NRC) or Michelle Arsenault (NSF) on a geology hike along the Billy Goat Trail, a popular and rugged hiking trail upstream from DC on the Potomac River, downstream from Great Falls. Michelle and Phil and I take turns leading this excellent hike. You'll learn about the Iapetus Ocean, Appalachian mountain-building, and the incision history of the Potomac River. You'll see potholes, amphibolites, metagraywacke, migmatite, and the mysteriously-straight Mather Gorge. The Park Service has just posted the spring schedule online here. Reserve your space today!

Labels: appalachians, dc, geologists, piedmont