That's a slab of the Shawangunk Formation conglomerate, from eastern Pennsylvania. I collected it a couple of years ago when I drove up to go fossil hunting at the Whaleback, but it wasn't until last year that I slabbed and polished it. (The slab measures 10 cm wide by 27 cm in length.) Then a couple of months to get around to scanning it, and finally a few months more before posting it. Sheesh.

It's a lovely quartz-rich clast-supported conglomerate, a ridge former in the Valley & Ridge province of the Appalachians. Like the Massanutten Formation, it's Silurian in age, and thought to be part of the "molasse" sequence shed off the Taconian mountain belt, first raised during the late Ordovician. It is interpreted as a relatively-high-energy fluvial system deposit; sediments laid down by rivers as the mountains next door were weathered and eroded.

Labels: appalachians, pennsylvania, quartz, sediment, silurian, valley and ridge, weathering

On our Historical Geology field trip to Washington, DC, this weekend, we were down at Chain Bridge Flats and saw some fresh flood mud deposited by the flooding Potomac. It was a gelatinous goo, like pudding, but had some lovely dessication cracks developing. Here are a couple of photos, courtesy of student Ana C., with a penny for scale in each:

Labels: primary structures, rivers, sediment

On the day before the GSA meeting began, I participated in a field trip to the Boring Volcanic Field, a zone of anomalously-located volcanic vents around Portland, Oregon. The field is named for the Boring Hills, adjacent to the town of Boring, Oregon, which is named for a dude named "Boring." Kim Kastens noted this funny name on the Earth and Mind blog recently. The USGS maintains an information page on the field here.

Today, some photos...

Atop Rocky Butte, field trip leaders Rick Conrey (WSU) and Russ Evarts (USGS Menlo Park) orient the group with a map highlighting the various units comprising the Boring Volcanic Field:


Mount Hood hides its peak in the clouds:


At our first outcrop stop, the field trip participants get out and look at the Boring rocks:


Here, a Boring lava flow overlies Troutdale Formation fluvial gravels:


Annotated version for the untrained eye:


In places, a "baked" zone of contact metamorphism can be seen in the Troutdale as it got scorched by the lava that flowed on top of it (bright red), but the characteristic red color was missing underneath one spot, the central overhang in this photo:

Weird, huh? Maybe the metamorphosed sediments need a certain amount of rain-mediated chemical weathering before they "blush"?

Well-rounded clast from the Troutdale: vesicular basalt from the Columbia River Plateau:


Another nice Columbia River flood basalt boulder, this one with phenocrysts of plagioclase, and a concentric zonation of texture (massive in the center, vesicular towards the edges):


Plus, you can find cobbles derived from further afield: gneiss (from Idaho?), quartzite (Belt rock?), etc:


Between cobbles of the Troutdale, you can see hyaloclastic sand (immature sand with lots of hydrated basaltic glass fragments, apparently produced by interactions of magma and water in the source area, upstream):


More hyaloclastic sand:


Oooh! A "crack panel" on the side of some cooling columns at another stop! These horizontal slats are produced in individual fracture-propagation events, and each one concludes with a little ridge called an arrest line.


Mafic pyroclastics that underlie the lava flows at this second stop:


More mafic pyroclastics, on a cinder cone in Mount Tabor Park.


This is a pretty neat outcrop: you can see normal faults cutting these angle-of-repose inclined volcanic strata, presumably forming in slumping events.


Annotated version of this same photo, highlighting a marker layer and its offset along the fault:


The weather was pretty grim for this trip, so that was a bummer. But it's Portland, right? What did we really expect? Anyhow, I enjoyed being introduced to this suite of rocks -- boring out of context, but interesting given their location well west of the main axis of Cascade volcanism. Unfortunately, the field trip didn't really address why the Boring rocks are there. I was expecting some sort of detailed discussion of the possibilities: an evaluation of different models for their generation and passage to the surface... but that really didn't happen in any substantive way. So it wasn't the most amazing field trip I've ever gone on, but it was a nice day of checking out a cool suite of rocks.

Labels: meetings, oregon, pleistocene, pliocene, sediment, volcano

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

Today I took a group of students to Sideling Hill, a syncline in western Maryland. Here are a few photos from the trip. All photos by my iPhone, via Facebook (which is why the quality is lower than my usual standards):

The group all kitted out at the Sideling Hill Visitor's Center (which was closed due to budget cuts in Maryland):


Jared points out fast-weathering shale layers betwixt slower-weathering sandstone layers:


Diamictite outcrop on the far western side of Sideling Hill:


More diamictite... enigmatic sediments...

Labels: maryland, nova, primary structures, sediment, structure, teaching, valley and ridge, weathering

Somehow, I've gotten a lot of reading done over the past six months. A lot of this reading consisted of books on climate change -- more on that in another post. But I wanted to share my thoughts on a few other books:

Sand - Michael Welland [blog]

Awesome. The perfect little book for those interested in geology. Looking at the world through a grain of sand. Very diverse, chock full of fascinating stuff that appeals to the intellect on many levels. Smart, erudite, funny. Recommended.

Stories In Stone - David Williams [blog]

A good read; like reading a compliation of feature stories in EARTH magazine; however, unlike Sand, no single unifying theme ties them all together. The overall idea is that the rocks we make our buildings out of have interesting backstories. The book is organized into a dozen or so chapters, each about a different building stone. Some are common (Indiana limestone), some are rare (petrified wood). All have got interesting stuff going on in terms of their geological history, human tie-ins, and architectural tweaks. If you live or work in a building, it's worth reading.

Your Inner Fish - Neil Shubin

Superb. Learned a ton about evolution's lingering fingerprints on our bodily blueprint. Did you know that the nerve which controls our larynx runs from the brain to the larynx via the heart? This unintelligent design is a vestige of the way our body develops from an embryo -- and can be traced directly to fish. There wasn't as much about Tiktaalik in here as I expected, but just enough to make the point.

Bones, Rocks, and Stars - Chris Turney [blog]

Really interesting, though the chapter on King Arthur didn't do much for me. But the rest of it is a great introduction to the various ways we figure out how old things are (Subtitle: "The Science of When Things Happened"). Great chapters on the orbital forcing of ice ages, carbon dating of Homo florensis (which Turney did), and Pleistocene megafauna extinctions. Recommended.

Glacial Lake Missoula - David Alt

Not so great as a book. Really more of a field guide, but not even all that great on that level. It essentially traces the geologic evidence of GLM "and its humongous floods" from Missoula north, west, south, and west again -- the path of the big Channeled Scablands-forming megafloods. A good resource for specific outcrops that illustrate parts of our understanding of this huge event, but not especially enjoyable to read.

Bretz's Flood - John Soennichsen

Much better -- a lovely biography of J. Harlan Bretz, the geologist from the University of Chicago who first documented the Channeled Scablands and deduced that they must have been carved by an enormous flood. A perfect little portrait of an academic's career. Bretz appears to have been quite a character! I really enjoyed the perspective this gave me on the whole "megaflood" idea.

Labels: books, evolution, floods, geologic time, sediment

Here we have some nice little glacial striations exposed in the Grinnell Glacier cirque in Glacier National Park, Montana. These grooves were carved by pebbles and other clasts within the glacial ice as it flowed over this outcrop of the Mesoproterozoic Helena Formation (part of the Belt Supergroup). Perhaps some of the same pebbles you see in this photo were responsible for acting as carving tools -- though the 'hand' that wielded them, Grinnell Glacier itself, melted away from this point sometime since 1939.

Also of interest to me in this photo is the lingering stain of water around the joint set in the upper right. I'm fascinated at the interplay between physical and chemical weathering, and seeing stuff like this emphasizes how even a simple hairline fracture can help funnel water, with all its destructive effects, deeper into the heart of an outcrop. Weathering is focused on these areas, and in another century this outcrop may look quite different.

Labels: glacial landforms, glaciation, joints, montana, national parks, pleistocene, proterozoic, sediment, snow, weathering

First off, I'd like to say a big "Thank you!" to everyone who joined in the basins discussion yesterday after my post comparing depositional basins and structural basins. I haven't had a post generate that level of chewy discussion in a while, and it pleases me to see folks chiming in.

So here's some additional thoughts: yes, structural basins are big synforms wherein the bedding dips in all directions towards the center of the structure. They are the opposite of structural domes. It seemed that this was a sticking point with several readers, who weren't familiar with "structural basin" used in this way. Chris indicated that the term "structural basin" isn't part of structural geology vocabulary in the U.K., and in many ways I agree with him when he says, "calling a structure which was never a site of sediment deposition a 'basin' seems rather silly to me." But that is what our textbooks and lab manuals refer to them as... That's why students get confused, and that was my motivation to draw the graphic delineating the differences. (I didn't invent this term! Ed appears to back me up on this.)

Suvrat called attention to the erosion that I included as part of my structural basin "model," and while that's not necessary for a structural basin to be called a structural basin, I included it to show that there was no basin-like topography necessarily involved. And that word, topography, is likely critical to the discussion. Shame on me for not mentioning it yesterday. (Ed mentioned that's how he distinguishes the two.) Here's the way structural domes and basins are expressed in the second edition of Steve Marshak's textbook Earth: Portrait of a Planet (reproduced here with his permission):


In the uppermost part of the image, you have both topographic and stuctural domes and basins. In the central part of the image, you see erosion-gutted (and differentially eroded) structural domes and basins that are not topographically basinal or domal. Brian asked an excellent question after yesterday's post, which was "where's a good example of a structural basin?" I didn't know of any great ones offhand, so I Googled it, and as it turns out, Wikipedia has a list on their page about "structural basins." (Tragically, the fourth hit on that same search turned up yesterday's blog post! I hate it when that happens.)

And this brings us to the most interesting part of the discussions: Lockwood was the first to say it: "Basins can be both, can't they? i.e., a structural basin can become a locus of deposition." Ah, yes! As my friend John Weidner likes to say about simple geological explanations, "Actually, it's more complicated than that." Are there depositional and structural basins? "Yes...."

"...but actually, it's more complicated than that."

The reality is that many basins are both structural and depositional. I hinted at this yesterday, when I said "[Depositional basins] can also self-perpetuate, as the heavy sediment keeps the crust sagging downward at that location." But I didn't launch into a full-blown discussion then because I was mainly interested in generating crisp thinking in my students: understanding that the term "basin" gets used (at least in our textbooks) to mean two different things, which have similar patterns but independent means of generation. Yes, the reality is that crustal sagging creating a lowspot is itself a structural phenomenon, which then has sediment accumulate atop it, which can encourage through its weight additional sagging, and additional sediment accumulation, and so on. Howard pointed this out in yesterday's comments. The layers at the bottom of such a "hybrid basin" will be structurally deformed at the same time sediment is being deposited at the top of the stack in the resulting topographic low.

So, really, what I outlined yesterday are end-members of a spectrum:


Reality has shades of gray! Yesterday's post was about the "black and white." Today, we discuss the spectrum in between.

How can we tell them apart? The classic test of whether a basin represents a sag in the crust and a hence a paleo-crustal downward flexure is to look at the thickness of the sedimentary layers. If they thin towards the edge and thicken towards the middle, then you've likely got some topographical low, and hence elements of a depositional basin. In contrast, a purely structural downwarp in the strata will not necessarily show any such changes in bedding thickness across the structural basin; so you'll see uniform thickness across (so much as such a thing exists):



Many basins have aspects of both of these -- sometimes they look structural further down and depositional higher up. The lower half of the Marshak illustration above is a map that shows the various basins and domes of the Midwest U.S. (Sometimes the domes are called 'arches' in they're more elliptical in outcrop than circular.) So are these regional-scale basins depositional or structural? Or both? Both, pretty much. These basins do show bedding thickness changes over time, and as I understand it, those times of increasing crustal flexure have been tied to the various episodes of Paleozoic mountain-building on the east coast. The Cincinnati Arch, for example, appears to have developed by the Devonian, since the layers older than the Devonian appear to be uniform in thickness across Ohio, but the Devonian sequence is thinner atop the arch and thickens to the southeast. (I'm no expert on Midwest geology; if someone cares to clarify and/or enlighten, please do!)

Eric made another excellent point: that sometimes we refer to the volume of sedimentary rock that was deposited in a depositional basin as a sedimentary basin. Hence the volume of sedimentary rock comprising the tortured strata of the Valley & Ridge province is sometimes referred to as the Appalachian Basin: not because it's either a depositional or structural basin today, but because it was a depositional basin in the past, before it got folded and faulted. Interestingly, the Marshak map also shows a non-folded, non-faulted Appalachian Basin northwest of the Valley & Ridge province. Hmm. You mean there's one term that geologists apply to two different things?

"No! Say it ain't so!"

Howard asked about the basins of the Basin & Range province. In my parlance, those would be strictly depositional basins -- structurally controlled, yes, but by brittle faults rather than crustal downwarping. They are sites of sedimentary accumulation, but do not show any kind of synformal structure. Thus, they don't qualify as "structural basins." Tricky business! ...Yes, they're basins; yes, they're structurally controlled. But they don't meet the definition for "structural basin."

And lastly, both Eric and Howard noted that there's yet another kind of basin: a drainage basin, a topographical feature through which runoff is collected, essentially synonymous with "watershed." To summarize the difference between a drainage basin and a depositional basin, consider this: a topographical basin which is primarily the site of erosion would be a drainage basin. A topographical basin which is primarily the site of deposition would be a depositional basin. Can a single topographical basin host both erosion and deposition? Definitely! Consider the Mississippi River drainage: eroding in the high country headwaters, depositing in the lowlands nearer the mouth of the river.

Thanks again for all the thoughtful comments, folks.

Labels: appalachian plateaus, appalachians, devonian, sediment, structure, valley and ridge, words

Definitely worth watching for Environmental Geology classes. (from the Times-Picayune)

Hat tip to Lisa for forwarding this to me!

Labels: louisiana, mississippi river, rivers, sediment

One thing I've noticed when teaching Historical Geology at NOVA and GMU over the past four years is that students get confused between basins. There are depositional basins and structural basins, and they're not the same thing, though they both sag downwards in the middle. The other day while driving out to the Blue Ridge for a hike, a lightbulb went off above my head. I knew what I needed was a graphic that explicitly laid out the processes responsible for each structure, and their development over time. I jotted down a reminder to myself on the lid of the Starbucks coffee cup in my car's cup-holder.

When I got home, I translated the scrawled reminder into action. In my spare time over the past couple of days, I've been composing the basin graphic with CorelDraw. Here's what I drew:



Depositional basins result when there's a low spot on the Earth's crust. Water flows into these crustal sags, carrying sediment with it. Gradually, they can fill in. Sedimentary inputs are shown with arrows. (They can also self-perpetuate, as the heavy sediment keeps the crust sagging downward at that location.) Layers stack up according to superposition: oldest on the bottom, youngest on the top.

In contrast, structural basins have a different story. There, we start with an accumulation of sedimentary layers, and then we deform them into a basin shape. This deformation is the result of tectonic stresses which warp the rock layers. Erosion can then attack the downwarped strata, planing the "nested cups" shape down to a roughly horizontal ground surface. Sedimentary outputs are shown with arrows. The resulting outcrop pattern is somewhat like a bull's-eye, with the youngest layers exposed in the middle and the oldest layers exposed on the outer part of the structure.

In a depositional basin, the downward central sag comes first, and the stack of sediment is a result of that sag. In a structural basin, the stack of strata comes first, and the central downwarp is produced second.

________________________________________
If any educators want a larger version of this graphic for use in teaching, let me know. I'll happily e-mail you one. Also, if anyone would suggest any modifications to the graphic to make it more accurate or more useful for communicating these ideas, I'd be happy to get that feedback.

Labels: art, sediment, structure, teaching, words

The Crazy Mountains are a range of mountains in south-central Montana, north of Livingston:

In this Google Map, you can orient yourself from recent posts by finding Bozeman, the Gallatin Valley, and the Bridger Range down in the southwest corner.

The Crazys are an Eocene intrusion, (Ar/Ar dates of ~50 Ma), and they are beautifully expressed on a geologic map as a radiating series of dikes around two central blobs of intrusive rocks (quartz diorites, etc.: dark pink on the map):

These igneous intrusions penetrated the Livingston Group, a series of volcaniclastic sedimentary rocks of late Cretaceous to early Paleogene age (hot pink on the map).

The day before my students arrived in Montana this summer, Lily and I took a hike in the Crazys, entering in the northern part of the range. We saw some cool dikes exposed along the road on the way in. Here's me pointing out the contact between a subvertical dike of porphyritic andesite cutting across subhorizontal layers of the Livingston Group:


Annotated version of the same photograph:


And here's a close-up of the rock making up the dike; mostly fine-grained and gray, but with some lovely big euhedral plagioclase feldspars as well:


That's about it for the geology I saw in the Crazys. Our hike kept us mostly in the forest, so clearly I'm going to have to go back some other time and spend more time there!

Labels: igneous, montana, mountains, sediment, travel, volcano

Yesterday, I asked you, "What is the origin of these red polka-dots on this apparently white sample?"



And I gave you a hint that this cobble was photographed at the "half drumlin bluff" site I detailed the previous day! And the fact that the word "apparently" was in italics was also a hint.

Well, here's the deal -- this is a red cobble, with a white coating. I think Paul said it was the Oswego Formation, a fine-grained reddish sandstone. I think what's going on here is that all the limestone powder in the till is readily dissolving during rain storms, and the dissolved calcite gets carried along in solution, flowing over and around larger clasts like this cobble. Then, as it dries out, it begins to precipitate a coating of calcite all over everything (maybe with entrained clay and silt particles too?). Then along comes a little sprinkle of rain, and individual raindrops splash away this encrusting solution from little circular areas, revealing the red rock beneath. I saw this same phenomenon on several cobble and boulder lithologies, not just the red ones.

What do you think? Plausible?

Howard came pretty close in the comments (though this is the top side of the cobble, not the underside), and as the sole guesser, I reckon that entitles him to the prize. Howard, send me your mailing address if you want a bumper sticker.

Labels: contest, new york, sediment

A quick contest:



What is the origin of these red polka-dots on this apparently white sample?

Hint: this cobble was photographed at the "half drumlin bluff" site I detailed yesterday!

First one to answer correctly is entitled to a free "GEOLOGY ROCKS" bumper sticker!

Labels: contest, new york, sediment

Labels: glacial landforms, glaciation, new york, sediment, travel

Some of the photos featured in my post on Green Sands Beach in Hawai'i have been added to "Hawaii Wow," a website that features intersting information about Hawai'i.

Labels: hawaii, minerals, sediment, travel

My third day in Montana this summer, Lily and I took a hike in the Bridger Range, going up the west side of the range via Corbly Gulch to a cirque opposite the "usual" route up Sacagawea Peak, which starts at Fairy Lake on the east side, then goes up into Sacagawea Cirque* and south to the peak. Instead, we went up Corbly Gulch and got a whole new look at Bridger stratigraphy. First, orient yourself with this topographic map:



The Fairy Lake route brings you to the ridge crest from the upper right (northeast), wheras the Corbly Gulch route brings you to the same ridge crest from the lower left (southwest). Now take a look at some satellite imagery:



The green line at upper right is the ridge crest; Sacagawea Peak is just off-screen to the right. It will not surprise you to learn that stratigraphic contacts strike NW-SE in this area. The forested left-hand part of the screen is underlain by Mesoproterozoic LaHood Formation, a coarse-grained formation in the Belt Supergroup. Then there's a little gap of grassy area and a thin line of trees atop a light-brownish layer. This is the Cambrian Flathead Sandstone, which is chock-full of interesting sedimentary structures and trace fossils. The prominent light-colored ridge-forming layer traversing the screen from upper left towards lower right is the Cambrian Pilgrim Limestone, which shows "fossil hurricanes" in the form of limestone-chip conglomerates.

Here's some of the trace fossils in the Flathead Sandstone:


Here's a limestone-chip conglomerate from the Pilgrim Limestone, which I interpret as a paleo-hurricane deposit: rip-up clasts from a carbonate bank tumbled and re-deposited together in a big jumble:


We hiked up to the ridge, and peered down into Sacagawea Cirque (getting pummeled by the wind!), but didn't feel like we had sufficient time to attempt summiting Sacagawea, since I had to be back on MSU's campus for an evening session as part of "Bahama Montana" class. More on that tomorrow...

______________________________
* The following week, my Regional Field Geology students proposed to rename Sacagawea Cirque as "Death Cirque," for reasons I will explain in due course...

Labels: cambrian, fossils, limestone, montana, msse, primary structures, proterozoic, sediment

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

So, it's a month until my Rockies class starts. I've been encouraging all the students to get in shape, because the high elevations, rough terrain and multimile distances we'll be hiking in Montana and Wyoming could really kick an east-coast flatlander's arse. So we've scheduled a few training hikes to help everyone physically prepare for the Rockies experience. Last weekend, we did a 5.5-mile circuit up the steep Little Devil's Stairs trail in Shenandoah National Park. I was joined by five Rockies students + one of their kids. Here's a map of the loop we did:



Here's a few photos of the hike, and the geology we encountered along the way:



John poses next to some jointed columns in the Catoctin Formation, a Neoproterozoic rift-related series of flood basalts (subsequently metamorphosed during Alleghenian mountain building).


End-on view of one of the columns:


Overhanging cliff showing columns weathering out along jointed surfaces:


Bob poses next to a cliff, helping me demonstrate how difficult it is to take a well-exposed photo in the jungle of the Virginia hardwood forest:


A wiggle in some columns:


Jared thought these columns were better than the first ones he saw, at Old Rag Mountain.


Here's me with a fifteen-foot-long section of columns, indicating that the flow from which this boulder was derived must have been at least fifteen feet thick, maybe more:




But it wasn't all columns. There was also a lot of column-less massive Catoctin Formation, and some nice inter-flow conglomerates which are interpreted as stream deposits that developed atop a cooled flow before the next flow erupted. These conglomerates imply a reasonable amount of time passed between successive eruptions of the Catoctin flood basalts. The lichens obscure the rock, but note for instance the fingernail-sized chunk of greenstone an inch above my hand:


More chunks in the conglomerate:


And more:


Jared guards the way forward:


The view from the top:

Labels: basalt, iapetus, igneous, metamorphism, nova, primary structures, proterozoic, sediment, shenandoah, structure, teaching

Today's my final field trip of the spring semester. My environmental geology students and I are going to see some acid mine drainage, some coastal erosion, and a coal-burning power plant.

The coal bit reminded me to share with you this cobble I found the other day in a deposit of cobbles along the Potomac River:




It's a well-rounded prolate cobble of bituminous coal! Of course, it makes sense that there would be coal cobbles in the Potomac's bedload, since the Potomac drains those portions of the Valley & Ridge province where such layers outcrop. But it's also reasonably fragile stuff, and I've never noticed in out here before. Usually those cobble beds are full of quartzite, flint, and the like.

Labels: coal, sediment

When I visited the exposures along newly-minted New Route 55 in West Virginia in March, I was so impressed, I decided to bring my structural geology students there on our trip. Now, after two stops in the Blue Ridge and a late afternoon anticlinorama, we woke, broke camp, and ate some great eggs and sausage (mine were swimming in coffee due to an accident with the French Press, but hey -- it all goes the same place, right Ben?) and set off to the west.

Hanging Rock Anticline roadcut:


Hanging Rock Anticline as viewed from the valley of the Lost River, where Old Route 55 wends and winds:


Ben, Dave, and Joe on the berm (note the thrust fault above their heads):


Plenty of primary structures to be seen here, too, like these trace fossils:


A hand-sample of trace-fossils (Arthrophycus, I think):


...or this beauty:


Small reverse fault with an offset of ~1 meter:


Here's a fossil (??) that I don't understand and cannot identify. I saw four of these out there. Can anyone (Tom, ReBecca?) help me identify this sucker and understand how it formed?










We moved on down the road a bit, to this lovely monocline (Jim & Jay for scale):


John, Karine, & Ryan take a closer look at primary and secondary structures in these strata:


Lovely flute casts:


Plumose structure #1:


Plumose structure #2:


Paleo-river channels incised into these strata (at the time of their deposition):


Reduction "halo" around a carbonaceous plant fragment fossil:


Ripple marks:


More plant fossils (these were the largest I saw):


Lots of carbon films of shredded up plant chunks:


Ball & pillow / flame structures:


Ditto, and note the graded bedding in the upper sandstone layer, too:


Great trip, everyone! Thanks!

Labels: field trips, fossils, primary structures, sediment, structure, teaching, valley and ridge, west virginia

Now that we've visited a couple of stops in the Blue Ridge province, it was was time for my Structural Geology class to head out to the Valley & Ridge province.

We made a brief stop to be introduced to the Massanutten Sandstone (Silurian quartz sandstone to quartzite) at Blue Hole, where we noticed this fault zone:


...But the main show was up in Veach Gap, where there's a zillion parasitic folds on the larger Massanutten Synclinorium. This was our third Field Study Area. The anticlines are beautifully expressed in human-sized outcrops, while the intervening synclines are lost in the subsurface:














In spite of this profound deformation, there are still some primary structures to be seen, like these Arthrophycus (?) trace fossils...


...and these external molds of articulate brachiopods:


As you might be able to deduce from the angle of light in these photographs, we hit this site late in the day, and then went back to camp at a site Dave knew of, by a lovely creek. Jim and Joe cooked us an amazing dinner of pasta and meatballs, and we hung out by the campfire a bit before bed. Sleep, and then up and at 'em the next morning to move on to our final Field Study Area... (more on that tomorrow)

Labels: fossils, primary structures, sediment, silurian, structure, valley and ridge

Wow... I can't believe it's taken me so long to process through my Billy Goat Trail photos from a month ago! Here's the final batch: a collection of images of potholes. Here's a typical pothole in metagraywacke of the Mather Gorge Formation:



Differential etching of the mica-rich and quartz-rich layers suggests that sand or silt is responsible for much of the carving of potholes: picture a liquid tornado with suspended grit, focusing abrasion on a specific area of the sub-river bedrock. Later, the brittle fins of quartz may be snapped off by shearing stresses when larger clasts smash into them, such as the pebbles and cobbles seen above.

Here's a trio of itty-bitty potholes in migmatite:


And, lastly, a nice waterfall-carved (now dry) series of chutes and plunge-pools, again carved into migmatitic metagraywacke:


...And a more zoomed-in shot to give a better sense of the complicated topography here:


I'm heading out on the Billy Goat Trail again today, and also Thursday and also Friday... a busy week of field-tripping. Hope you can make it outside too!

Labels: geomorphology, hydrodynamics, rivers, sediment

Yesterday, four Honors students and I went out to West Virginia's route 55 (between Wardensville and Moorefield), to look at some sedimentary strata and associated tectonic structures. Our guide was my friend David Dantzler, an enthusiastic amateur geologist. Here's a map of the terrain we traversed:



As you can see, this is part of the Valley & Ridge province, an area of the country defined by Paleozoic rocks that were folded and thrust-faulted during the Alleghenian phase of Appalachian mountain-building. Recently, a new road has been constructed traversing these valleys and ridges. It's a bit of a boondoggle, a pet project of West Virginia senator Robert Byrd which funneled federal dollars into the Mountain State, ostensibly to make it easier for the chicken farmers of Moorefield to get their birdie bits to market on the east coast.

This image ought to give you a sense of the project's scale (big bridge), and how much use it gets (no one on the bridge):


But the U.S. taxpayer's loss is the geologist's gain... There are some pretty spectacular new exposures of Valley & Ridge rocks along the new route 55. Here's the NOVA van parked at an outcrop of Tuscarora Sandstone that is arched up into a broad anticline. Again, notice how few people are driving on route 55 here:


Ooh, look: heavy traffic!


Contact between the lower Tuscarora Sandstone (a Silurian-aged extremely pure quartz sandstone, variably fused to quartzite), and the overlying (darker-colored) formation, which is either the Rose Hill Formation or the Mackenzie Formation at this location:


We found oodles of cool trace fossils:







But it wasn't just sedimentary layers. There were also some cool tectonic structures, like this joint in the Tuscarora, showing a beautifully developed hackle fringe:



Here's some "pencil cleavage" where fine-grained shale develops cleavage that intersects the planes of fissility, causing it to fracture in long slivers:



I slammed on the brakes for this one: an awesome anticline...


I forced David and the students to act out the orientation of the bedding planes at this anticline:


Honors student Jason points out a small thrust fault in the outcrop above him: You can see the offset in a greenish/gray shale layer:


In case it wasn't obvious above, here's a zoomed-in shot, with the offset layer highlighted (the miracles of Photoshop!) and the fault labeled:


We all had a grand day outside, and the rain held off until our return trip, which was pretty great. Thanks to David for showing us these rocks, and thanks to my students for being smart and inquisitive and into field trips.

Labels: appalachians, devonian, field trips, fossils, mountains, nova, politics, primary structures, sediment, silurian, structure, teaching, valley and ridge, 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

Amazing stuff...



I found out about this incredible art via Michael Welland's book Sand: the Never-Ending Story, which I just finished reading. [The book is superb, and everyone should read it, but more on that later.] For the moment, just watch this incredible thing. This is art, real art: simple in the extreme on one hand (a ball rolling through sand), but complex in the extreme on the other hand (the two dimensional images that emerge and evolve over time are terrific), and its underlain by some reasonably complex computing. Here's artist Bruce Shapiro talking about his work:



Like what you see? Then download this video and watch it. Showing the "Sisyphus III" sand plotter in time lapse photography set to music, you really get a sense of what this thing is capable of. ...Mind-blowingly cool.

Labels: art, books, sediment

As you'll recall, when I left off with my Ecuadorian travelouge, Lily and I had summited Pasochoa, and then taken a day-hike in Cotopaxi National Park. Next up, a new mountain that has about the same elevation as Mt. Whitney (highest peak in the lower 48 United States): about 14,500 feet. To climb this extinct volcano called Ruminahui (Roo-min-ya-wee), we headed up a ridge between two adjacent glacially-carved valleys.















Me with clouds and background glacial valley:


Diego (our guide) on the trail:


Up on top, there was less vegetation, but more cloud... and snow was falling.

The bedrock was a volcanic breccia that had been cut by numerous andesitic dikes:






You can see some blurry snowflakes in the previous photo. Here's a cold-looking Lily with her boots on an andesitic dike:



Here's a couple of close-ups to show the cross-cutting relationships between the andesite dikes and the volcanic breccia:





Here's a short, not-especially-great video wherein I point out a few things that don't really show up all that well. Still, you get to see it snowing!

A big "thanks" to NOVA's king of digital video, Richard Attix, who helped me rotate this video and crop out some unintended footage from the raw video we shot on the mountain that day.

Cold hikers:



"Sheesh! It's cold up here!":



On the way down, we also took some time to check out the plants. Here's one called "Orejas de conejo" ("Ears of the rabbit"):

Here's one that smells exactly like chocolate!


In fact, Lily was able to harvest this chocolate bar from it!

Okay, not really. It's money that grows on trees, not chocolate bars.

So that's the story of our second successful summit... now there was only one more to go... the legendary Iliniza Norte. Photos from that hike in a couple of days...

Labels: ecuador, glacial landforms, mountains, plants, sediment, south america, travel, volcano

Yesterday I mentioned finding a new (to me) outcrop of the Martinsburg Formation's graded beds (turbidite sequences shed off the late-Ordovician Taconian Orogeny here on the east coast of North America). Today, I'd like to share a few images of where John Graves and I went next: up into the heart of the Massanutten Synclinorium, the Fort Valley. To remind you of the relationship between the Shenandoah and Fort Valleys, here's a Google Map I've posted before:



There, defining the ridges of Massanutten Mountain (and thereby separating the lower Shenandoah Valley from the upper Fort Valley) is the Massanutten Sandstone, a Silurian-aged quartz sandstone (in some places it's a quartz-pebble conglomerate) that is correlated to the Tuscarora Sandstone further west in the Appalachian Mountains' Valley & Ridge province.

The Massanutten can show some nice primary structures, including some of the oldest known terrestrial plant fossils (preserved as fragmentary carbon films) and cross-bedding like this:



With regard to the cross-bedding, note that this is "reverse" cross-bedding, which records shifts in current direction over time. At the bottom of the sample, the current was flowing from left to right, and at the middle and top of the sample, it was flowing in the opposite direction, right to left. This sample shows well the distinctive shape of cross-beds: they are tangential to the main bed at the bottom, but are often truncated on top, making them superb geopetal indicators. (They tell you whether your rock is right-side-up or up-side-down.)

I took John on a hike up the Veatch Gap trail, because I wanted to show him the awesome anticline in the Massanutten Sandstone that NOVA adjunct geology instructor Chris Khourey and I had found on a reconnaissance trip out there in May of last year. John and I took a "group shot" with the fold:



And here's John showing those Montanans that we do actually have some cool geology out on the east coast:



So, what's going on here? Well... the Valley & Ridge province of the mid-Atlantic region is defined by folded (and thrust-faulted) sedimentary strata. These folds were produced about 300 to 250 million years ago, during the Alleghenian phase of Appalachian mountain-building. The tectonic cause of this deformation is interpreted to be North America's collision with Africa, closing the Iapetus Ocean and completing the assembly of the supercontinent Pangea.

More locally, the Shenandoah Valley and Massanutten Mountain are structurally underlain by a great fold, the Massanutten Synclinorium. Synclinoria are different from mere synclines because they are more complicated: the overall synclinal shape is "decorated" with numerous smaller anticlines and synclines. It's a big trough-like shape, but wrinkles are "parasitic" on the main fold. So, even within the big "canoe" shape of the Massanutten Synclinorium, there are little bulges and wrinkles that go the opposite direction. This anticline is one of them.

At that point, having seen the anticline, we weighed whether to keep hiking or not.

We opted to press on... and I'm so glad we did. ... Twenty feet further down the trail, we saw another two anticlines!



At its base, this one had a small cave I could crawl into:



And: a short distance further we found a hiker's shelter with an apt name:



Ha! I love it.

More tomorrow, when I'll revisit the issue of plumose structure and hackle fringes.

Labels: appalachians, geology, mountains, msse, ordovician, primary structures, sediment, silurian, structure, valley and ridge, virginia

Yesterday, I mentioned what my MSSE advisor John Graves and I saw along the Billy Goat Trail on Saturday afternoon. Today, I'd like to share some images and insights from our Sunday field trip, out to the Shenandoah Valley and the Massanutten Synclinorium which underlies it.

I would like to thank Rick Diecchio of George Mason University for sharing some key outcrop knowledge with me. I've found that information about good outcrops can be very difficult to obtain unless you know somebody who knows. The information is primarily passed on through the oral tradition, rather than written in sufficient detail in peer-reviewed literature or in field guides (...or posted on geoblogs?).

Anyhow, back in December, on our drive down to the Blue Ridge / Valley & Ridge Symposium in Charlottesville, I told Rick I was organizing a new Massanutten Synclinorium field course. It's a place he's very familar with. He recommended a good outcrop to see the turbidite sequences of the Martinsburg Formation, a late Ordovician clastic unit made of debris shed off the rising Taconian Mountains to the east. Rick drew me a map in my field notebook, and on Sunday I was finally able to schedule a visit. Since John is unfamiliar with the stratigraphy and structure of the Shenandoah Valley (or the east coast in general), we also stopped at a lot of the other stops I'll be taking students to, including the classic "Tumbling Run" section.

Today I'd like to share a sets of photos with you from this new (to me) outcrop of the Martinsburg Formation. Tomorrow I will share another set from the next layer up in the stratigraphic stack, the Massanutten Sandstone. Both outcrops a pleasing combination of sedimentary stratification and structural geology.

Here's the Martinsburg Formation outcrop, just west of the Shenandoah River's North Fork:


This, like the "Pet Store Anticline" that I have previously blogged about, is an excellent place to look at bedding/cleavage relationships. The beds are dipping east, but the cleavage dips steeply to the west, implying the outcrop's position within a much larger (kilometers-wide) cleavage fan.

Here's a eye-catching outcrop that shows the beds weathered out differentially, while pervasively cut by ~vertical metamorphic cleavage:


More beds, of alternating sand and mud, steeply dipping in the Massanutten Synclinorium:

Note how the muddier portions show cleavage development better than the sandier strata.

More pervasively-cleaved muddy layers:


Here's one that confused me. In this predominantly-sandstone layer, you can see that the cleavage is better developed on the right, lower side of the bed. Does this mean that the right, lower-side of the bed is more mud-rich? (and sand-poor?) It did appear to be finer grained. If so, does this imply this bed is upside-down? Ordinarily, I would have thought to only look for the primary sedimentary structure as a geopetal (right-side-up) indicator, but this is the first time it has occurred to me that structural susceptibility based on mineralogy (in this case, susceptibility to cleavage development) could be used as an indicator of younging direction. I should note that this particular photo was taken downhill of the main outcrop, and may well be overturned. It's a synclinorium, after all, not a smooth syncline!


In this photo, the turbidite sequences of the Martinsburg Formation show a cool feature, a primary sedimentary structure known as cross-bedding:

Note that this photo is taken with the photo's long axis ~parallel to bedding, but the reality of the outcrop is that this is all steeply dipping, rotated 90 degrees clockwise (see the inset for "true" outcrop orientation).

...But wait! There's stuff dipping to the left, and stuff dipping to the right! Which one is this purported cross-bedding? Try this labelled version to sort it all out:

Note how at the bottom, the cross-beds curve tangentially to subparallelism with the main bed. They are truncated at top by the overlying layers. This is a good geopetal indicator, and the photo is oriented in depositional position, with the top at the top. Furthermore, if you reconstruct the current direction from these cross-beds (after the strata have been "unfolded" and restored to their original horizontal orientation, it would have come from the east... that is, from the orogen itself (the roots of which are exposed along the Billy Goat Trail.)

The intersection of rock weaknesses along the planes of bedding and planes of cleavage can result in the rock fracturing into long pencil-like bits, a phenomenon known as "pencil cleavage." This is my Freddy Krueger impersonation using the Martinsburg's cleaved "pencils."


John puts his hand up to give a sense of scale to the axis of this small fold in the steeply-dipping strata:


I was all agog over this outcrop, really digging the relationship between the structure and sedimentological elements in the rock. Best of all, it's a very short drive from Tumbling Run, and will replace the hike to the Buzzard Rock outcrop in my Massanutten field trip in April. (For NOVA-area readers, there are still four spaces open in that class...)

Labels: appalachians, mountains, msse, ordovician, primary structures, sediment, structure, valley and ridge

Today's a bit of an art theme, so I had to pass this on...

Labels: art, geology, sediment

Last Friday was the first day of Structural Geology at George Mason University. Though I'm a full-timer at NOVA, GMU talked me into teaching Structure this semester, too. I've done this once before -- my first job out of graduate school, in fact. Then (in 2005), it was very stressful for me, and I'm not sure that I did a very good job. Now, though, I'm much more confident as an instructor, and I feel like I've got a better grasp of some of the essential ideas and techniques: both structural and pedagogical.

For the first day of class, I took a page from Kim of All My Faults Are Stress Related, who recently described a simple but effective "first day of structure" exercise in a post. Inspired by this idea of nurturing structural curiosity right from the start, I gathered up a collection of 36 samples of deformed rocks (plus a few non-deformed ones as "decoys") and laid them out on tables in our classroom:

Most of them were samples from my personal collection, which resides in my office at NOVA, but there were NOVA teaching lab samples too, and I added a few more interesting ones I found at Mason, like this ptygmatic fold in a granite dike:

The instructions to the students were twofold: First, visit each sample and describe it as fully as possible, noting in particular its "structural significance" (which I declined to define more explicitly). Then, once everyone had done that, get together as a whole class and organize these samples into groups based on common features. How many groups? Which features? They had to decide.

I took as my mantra a quote my friend Bridget (a writing instructor at NOVA) found:

"Teaching should be as experimental as writing." -Donald Murray

So I was conducting an educational experiment...

Starting the class in this way felt unfamiliar to me -- everyone "knows" that the first thing you're supposed to do is distribute the syllabus and spell out the gameplan for the semester. Or perhaps start with an introductory lecture. So it was kind of eerie and uncomfortable for me to sit still and quiet off on the side while a roomful of eager students (that I had only just met) went to work.

I sat back and made observations. One student was miming squeezing and stretching rocks with his hands -- "replaying" the stresses that he interpreted must have acted on the rocks to leave behind such structures. (Kim has another post up, just today, about the role of gesturing while teaching and learning geology.) I was pleased when (umprompted by me) they started using supplies like hand lenses, rulers, percentage charts, and hydrochloric acid to quantify the samples' characteristics.

Another student picked up a metaconglomerate with stretched pebbles whose boundaries were somewhat indistinct. His pen moved over the surface of the sample, visually tracing out the place where one stretched pebble stopped, and the next began.

Later, a student set aside a chunk of slate with plumose structure on its surface. With raised eyebrows, he said, "I can't say much about that!" A few minutes later, the sound of stippling resounded in the room as one student sketched a grainy sample.

Periods of quiet work were interrupted periodically with joking commentary. The students in this class (mostly guys) appear to have really bonded with one another during previous geology classes. They are all seniors, with the exception of one geography graduate student. It's good to see that they are comfortable with one another.

During the groupwork portion of the exercise, when the students were organizing the samples into clusters based on shared characteristics, I continued my silent observations. "Let's organize them by stress direction," one student said. "But not fault direction?" asked another. "How about directionality, regardless of what it's direction of," came the reply.

They ended up choosing these titles for their groups: "Slickensides," "Bends and folds," "Smashed together," "Tension," and "Undeformed." It was cool to watch this process play out. I had put out one sample of tension gashes in a limestone (extensional fractures infilled with calcite). The sample was one of the few that I had labelled. That went into the "Tension" group, of course. But what about that other sample with the quartz veins? Was that the same kind of thing? It's a different mineral...

The most classic exchange went like this:

Student 1: "I'm confused."
Student 2: "It [the organizational system] made sense at first."
Student 1: "...Like a lot of organizational systems in geology!"
(laughter)

Finally, once consensus has been achieved, we all walked around to the various piles of rock and I talked in a general sense about the structural importance of each one. The students appeared to be pretty engaged with this discussion: after all, they had invested some serious time in trying to figure these samples out; now they wanted to know what they really meant. My discourse on the samples stretched to about an hour. All told, the whole lab, grouping, and ensuing discussion lasted about three and a half hours. I felt really good about the exercise as a way of generating structural thinking during our first few moments (and hours) of class. I preferred this way of starting class to the traditional approach.

Satisfied that we were off to a good start, I passed out the syllabus.

Labels: igneous, metamorphism, sediment, structure, teaching

An article posted last hour on washingtonpost.com by Joel Achenbach examines an upcoming paper in Science that explores the idea of an impact triggering the Younger Dryas glacial advance as well as ending the Clovis culture and triggering the extinction of the Pleistocene megafauna. The evidence is nanodiamonds in sedimentary deposits from 12,900 years ago. Read the article, and wonder how Joel Achenbach finds out about this stuff a day before it's published. How does he get his hands on this article with enough time to compose a newspaper piece about it, but the rest of us have to wait until tomorrow to read the original paper?

Labels: anthropology, climate change, critters, diamonds, ice, news, sediment

I've got a few more stories to tell from Hawai'i... Today I'd like to share the tale of a backpacking trip that my friend Lily and I took along the northern coast of the big island. From the road's end at the Pololu Overlook, we descended into the Pololu Valley, across its excellent beach, then up the adjacent ridge to the east, down into the next valley, up another ridge (and further east), and then down into the third valley, where we camped.

The route is shown on this Google "My Maps" map:


Here's a look eastward into that final valley:


Descending into the final valley:


The view from our campsite:


The substrate of our campsite: a poorly-lithified conglomerate:


The thing that stands out in my mind most about this excursion was a landslide scar that had cut off the trail at one point. This landslide occured in the middle valley (between Pololu and our campsite valley). The landslide scar is nice and visible in the lower-left of this Google Maps image:


It happened in 2006, triggered by the big earthquake that struck the big island that year. It was one of several landslides that were set off by that shaking. (Wikipedia has a nice "live-action" photo of another cliff collapsing up the coast at Waipio.)

Here's the landslide scar viewed from the east, looking west (on our hike back towards Pololu):


Another shot from the same perspective shows the run-out of debris below the source:


The tricky thing about this was that we had to get past this landslide, since it wiped out the trail. On our way in, we somewhat stupidly climbed down the face of the landslide itself, gingerly picking our way down the steep slope, so we didn't trigger any further mass wasting. Here, for instance, is a poorly-put-together composite photo showing Lily descending into the valley:


(On the way out, we found some ropes in the vegetation next to the slide, and hauled ourselves up those rather than getting on the slide surface again.) But on the way in, when we got to the bottom, we weren't sure where the trail was, and plunged through some dense bamboo forest. I felt like I was in LOST, where the characters are perpetually fighting their way through similar vegetation:


Eventually we found the trail, and continued along. Because of the landslide blocking access, this part of the trail hasn't been used as much for the past two years. Lots of pandanus leaves had been shed off and blanketed some parts of the trail. Hiking across these dried pandanus leaves was a noisy affair:


On the eastern side of the ridge between "Landslide Valley" and "Campsite Valley," we saw this two-inch-wide crack opening up along the trail, parallel to the ridge/valley trend. The edge of the ridge was about twenty feet away towards the east (direction my boot toe is pointing). Certainly something like this portends a future episode of mass wasting...

Labels: hawaii, landslide, mass wasting, sediment, travel

As I mentioned yesterday, the Virginia Department of Geology and Mineral Resources has an excellent rock garden outside their office in Charlottesville, displaying a diverse suite of large rock samples from across the state's five physiographic provinces.

Here's Rick Diecchio (George Mason University) providing a sense of scale for the rock garden:


Here's a few of the samples that caught my eye, with my shoe providing a sense of scale (size 12, specifically) in each image...

Aquia Formation sandstone with Turitella fossils (Paleocene); King George County:


Balls Bluff Siltstone with mudcracks (Triassic); Culpeper County:


Conococheague Formation collapse breccia (Cambrian); Augusta County:


Cranberry Gneiss (?) showing well-developed lineation (Mesoproterozoic); Grayson County:


Kyanite quartzite (probably Ordovician metamorphic age); Prince Edward County:


Fossil Sigillaria tree trunk from the Wise Formation (Pennsylvanian); Wise County:


Unakite, the state rock of Virginia according to some (Mesoproterozoic); Rockbridge County:


Here's a link to the PDF (1.82 MB) with all the details about all the rocks in the garden, an impressive achievement just like the symposium.

Labels: conferences, fossils, igneous, meetings, metamorphism, primary structures, sediment, virginia

This weekend, I was procrastinating working on my final paper of the semester for my online MSSE education class, and decided to search "geology" on YouTube. (I knew there were some gems there, as Bryan reminded me earlier today.)

Anyhow, that search brought me to this piece of utter silliness, which I now share with you:

Labels: art, humor, sediment

This image came to my attention the other day via Lutz's Geoberg blog. It's one of the high-res images provided by the newly-launched satellite, the GeoEye-1, which is supplying new images to Google*. The image shows a marginal lake associated with an alpine glacier in Kenai Fjords National Park, Alaska (just south of Seward):


The top of the above image is not north; it's southwest. Mentally rotate it, and you can see that the resolution is a lot better than the current level on Google Earth and Google Maps:


The thing that struck me about the new GeoEye image, aside from its beauty, is the distinct pattern of iceberg sizes in the lake: freshly calved off the glacier, the biggest icebergs are close to their source, while further away the icebergs are smaller. This pattern struck me as being analogous to sediment. Fresh from its source, sedimentary particles are at their largest size, and the further away they travel, the more weathering they experience. This weathering (in particular of the physical variety) tends to break them down into smaller pieces. Adjacent to an orogenic belt, for instance, you tend to find deposition of sedimentary particles shed off the uplifting mountains. As a general rule, these are of the largest sizes and the greatest volume closest to the source, and then particle size and stratum thickness both diminish with increasing distance from the orogen.

For a North American example, consider the Catskill Clastic Wedge, a tick pile of sediments shed off the late Devonian Acadian Orogeny along the east coast. Here's a cross-sectional view** (pre-Alleghany Orogeny deformation) of the wedge, running from the Bay of Fundy west to Michigan:


Same pattern! Coarse stuff, and more volume of stuff, close to the source. Finer stuff, and less volume of stuff, further from the source. Just like the iceberg, except the weathering of the icebergs is mainly thermal, while the weathering of the sediments is physical, accompanied by depositional sorting by the transporting currents of water.

__________________________________

* An original version of this post misidentified Google as the owners of the GeoEye-1, as opposed to the company called GeoEye, which sells images to Google. Thanks to Bruce Haley for the correction. (updated 8:14AM eastern time on Dec. 9, 2008)
** Image redrawn (by me) from an original in Prothero & Dott (2003).

Labels: alaska, analogies, art, canada, glacial landforms, glaciation, maine, michigan, mountains, new york, satellite imagery, sediment

Yesterday, I showed you some sand, including some green sand from Green Sands Beach on the big island of Hawai'i. Today, I'll show you some more images from Green Sands. Let's start by orienting ourselves: We're on the south side of the island, just east of Ka Lae (a.k.a. South Point). Here's a Google Map of the cove (Mahana Bay) where Green Sands Beach (a.k.a. Papalakoa Beach) is located:


To get there from the Ka Lae parking area, you can either drive a four-wheel-drive vehicle over some very rough "roads" or you can hike about 2 miles along the coast. When I visited last week, we hiked. It's a pleasant walk, and there's plenty of green sand to be seen en route to the official Green Sands Beach. Here's the coast: basalt and grassy pastureland, with plenty of wind:


A view looking down into the cove where the green sand beach is located:


So, just why is the sand here green? It's full of olivine, which is weathering out from the local rocks. At first, I assumed the source was the local porphyritic basalt. The fine-grained basalt contains many large phenocrysts of olivine, and when the basalt breaks up, these dense grains tend to be concentrated together. Here's some of that olivine-rich basalt:


But apparently the major local source of olivine are some ash/lapilli layers that make up the prominent headland on the cove's eastern edge, as seen as the "backdrop" in this photo:


A close-up of the sand on the beach, with my fingertip for scale:


And a (repeated showing) of a handful of the stuff:


These green grains don't last especially long -- olivine isn't stable over geologic timescales at the earth's surface, and so it chemically degrades and weathers away. Thus, green sand beaches are extraordinarily rare on the planet Earth (according to Wikipedia, there are two: this one, and one in Guam). You've got to have a source of olivine right there, continually adding new green grains to the mix at a rate which matches or exceeds the rate at which they are being chemically broken down.

On the back side of the beach, draped up against the outcrops of ash and lapilli, is a big slope of sand piled up at the angle of repose. I really liked the patterns made between the olivine and the dark grains as small "avalanches" flowed down the "slip face" of this pile:


I even made a pointless little movie showing these mini-avalanches of green sand:

...Or not pointless? Maybe the sandman, new on the geoblogoblock, can tell me more about what's happening here.

A poorly-lithified chunk of green sandstone (cemented with halite from seawater, apparently, as it crumbled readily in my hands):


Some green sand on a basalt cobble (which itself hosts plenty of olivine phenocrysts):


And now a closer look at some of these ash/lapilli layers which are supposedly the main source of all this olivine. These ash layers were erupted by a cinder cone called Pu'u Mahana, and apparently date to 49,000 years ago:


Some of these layers are better lithified (probably due to welding, a phenomenon that occurs when pyroclastics are erupted at higher temperatures and then deform around one another as the particles settle) and thus stand out as little 'shelves' that are more resistant to erosion by the waves and wind:


Close-up of the ash/lapilli layers, with my fingertip again providing a sense of scale:


After an hour of swimming and relaxing on the beach, we climbed back up to the plateau above the beach, where we noticed this contact between lower ash/lapilli layers and overlying basalt flows:

Notice also all the white stuff filling in fractures here. I'm betting it's calcite, especially considering the little stalactites hanging down, but I didn't have any acid with me, and I neglected to collect any to confirm that assumed identity once I got home. Mea culpa.

Hiking back along the coast to the west, we encountered more beautiful olivine basalt. Porphyritic and vesicular, this stuff just about made me cry, it was so beautiful:


I was delighted when we detoured along one of the little unnamed coves between the official Green Sands Beach and the car, and found this:


This green sand was greener than the official Green Sands Beach:


A reprise of yesterday's image of this beautiful stuff:


We noticed some footprints of a mongoose (introduced species) crossing the green sand:


Close-up of the mongoose tracks:


There was also a nice accumulation of basaltic cobbles (some porphyritic, some not, almost all vesicular) mixed in with chunks of coral:


Wow. What a cool place! Unique in my experience, and pretty close to unique in the world. If you're ever on the big island, you've got to check it out. As a geologist, visiting Green Sands Beach imparts big bragging rights: it will make all your friends green with envy!

Labels: basalt, hawaii, oceans, sediment, travel

As I mentioned a few posts back, I spent the week of Thanksgiving on the big island of Hawai'i. I had an exam scheduled in one of my classes, and I pre-recorded the lecture for my other class (via Smartboard), so I was free to kick back and relax on my travels. However, I find it's difficult to turn the inner geologist off, and so I spent a lot of my time checking out the cool geology of this unique island. I've got a lot of photos to share and stories to tell, but I'll start off simple: here are sand samples from four beaches in Hawai'i:






As you can no doubt tell, these sands are dominated by, respectively from top to bottom: calcareous hash (fragments of shells and corals), basalt fragments, olivine crystals flavored with basalt fragments, and a greater proportion of olivine crystals. They are respectively from "Sixty-Nines" Beach (west side of island; named for the milepost on the nearby road, so get your mind out of the gutter), Punaluu Harbor (south side of island), Green Sands Beach (south side of island), and a nameless cove between Green Sands and Ka Lae (a.k.a. South Point, on the south side of the island -- and in fact the southernmost point in the entire United States).

Labels: hawaii, minerals, oceans, sediment, tech, travel

Saw this amazing image of an alluvial fan in Iran yesterday on NASA's Earth Observatory's "Image of the Day":



All those elongated rectangles called to mind the famous oil painting The Kiss by Gustav Klimt:



Clearly, another case of nature imitating art!

Labels: art, satellite imagery, sediment

I've already posted some images from the VCCS Science Peer Conference a week and a half ago. Outside the offices of the Wintergreen Nature Foundation, they've arranged a series of large charismatic rock samples from the region. Some of them are from the Blue Ridge (where Wintergreen is located) and some are from adjacent physiographic provinces. These samples are from the Valley and Ridge province, showing some cool features sometimes found in sedimentary rocks.

First, some articulate brachiopod fossils in quartz sandstone (internal/external molds). This wasn't labelled as to its source formation, but it looks a lot like the Oriskany Sandstone, a major ridge-former in the Valley and Ridge. Quarter for scale.


Second, a breccia in limestone. (FYI, Andrew's Oakland Geology blog has another nice image of breccia today.) Perhaps a collapse breccia? Again, the sample wasn't labelled, so I have no idea which formation it was derived from. The white in-filling is calcite. Quarter for scale.

Labels: conferences, fossils, sediment, valley and ridge

...So let me ask you something, especially you sedimentary geologists...

This is a sample of the Martinsburg Formation, a clastic unit shed off the Taconian Orogeny and into the adjacent basin. It's exposed in the modern-day Shenandoah Valley, where it overlies Ordovician carbonates, and is overlain by the Silurian Massanutten Sandstone (which is correlative to the Tuscarora Formation). It's essentially a graywacke, showing rhythmic bedding traditionally interpreted as turbidite deposits. I collected this sample in the Shenandoah Valley a year and a half ago, on a camping trip with my family.

Then I put it on the NOVA rock saw and sliced it in half. This chunk went to my dad's back yard, where I ground it down and polished it up. The result is a decent look at the internal structure of the unit (you can click on it for higher resolution):


Note the pretty uniform weathering rind wrapping around the whole thing, like crust on a loaf of bread.

UPDATE: Woe is me; I forgot to include a sense of scale. The sample measures about 10 cm (~4 inches) on a side.

Here's the thing that gets me... While this portion ('upper' 2/3 of the sample) shows a clear fining-'upwards' sequence....


...this portion of the sample (lower 1/3) appears to show a coarsening-'upward' sequence:


In other words, in this 'graded bed,' the coarsest grains appear about 1/3 to 1/2 of the way 'up,' from 'bottom' to 'top'... What gives? This isn't part of the traditional Bouma sequence, is it? How does a bed like this form?

I'd appreciate any enlightenment you can offer.

Labels: primary structures, sediment, valley and ridge, weathering

Yesterday, I pulled up the Venetian blinds in my office window at NOVA, and this is what I saw:


Naturally, I had to take a photograph. It's puuurty.

While I had the camera out, I figured I'd shoot a few photos of the rest of my office, since it's full of all sorts of interesting clutter. Rather than explaining what all the doodads are in these photos, I figured it would be more fun to just post them and see if you can identify them all:















Have fun!

Labels: fossils, igneous, maps, metamorphism, minerals, nova, plants, sediment

Driving back from the GSW fall field trip this past weekend, I took this photo out of my car window. Considering the vehicle was in motion, I'm pleased with the decent quality of the photo:


This house clearly shows graded bedding! There are many ways to get graded bedding showing a fining-upwards sequence of deposition, but my favorite is deposition by turbidity currents, dense sediment-water flows that drop the heaviest stuff (usually the biggest particles) first. Then as the water calms, progressively finer and finer particles settle out of the turbid water.

I was predisposed to look for graded bedding in buildings, because one of my students/ colleagues/ friends, Dr. John Weidner, took this photo earlier in the year and shared it with me:

This would be a case of inverted graded bedding, a coarsening-upwards sequence. Did this house prograde out into a basin like a delta? Or was it deposited by a turbidity current and then later tectonically overturned?

Labels: humor, primary structures, sediment

Yesterday was the Geological Society of Washington's fall field trip. A group of about twenty of us went down to George Washington Birthplace National Monument, a stretch of land in the Virginia Coastal Plain, about an hour east of Fredericksburg. The trip was lead by Wayne Newell of the USGS in Reston and Rijk Morawe of the National Park Service.

Here's a map of the Monument, adjacent to a small bay formed as the valley of Popes Creek flooded with post-glacial sea-level rise (essentially the story of the entire Chesapeake Bay in miniature):


Wayne and Rijk are studying the coastal processes here in an attempt to use the Popes Creek as an analogue for Chesapeake Bay processes in general. One of the reasons they really like it is because unlike other small bays in the area, it has a spit (almost a baymouth bar) protecting it from the ravages of the tidewater Potomac (which it flows into). Here's the spit heading southeast across the mouth of Popes Creek Bay:


This rotted old wooden seawall was erected along the coast in the 1960s. This is on the Potomac, just upstream from the Popes Creek Bay. Effectively, this seawall serves as a "before" line, a marker which conveys the shoreline's former position. You can see how much erosion has taken place since then:


I'm less interested in these coastal dynamics, though, than I am in the bedrock geology. There were some bluffs along the river which exposed the Miocene Calvert Formation (clay-rich lower unit) topped by a foot-thick diamictite unit, and then well-rounded river gravels on top of that:


Here's Merily (sp?) from AGI checking out the sequence of strata:


My favorite part of the trip was looking at the variety of cobbles on the beach. These cobbles are derived from all of the mid-Atlantic's physiographic provinces within the Potomac River's watershed (Valley & Ridge, Blue Ridge, Culpeper Basin, Piedmont, Coastal Plain). All those physiographic provinces have been weathered to produce the sediment that the Coastal Plain is made of. In spite of their diminutive size, they give insights into the geologic history of Virginia over the past billion years. So if you're familiar with Virginia geology, you will see some familiar rocks here.

For instance, there were a lot of these Skolithos-bearing quartzite cobbles. These are pieces of the Antietam Formation, a meta-quartz-sandstone that crops out in the Blue Ridge province, many many miles upstream:


Skolithos is the name given to vertically-oriented cylindrical burrow trace fossils, which start showing up in the Cambrian period of geologic time, indicating the evolution of vascularized bodies among animals. They are usually interpreted as worm burrows. This cobble shows several different diameters of Skolithos tubes:


Here's a cobble of another distinctive Blue Ridge rock. This amygdular meta-basalt is a piece of the Catoctin Formation, a sequence of (mainly) mafic lava flows that erupted as the supercontinent Rodinia was breaking up in the Neoproterozoic era of geologic time. The white spots you see are amygdules: vesicles that have been filled in by mineral deposits. When lava erupts, it degasses. If the lava cools into extrusive igneous rock before the bubbles have a chance to pop, little round holes are preserved in the rock, like Swiss cheese. We call these "vesicles." When vesicles get filled in with deposits of minerals (from groundwater passing through the rock), they are called "amygdules," from the Latin for "almond," which I guess they resemble in an ellipsoidal sort of way:

(I showcased a very similar cobble here in March of this year.) Like the Antietam Formation cobbles, this Catoctin Formation cobble originated in the Blue Ridge province, and has tumbled dozens of miles downstream to end up out here on the Coastal Plain.

Here's one from even further away! This is a cobble of flint from one of the limestone units out in the Shenandoah Valley, the easternmost valley of the Valley & Ridge province. (I've previously posted on those rocks, too.) While the limestone which originally hosted this flint nodule has weathered away, the flint is microcrystalline silica: very hard, very chemically stable. It's a common cobble to find surviving out here in the Coastal Plain:

We also found some rocks that are distinctive occupants of the Culpeper Basin, a Triassic-Jurassic rift valley upstream. Here's a chunk of the Manassas Sandstone Formation, another rock that has been previously mentioned on this blog:


The rock I spend most of my time thinking about is the metagraywacke of the Mather Gorge Formation. (For one mention on NOVA Geoblog, click here.) Here's a piece of it that looks identical to the rocks you'll see near Chain Bridge, DC, or along the Billy Goat Trail (Potomac, Maryland):

This rock was metamorphosed ~460 million years ago, in the late Ordovician, although the original sediments are older than that: perhaps Cambrian or late Neoproterozoic in depositional age. This sample even had a little bit of hydrothermal quartz stuck to it, a common feature of Piedmont metamorphics...

Having covered clasts derived from the Valley and Ridge province, the Blue Ridge province, the Culpeper Basin sub-province, and the Piedmont province, there's nothing left in the Potomac River watershed except for the Coastal Plain itself. And sure enough, we saw Coastal Plain clasts too. Here's a chunk of the Calvert Formation that GSW Field Trip Chair Bill Burton found: He cracked it open and found a shark tooth fossil inside:

This is the first time I've ever seen a tooth preserved as a carbon film. Except it wasn't really just a film, it was more a three-dimensional external mold with a carbon film, and little nuggets of carbonaceous material rattling around inside. Shark's teeth are pretty common in Miocene deposits on the Coastal Plain, including C. megalodon teeth, but this style of preservation was pretty novel for me. If you're into fossil collecting, don't go to George Washington Birthplace National Monument, because collecting isn't allowed there. However, nearby Westmoreland State Park offers legal fossil collecting opportunities. It's about ten minutes further south.

I'd like to thank the field trip leaders and Bill Burton for organizing the trip. I enjoyed the excursion!

Labels: coastal plain, field trips, flint, fossils, gsw, rivers, sediment, virginia

Several years ago, (former) NOVA student Theresa R. put together a nice little webpage with rock and mineral photos. My favorite part is a "gray rock quiz" at the end. Check it out and see how well you do!

Labels: igneous, metamorphism, minerals, nova, sediment, websites

Yesterday I noted that there's an interesting pattern to be seen as one crosses DC's Duke Ellington Bridge:

After sharing these photos yesterday, I posed question for you: What's up with the coloration of these exposures? Why are they black on top and white on bottom? It's the same rock (Indiana limestone), so why the difference in color?

The answer has two parts. First, the calcite (calcium carbonate) which comprises the limestone is sitting out there in the air, and is subject to rain and what-not. Some of that rain has sulfuric acid in it, and that dilute sulfuric acid reacts with the calcite, producing a thin layer of gypsum (calcium sulfate). Those itty-bitty crystals of gypsum have bladed habits, and those bladed crystals are really good at trapping soot and dust. So while the calcite underneath isn't as effective as a soot-trap, the thin layer of chemically-altered gypsum on the surface of the blocks rapidly accumulates dark-colored particulate matter.

So that explains the dark color, but what about the lighter-colored lower portions? Is it simply that they aren't exposed to as much acid rain? Perhaps because they're further down on the "outcrop"? Nope... though that's clearly a consideration (note the thin white vertical lines below some of the stars), it wouldn't explain the abrupt transition from dark colored above to light-colored below. So: what gives?

It's here that context plays an important role. This is an urban location, an outcrop in the city. Like many flat surfaces in the city, it's subject to being tagged with graffiti. Periodically, the City sends along a crew to power-wash the bridge's graffitied surfaces. When they do this, they strip away not only the spray-paint, but also the gypsum and its trapped soot! Because graffiti artists can only reach so high, the city only power-washes so high, and the upper portion of the bridge "outcrop" is both unmolested by graffiti and uncleaned by the City. It records a continual accumulation of gypsum and soot, but the lower portion has its proverbial slate cyclically wiped clean!

I'm on a field trip this weekend (I wrote this post on Thursday and set it to publish while I was away), so I don't know who won the prize (a "GEOLOGY ROCKS" bumper sticker!) but as soon as I get back, I'll settle up with the clever winner. In advance, I'll congratulate you: Nice job!

Labels: contest, dc, limestone, minerals, sediment

To walk from the Woodley Park neighborhood of DC to my neighborhood (Adams-Morgan), you have to cross the deep gorge of the Rock Creek Valley. To do this, walk east on Calvert Street over the Duke Ellington Bridge.


My question for you: What's up with the coloration of these exposures? Why are they black on top and white on bottom? It's the same rock (Indiana limestone), so why the difference in color?

First person to post the correct answer in the comments section below gets a "GEOLOGY ROCKS" bumper sticker as a reward. Full answers tomorrow in a separate post...

Labels: contest, dc, limestone, sediment

Labels: appalachians, climate change, global warming, new york, rain, sediment, snow

Yesterday morning, I took a jaunt with a local amateur geologist, Owen P., to go look at some outcrops in streambeds in and adjacent to Silver Spring, Maryland.

Owen wanted me to look at these surfaces, our local unconformity between foliated metamorphic rocks of the Piedmont below, and unconsolidated sediments of the basal Coastal Plain above (cell phone for scale):
The lower rocks are metagraywacke schist of the Sykesville/Laurel Formation (different aspects of the same thing, as far as I am concerned, and not worthy of two different formation names). They were metamorphosed during the Taconian ("Taconic") Orogeny, ~460 million years ago. These rocks were then eroded, and new sediments deposited on top of that eroded surface -- this is an unconformity like the ones I posted about over the past couple of days out in Wyoming and Arizona.

My host thought the layer above the unconformity might be tsunami deposits associated with the Chesapeake Bay bolide impact at 35.5 million years ago. However, that's not what I saw. Instead, the high proportion of angular quartz, and the fact that it was clast-supported rather than matrix supported, suggested to me that the upper layer was a gravel deposit from this very stream. It was good for me to see such a collection of angular clasts atop the unconformity -- on hilltops in DC, I'm used to seeing the Potomac Formation in this position. It's a Cretaceous-aged river deposit, with a real mix of sand, clay, and well-rounded (mainly quartzite) cobbles.

Another look (with cell phone for scale):


After I explained why I didn't buy the tsunamite hypothesis, but encouraged him to keep looking, Owen took me to another cool location, on Northwest Branch (a creek) just outside the Beltway at Burnt Mills Park. Here's a location map:


There, we found an outcrop of migmatitic metagraywacke very reminiscent of the one I visited on Four Mile Run in Arlington, VA, in March of this year. Cutting down, Northwest Branch has exposed a complex of clearly metasedimentary, clearly granitic, and not-so-clearly transitional migmatitic rocks. It's pretty cool, and not only because some of the potholes went all the way through the rock, making wormhole tunnels that a geologist can (and will) crawl through...


I found a couple of cool igneous contacts. Here's a dike of granite cutting through metagraywacke. I like this outcrop because it shows that these things are in fact filled-in cracks, and cracks have a propagating edge, a tip. Most granite dike exposures don't show this fracture edge, but this one does. In spite of the graffiti, it's a good look at that process caught in the act.


And here's a nice example of cross-cutting relationships. Host metagraywacke (notice the pebble-sized clasts of various lithologies in the upper left) is cut by two granite dikes: first a finer-grained, darker-colored one, and then by a coarser-grained, lighter-colored one. Beauty!


Thanks to Owen for showing me these outcrops -- I appreciate the interest and the invitation!

Labels: granite, maryland, metamorphism, migmatite, primary structures, sediment

As a follow-up to yesterday's post on the "Great Unconformity," today I offer a few more shots of unconformities in the Grand Canyon, including (at the end), an angular unconformity...

First, here's a close-up of the contact between the Vishnu Schist and the Tapeats Sandstone:


Slightly blown-out because I was shooting into the sun, and the outcrop was in shadow, but that's why God invented Photoshop:


Same thing, but with the direct light, it's texture (rather than color) that allows you to discern the difference between the two rock units:


The Great Unconformity is visible here, with a boatload of river rafters for scale:


Same thing:


Same thing again...


Okay, here's something different. A waterfall shot. People apparently love waterfalls. Every place I went this summer with a waterfall, there were oodles of folks gathered around, and much flapping of camera shutters. I must be dim, because I kind of don't get it. Water flows downhill... What's the big deal? Anyhow, here the waterfall actually shows us something interesting: note where it emerges from:

That's right -- from the unconformity. Apparently, this is due to the stubborn resistance of the crystalline basement rocks, which are tougher to erode into than the overlying sandstone. The creek cut through the sandstone, but hasn't yet cut through the Vishnu Schist and Zoroaster Granite. However, the Colorado River has, and as the creek flows into the river, there's a difference in the elevation of the two bodies of water. Hence, the waterfall.

I went for a pretty amazing swim in the pool at the base of this fall: the water was cool and bracing, and the wind created by the waterfall was amazingly powerful, actually blowing swimmers downstream! Just the thing after a hot hike.

Lastly, a different aspect of the same unconformity, also seen in the Grand Canyon. Don't look in the foreground, but high up on the distant ridge. This one is an angular unconformity, with sedimentary rocks below the ancient erosional surface as well as above.

In this case, the angular unconformity separates the Grand Canyon Supergroup from the Tapeats. The Tapeats, as we've seen, is Cambrian (~543-488 million years old). The Grand Canyon Supergroup (1.25 billion to 825 million years old) was laid down on the basement rocks first, then faulted and tilted 15 degrees. These tilted blocks were then eroded. On many, the Grand Canyon Supergroup was totally burnished away, re-revealing the underlying basement rocks. In the more down-dropped blocks, however, little protected packages of the Supergroup were preserved. When sea level rose anew in the Cambrian, it deposited the Tapeats Sandstone. In some places, the Tapeats sand was laid down on granite and schist, and in other places on these tilted layers of the Grand Canyon Supergroup. Same erosional surface; different rocks below it in different locations.

Here's a Flash animation showing the various steps it took to put the Grand Canyon together, including the erosion that gave rise to these various unconformities.

Labels: cambrian, grand canyon, primary structures, proterozoic, sediment

Sorry for the long delay in posting here. Turns out they don't have Wi-Fi at Phantom Ranch.

After my time in Zion (did Angels Landing and a few other small hikes while there), I scooted down to Las Vegas, Nevada, to pick up my father and two brothers. They had flown in there, and after one day were already tired of the city. I was ready to leave five minutes after I got there, which is always how I feel about Vegas. Somehow, circumstances keep conspiring to bring me back there, though...

We drove out of the Basin & Range and up onto the Colorado Plateau, and spent the night at Cliff Dwellers, a lodge near Marble Canyon. I was really impressed with their food and drink. We had an amazing meal, washed down with several pitchers of Newcastle Brown Ale! In the morning, we gathered up our gear and put onto the river. Our trip consisted of two rafts outfitted with side tubes and motors and guides. One raft was entirely made up of a family from Charlotte, North Carolina, including the glass artist Wayland Cato, III. The Bentley's raft was augmented by a family from Littleton, Colorado, two oil men from Oklahoma, and a couple of veteran river rafters from northern California. It was a motley crew, but we started having fun immediately.

We launched at Lees Ferry, in the Kaibab Limestone, and then descended in both elevation and geologic time. At our first lunch stop, in the Coconino Formation, I was astonished at several synapsid reptile trackways protruding from the underside of the paleo-dune slipfaces overhead. I took some photos, but because of the aforementioned software issue, I won't be able to share them until I get back to DC in August. As the first couple of days went by, we just went deeper and deeper into the Paleozoic stratigraphy of the Colorado Plateau. Of all the formations, my favorite was the Bright Angel Shale, which has many beautiful colors in thin layers throughout (not to mention oodles of trace fossils). I was particularly pleased to play frisbee in a "cave" in the Redwall Limestone, a place that I have shown photographs of to my students, but never actually seen before. It's a HUGE cliff of the Redwall, and then this seemingly small cave etched into its base (and filled with sand), but the cave could easily swallow my building at NOVA: it's big!

At some point, we crossed a major fault, and were instantly dropped down about a billion years in geologic time. Once we got into the Grand Canyon Supergroup and the metamorphic and igneous basement rocks, my geologic interest really went wah-wah. The Vishnu Schist and Zoroaster Granite make a stunning contrast: really beautiful pink cutting across dark grey. I introduced my raft-mates to the idea of the Mazatzal Orogeny, and we discussed how boudinage forms. There were faults and folds galore: structural paradise. I loved it.

Did I mention the rapids? There were rapids. The water was COLD, thanks to Glen Canyon Dam(n). But the sun was hot, and we dried out quickly. Meals were gourmet, though the campsites were spartan (you had to poop in a box that got packed onto the raft each morning: leave no trace!). We slept out under the stars every night, sometimes dealing with blowing sand.

We took several hikes up side canyons to see waterfalls and go swimming. Several of these were good and physically challenging, which is what I wanted. I enjoyed swimming and playing "three-dimensional frisbee" in Havasu Creek, and doing cannonball jumps in the weird blue of the Little Colorado River.

The final day on the river, we came to the western section of the Canyon where recent lava flows (basalt) have cascaded over the rim and down into the canyon. This is famous for producing one of the toughest rapids in the whole Grand Canyon: Lava Falls. But it was awesome to float by and see umpteen gazillion columnar joints, and whole feeder canyons plugged up by basalt. Pretty cool!

Our final morning, we were helicoptered out of the Canyon to a ranch on the rim. This was my first time in a helicopter, and it was giddy and amazing. I want to fly! From the ranch, we transferred to small fixed-wing planes, and I said goodbye to my family. They went back to Vegas, and I flew back to Cliff Dwellers, where my Prius (and a shower!) awaited.

Labels: fossils, geology, grand canyon, minerals, primary structures, rivers, sediment, travel, volcano

Cleaning up my hard drive today, before switching over to the laptop for my summer travels. Thought I would share a few annotated photos from my "Geology of Glacier National Park and surrounding areas" class that I took last summer.

Here's Chief Mountain:


On the trail to Firebrand Pass, here's the contact between the Altyn Formation (lowest of the Belt Supergroup exposed at Glacier) and the overlying Appekunny Formation:


The Purcell Sill is a readily recognizable feature high on the glacially-carved walls of Glacier National Park. This shot is from the trail on the way up to Grinnell Glacier:


Here's a shot from Sun River Canyon, showing one of the many imbricate thrust faults there, with some glacial till thrown in as a bonus feature:


Just outside of Sun River Canyon, we saw some nice recumbent drag folds on some thrust faults in the Cretaceous rocks:


This one was from early in the trip, on the road from Helena up north towards Glacier. Specifically, we stopped in Little Prickly Pear Canyon, near Wolf Creek, and saw these chevron folds in the Cretaceous rocks there:


Along those same lines (folded Cretaceous strata), here's a gorgeous fold just outside the park's boundary, on the road leading north from Two Medicine towards Many Glacier:


No annotations on this one, but I wanted to share it anyhow: a blind thrust / drag fold complex, in the Grinnell Formation (exposed on the trail up to Grinnell Glacier):


Lastly, some snow photos. I took this shot on my way up the trail to Grinnell Glacier, because the holes in the snow reminded me of the scary mask face from the Scream movies. But then on the way down, I realized I had the opportunity to document how much snowmelt occurs in six hours of Glacier NP summer weather. Hence, the bottom "after" shot:


That's it for today... Enjoy!

Labels: field trips, montana, msse, plutons, proterozoic, sediment, structure

In December of 2005, I went out to The Sea Ranch, California, for Christmas. (The Sea Ranch is one of those towns that is officially called "The" something, kinda like The Plains, Virginia. Sorta weird, but there it is.) I want to share an experience I had there, because it gave me an important perspective on my own 'native' geology back in the mid-Atlantic region. It was a significant moment of understanding for me. Let me walk you through it...

The following collection of images are what I saw walking a mere 1 mile up and down the coast from the house where we were staying. I hope you will be struck by the incredible diversity of rock types seen here (as I was):

Conglomerate:




Siltstone and shale interbedded (vertical bedding):


Siltstone and shale interbedded (anticline):


Siltstone and shale interbedded (syncline):


Mudchip conglomerate (mud chips are "rip-up" clasts due to scouring of a muddy location by a sudden intense current, which carries much larger particles like the sand that now surrounds the darker, finer-grained mud chips):


Quartz-rich sandstone:


Graywacke (showing mouthwateringly beautiful graded bedding):


A zoomed-out shot of that graded bed:


Various sedimentary layers (sandstone, silstones, shale partings):


And a close-up of a few small faults that cut through them:


And it's not just sedimentary rocks. Here's some greenstone (metamorphosed basalt). Note the cluster of amygdules (infilled vesicles) in the center:


The greenstone is green due to a lot of chlorite, but it also shows some nice epidote:




Looking north up the coast from our rental house, you could see greenstone and conglomerate intermingled on the 10m-scale:


This is in the small cove directly in front of our rental. There are three different rock units seen here (greenstone, conglomerate, clayey sand), all indicating different things. Note the big clast of greenstone "hovering" in the clayey sand part:




So after taking a walk along the lovely coast there, and seeing all this stuff, I thought "Wow."

The tremendous diversity of rock types along this section of the Sonoma County coast was due to tectonic shuffling of rock types at a subduction zone. In the Mesozoic, this part of California was at a trench where the Farallon Plate was being subducted to the east underneath North America. Melting at depth produced magma, which resulted in the Sierra Nevada continental volcanic arc (excellently reviewed by Geotripper in his "Under the Volcano" series examining the Sierras). But at the trench itself, all the sediments at the edge of North America were being compressed and squeezed and mixed up with the sediments being scraped off the subducted oceanic slab. Some knobs and bumps of basalt even got scraped off the Farallon Plate and added into this jumbled mess. Altogether, this big pile of debris from the convergent boundary is referred to as an accretionary wedge. "Accretionary" because it got accreted, or added, onto the western edge of North America. "Wedge" because that's its overall shape in cross-section.

When subduction ceased (due to the subduction of the East Pacific Rise), the Farallon Plate was gone at this latitude, and the Pacific Plate and the North American Plate were now in direct contact for the first time. As time went by, the accretionary wedge reacted to now longer being dragged downward, and it began to isostatically rebound. It bobbed upward, and brought its 'melange' (French for mixture) to the surface. The uplifted accretionary wedge is the California Coast Ranges, a fantastic place for varied geology mainly because of the tectonic "shuffling" that happened here during the Mesozoic.

So, I mentioned that seeing all this diversity in so short a hike really impressed me. But the insight it gave me is that the same thing happened on the east coast. Where I live and work, in DC and Virginia, an accretionary wedge developed during the early Paleozoic, just like in California, with the exception that ours got subsequently squeezed and metamorphosed in a series of mountain-building events. It's a bit more difficult to recognize, partially due to that metamorphism and partially due to all the @#$%ing vegetation obscuring the underlying bedrock. But it's there: we have metagraywacke, with relict graded beds, metabasalt, quartzite, schist ("meta-shale") and metaconglomerate: it's everything I saw in California with a metamorphic overprinting!

"Wow," I thought again.

Here's some shots of DC-area rocks that are analogues for the ones I've already showed you in California:

Metamorphosed mud-chip conglomerate (near Chain Bridge, DC):


Metamorphosed quartz-rich sandstone (the Sugarloaf Mountain quartzite, MD):




Metagraywacke showing metamorphic chlorite, garnet, and pyrite (both from DC):




Graded bed preserved in metagraywacke (Billy Goat Trail, MD):


Metabasalt (amphibolite, again from the Billy Goat Trail, MD):


Metaconglomerate (Klingle Road, DC):




The experience comparing the two coasts greatly enriched my understanding of tectonics and subduction, and gave me perspective on DC's geologic history. Two different accretionary wedges, two coasts, two eras... but one underlying process. That's what really hit home. Geology repeats itself. It gave me a renewed interest in my local geology. Everyone always hears about what great geology California has (and it does), but doggone it, DC pulled that same trick millions of years earlier, and experienced a series of orogenies immediately afterwards (which California can't claim!).

If it's true that "the best geologist is the one who has seen the most geology," then I became a better geologist that day on the Sonoma coast.

PS - I think it's funny to note that I didn't put a sense of scale in any of the California pictures, but that most of the DC area pictures do have one. I think that says something about my development as a geologist and educator too...

Labels: basalt, plate tectonics, primary structures, sediment, structure

Last week, I mentioned some cool conglomerates I saw when NOVA adjunct instructor Chris Khourey and I did some field scouting. The main purpose of that trip was not to focus on the Culpeper Basin's boundary conglomerates, however, but the "Great Valley" of Virginia's Valley and Ridge province. The "Great Valley" is usually called the Shenandoah Valley in Virginia, because the Shenandoah River flows north through it. (Topographically, it continues north into Maryland, but the Shenandoah River isn't found there.) Sitting in the middle of the valley is a mountain range, Massanutten Mountain. And in the middle of Massanutten, there is another valley, the Fort Valley. As you can see below, Massanutten is a fence-like ridge separating the higher Fort Valley from the lower Shenandoah Valley:

In fact, rumor has it that the name "Massanutten" is a native American term for "basket." This describes the overall shape of the mountain/valley quite well. It probably won't surprise you to learn that this valley-in-a-mountain-in-a-valley pattern is due to differential weathering of folded sedimentary layers. In fact, the entire Great Valley is one big downturned fold, a syncline. Actually, it's not a perfectly smooth fold -- there are some wrinkles and minor folds within the overall down-turned structure, so we call it a synclinorium. The oldest rocks are therefore at the eastern and western edges of the Great Valley, and the youngest rocks are at the center of the Massanutten Synclinorium, up in the Fort Valley. It turns out that some of these rock layers are easily eroded, and some are tough. Of particular note is the Massanutten Sandstone, a quartz-rich, well-indurated rock that is responsible for the ridges of Massanutten Mountain. It weathers away more slowly than the shales and carbonates (limestones) above and below it. Here's a cross-section view to show how the subterranean structure influences the surface topography:

The map view up above (using Google Maps' super-cool new terrain feature) and this cross-section also show the difference in landscape texture (and geologic cause) of the Blue Ridge province in the SE corner of the images.

In discussing the geology of the area, I'm going to mix my pictures from Thursday's scouting expedition with photos from Saturday's actual field trip with my Audubon class.

Let's start at the beginning. The first stop was in the Conococheague Formation, a late Cambrian limestone. Our field trip stopped at a nice exposure near Mulberry Run, west of Strasburg, VA. Here's the crew looking close at the outcrop, and trying out their geo-interpretive field skills for the first time:

Albert tests the outcrop with some dilute hydrochloric acid. It fizzes!

Soon, we spot the first of several stromatolites:

There are also some nice spherical grains of calcite called ooids (or ooliths). These form in wave-influenced carbonate banks today, like the Bahamas.

Interpretation of this environment then? Looks like a nice passive margin, far from any major terrigenous inputs (i.e. mud or sand). Warm tropical temperatures leading to the chemical precipitation of lime mud from seawater.

What comes next? On to stop #2, the Tumbling Run section* south of Strasburg, we see a nice long exposure of the New Market, Lincolnshire, and Edinburg Formations, a series of Ordovician limestones, all dipping nicely towards the axis of the synclinorium. (Last semester, one of my Honors students looked at silicified trilobites in the Edinburg Formation.) As you walk downhill (and up-section), you see a change in the limestones. They get darker in color, and they start splitting into thin sheets along clay-rich layers. Uh-oh, we're getting an increasing clastic influence on these sedimentary rocks. They no longer record pristine, Bahamas-type environments. Now the limestone is mixing with shale. Where is all that mud coming from? A hint may be found in several bentonite layers, weathered volcanic ash deposits. There's some volcanoes getting closer to the area, it looks like.

In the late Ordovician, the east coast of North America experienced the first of three episodes of Appalchian mountain-building. Geologists infer that the Taconian Orogeny was caused by the collision of a volcanic island arc (like modern day Indonesia) with the east coast. The Tumbling Run section shows well the increasing clastic influence of the growing Taconian Mountains to the east.

It's also good for some small but interesting tectonic structures. Check out this conjugate pair of en echelon tension gash arrays:

The black nodules you see along bedding in the above image are flint nodules, very characteristic of the Lincolnshire Formation. If you get close to them, you'll find that they exhibit different mechanical properties than the limestone that surrounds them. They are more likely to break (brittle behavior) than flow (ductile behavior):

But let's get back to the stratigraphy, shall we? (It just doesn't do to get distracted by these minor structures!) Our next stop was to look at the Oranda Formation (calcareous shale), indicating heavy clastic influence (but still a bit of carbonate). Then, after a lovely lunch at the Strasburg Emporium, we headed off to the Buzzard Rock Trail, to look at the Martinsburg Formation. The Martinsburg is a nice thick batch of fine sand and mud interpreted as turbidite deposits. Various pieces of the Bouma sequence can be seen throughout the formation, including graded beds, ripple marks, and cross-bedding. This picture conveys these alternating lithologies, representing fluctuating current strength as turbidity currents periodically brought coarser sediment into the deep (low-oxygen, as indicated by the dark color) basin.

Now, keep in mind that all these sedimentary layers later got folded during the final phase of Appalachian mountain-building, the Alleghenian ("Alleghany") Orogeny. At that same time of intense deformation, some of these mud layers began to convert to slate. The outcrop on the Buzzard Rock Trail shows this pretty well, in spite of being covered by lichen, algae, moss, and other horrible rock-obscuring growths:

The sandy layers outcrop as stiff, blocky strata. But look to the right of the quarter: in the muddy layers, a penetrative cleavage has developed, subperpendicular to the compressive stress. Here, let me draw for you what I saw at this outcrop:

The clay minerals in the mud are more susceptible to being alligned by tectonic forces than the grains of sand in the coarser layers. So the shaley intervals exhibit a more pronounced cleavage than do the sandy intervals.

But again, I'm getting distracted by the tectonic overprinting! This trip is supposed to be about stratigraphy, pure and simple. Doggone it! Okay, moral of the Martinsburg: no more carbonate by the late Ordovician. Instead, this sedimentary basin is getting filled with clastic debris shed off the Taconian Mountains** to the east.

Next layer up is the Massanutten Formation: mainly quartz sandstone, but also some quartz pebble conglomerate. We see it by entering the "basket" via a water gap near Waterlick, VA. Driving south (uphill) along Passage Creek, we were soon surrounded by looming cliffs of quartzite. It represents fluvial and beach facies as the depositional basin was filled to the brim. Here's a boulder of the conglomeratic portion:

Here's some nice cross-beds in the sandy portion exposed near Blue Hole, about 4 miles south of Waterlick, VA:

Other Massanutten Formation features include some fossils. Here's some poorly-preserved brachiopod external molds:

And here's some Arthophycus horizontal trace fossils, probably made by polycheate worms:

Okay, I can't resist this tectonic structure: an awesome anticline exposed along the Veatch Gap Trail (eastern part of the synclinorium, where a small anticline in the Massanutten Formation is superimposed on the larger synclinal pattern):

Beyond the Massanutten Formation, we are in the Fort Valley proper, inside the "canoe" shape of the Massanutten Mountain ridge system. Next layer up is some upper Silurian / lower Devonian carbonates, representing a return to passive margin sedimentation after the end of the Taconian Orogeny and the erosional beveling of those ancient mountains. Unfortunately, there are no good places to stop on the narrow Fort Valley Road, so I don't have a picture of them to share. Trust me, though: they're there.

The next good stops are of Devonian shales. There's some nice ones exposed across the road from Elizabeth Furnace. More mud? From whence does it come? We interpret this again as the onset of an orogeny, in this case the Devonian-aged Acadian Orogeny, which dumped a big thick wedge of sediment into the Appalachian Basin. Here's a shot of the Needmore Formation, one of these shales with distinctive trace fossils highlighted by iron oxide:

The overlying Mahantango Formation (Devonian) is a siltstone that bears a decent number of body fossils, like these brachiopods:

Here's something that may be the back of a trilobite (if I'm not imagining the lobe to the left of the central line of knobs), or maybe a crinoid (if the "central" line is all there is):

Here's what appears to be the (vertically-oriented) trace fossil Daedalus, which I learned for the first time this spring in Silurian rocks near Buffalo, New York:

Finally, at the top of the stack, near Seven Fountains, there are exposures of more bentonite, in this case the Tioga Bentontite, a major stratigraphic marker bed throughout the Appalachians. Here's a shot of the bentonite exposure on the Fort Valley Road near Seven Fountains:

Here's Chris looking at the outcrop:

To summarize the Fort Valley portion of the story: after the Taconian Orogeny ends, we get a brief period of tectonic calm and passive margin sedimentation (carbonate), and then a return to orogenically-induced clastic sedimentation (augmented with volcanic eruptions). In the sedimentary sequence of the Massanutten Synclinorium, this records the onset of the Acadian Orogeny. The actual deformation of all these sedimentary horizons into a synclinorium shape was accomplished by the Alleghenian Orogeny: the much bigger mountian-building episode triggered with Africa and North America collided in the latest Paleozoic.

Hope you enjoyed joining us on this trip. Virginia's got some great geology, eh?

* For the Tumbling Run section, I highly recommend this excellent field guide:

Fichter, Lynn S., and Diecchio, Richard J., 1986, "The Taconic sequence in the northern Shenandoah Valley, Virginia." In: Geological Society of American Centennial Field Guide - Southeastern Section, p.73-78.

** Note I don't say "Taconic." The Taconic Mountains are a modern topographic feature in New York. They exhibit Taconian rocks well, and the orogeny is named for them, but the Ordovician Taconian Mountains would have been much bigger and more areally extensive.

Labels: fossils, primary structures, sediment, stratigraphy, structure, teaching, valley and ridge

The Culpeper Basin is a Mesozoic (Triassic/Jurassic) rift valley in northern Virginia.

As Pangea was breaking apart, a series of normal-fault-bound basins stretched open in an NW-SE direction (giving them long axes that run NE-SW). Some of them connected together in a NE-SW direction, and kept spreading further and further open. Through continued seafloor spreading, these became the Atlantic Ocean basin. Some did not keep opening, and essentially filled in with dirt. Those are the ones that are still preserved up on the North American continent today, including the Culpeper Basin. These basins vary in size, but they run up and down the coast of eastern North America, from Newfoundland down at least into the Carolinas (presumably there are more buried beneath Coastal Plain layers even further south than that). Collectively, these basins are referred to as the Newark Supergroup. They are characterized by immature sedimentary rocks and mafic igneous rocks.

Here's an E-W cross section through the Culpeper Basin, by Chuck Bailey at W&M:

Structurally, then, the basin is a graben, bounded east and west by normal faults.

The igneous rocks in the Culpeper Basin are mostly diabase, but there are some basalt flows too. The sedimentary rocks are a motley mix, including arkose, red siltstones, and lake deposits including siltstones and anoxic black shales. Along the eastern and western boundary faults, we also find coarser sediments that have been lithified into conglomerates. Sediments flowed into the basin from source areas both to the east and west, so you would expect the conglomerates along each edge to look a little different. Indeed, they do!

A modern analogue for the Culpeper Basin is the Afar Triangle region of northeastern Africa (Ethiopia, Eritrea, and Djibouti). Note the sedimentary influx from both the east and the west. Note the lakes, and note the mafic extrusions:

Back to the Old Dominion: I've mentioned the Culpeper Basin's eastern boundary fault before, back in March, when I posted this picture of the conglomerate that outcrops in Clifton, Virgina. It is characterized by lots of clasts of highly-foliated metamorphic rocks (derived from the neighboring Piedmont).

...But I haven't talked about the western boundary fault much. And since I visited it yesterday, today's the day to talk about it.

One of these western Culpeper Basin conglomerates is kind of famous. It's the Leesburg Conglomerate, and it outcrops near Leesburg. It's mostly limestone cobbles and gravel, with some quartzite, too, set in a red matrix. It's a beautiful rock. Here's a couple of field photos taken on Route 15, a mile or two north of Leesburg proper:

The Leesburg Conglomerate was used in the awesome columns in the U.S. Capitol's Hall of Statuary (topped by the much less interesting Carrara Marble of Italy).

Yesterday, NOVA adjunct geology instructor Chris Khourey headed out to Thoroughfare Gap (see map below) to check on a couple of field sites. Thoroughfare Gap is a water gap in the eastern limb of the Blue Ridge Anticlinorium, and it's also the western boundary of the Culpeper Basin. Both Interstate 66 and Route 55 pass through this striking landscape feature:

Note how different this looks as compared to the Leesburg Conglomerate. One thing that immediately jumps out at you when you see an outcrop of it is the large proportion of the cobbles that are pieces of the Catoctin Formation basalt (see more photos of the Catoctin in Monday's post on rocks of Shenandoah National Park). Here's a couple of close-up shots of such cobbles, bearing distinctive amygdules (filled-in vesicles):

But there's also plenty of limestone cobbles and gravel in there too, as this photo shows:

As with the Leesburg Conglomerate, the Waterfall Conglomerate's limestone inclusions are likely coming from the Cambrian & Ordovician carbonates exposed today in the Shenandoah Valley and other valleys of the Valley and Ridge province. More on that later this weekend, when I'll post some shots from the Massanutten Synclinorium.

Labels: basalt, culpeper basin, limestone, mesozoic, sediment, virginia

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

Later this month, I'm leading a tour for "Walkingtown, DC" a twice-annual event sponsored by Cultural Tourism DC, a nonprofit organization. My tour is called "History Before History: the geologic saga of Washington, DC." I'll be leading the tour on both Saturday, April 26, and Sunday, April 27, from 1-4pm. If you're in the area, consider coming along. We'll be discussing the deposition of sediments in the Iapetus Ocean, generation of an accretionary wedge, the Taconian Orogeny, the Rock Creek Shear Zone, emplacement of the Georgetown Intrusive suite, and finally the erosion of the young Appalachian mountains and the deposition of dinosaur-fossil-bearing river gravels atop the unconformity: the Potomac Group. As a bonus, we'll even visit a thrust fault which ruptures the unconformity at the intersection of Adams Mill Road and Clydesdale Place, NW. It's a nice little jaunt through prehistory. However, this hike was extremely popular last year: we had ~300 people show up! So I've asked Cultural Tourism DC to institute a reservation system this time around: I'm limiting participation to 30 people per day. Act now to reserve your place by calling or e-mailing Cultural Tourism DC.

Here's two pictures of the mad crowds last spring. I get the heebie-jeebies just thinking about it:

Labels: action, dc, dinosaurs, geology, granite, metamorphism, sediment, unconformities

A few more photos from the Buffalo trip last week... All of these were taken by Victoria, my Honors student.

Here's some malachite in the sandstone of the Whirlpool Formation: the field trip leader suggested this was due to brine flow through these rocks during the Alleghanian ("Alleghenian") Orogeny:

Herringbone structure ("reverse cross bedding") in the Gasport Formation, overlying the DeCew Formation, which appears flat-lying and calm in this photo, but just below this shows disrupted bedding suggestive of seismic activity:

I showcase a sample too big to lug back to the van (ripple marks):

Watch where you stand! In the Niagara Gorge, we see some evidence that the Gorge is widening through mass wasting processes. Here's a small gap / scarp opening up as a block of rock to the right slumps down into the Gorge:

Lastly, on the trip home, we had an obligatory getting-stuck-in-the-mud moment:

Eventually, we got unstuck and headed back down the road!

Labels: geologists, mass wasting, meetings, nova, primary structures, sediment, teaching, travel

On Wednesday, two students and I participated in an excellent field trip examining the sequence stratigraphy of the Niagara region. We saw uppermost Ordovician rocks (the Queenston Formation) and then a dozen Silurian formations, some of them only 3 meters thick, stacked atop on another in a stereotypical layer cake fashion.

The trip was led by Carl Brett, who did a great job. I wanted to showcase here a few of the photos I took that day. Here's Carl showing us Arthophycus trace fossils (interpreted to be the burrows of polycheate worms):

At Outwater Park, we found fossil stromatoporoid reefs. Stromatoporoids were primitive, layered sponges. These ones show glacial striations across their surface, a result of the outcrop being scraped by glaciers during the recent Ice Ages:

At another stop (on Lockport Junction Road) , there was a Leperditia ostracode-rich layer. Ostracodes are small arthopods, kind of like krill, but with bean-shaped shells.

At Pekin Hill, we looked at the Goat Island Formation, which showed ripped-up stromatoporoids deposited within it.

Here's a stromatoporoid that tumbled loose from the slope. I'm bringing this one back to Annandale to use as a teaching specimen. Note the upward-bulging dome of the stromatoporoid's internal layers.

One of our most amazing stops was hiking up into the Niagara Gorge. This is at the downstream end of the Niagara Escarpment, where the Falls once were. The adjacent town is Lewiston.

Here's Laura and Victoria in the Gorge, overlooking the Niagara River:

Now for some fossils from the Rochester Shale and other units exposed in the Gorge. Carl brought these out to show us what we might find. Here's a mouthwatering slab showing Dalmanites trilobites:

And a golf-ball sized cystoid (relative of crinoids, blastoids, and other echinoderms):

He had some Lingula dwelling traces, too. Lingula is a common inarticulate brachiopod that dwells / dwelled in vertical burrows beneath the seafloor mud:

Here's a shot of a crinoidal grainstone. This limestone is almost entirely made up of "sand" generated by broken up crinoid skeletons:

Some spectacular trace fossils (ichno-genus unknown) on a slab that was catching the rays of the sun just right:

And a close-up of the same slab:

And lastly, a nice slab showing tool marks:

It was really a great trip -- perfect weather, fascinating rocks, good company, and I felt nice and tired at the end of the day.

Labels: fossils, meetings, new york, nova, primary structures, sediment, silurian

On Tuesday afternoon, four students and I drove from Annandale, VA, up to Buffalo, NY, for the NE section meeting of the Geological Society of America. On the way, we crossed the Pennsylvanian Appalachians, and pulled over to examine some beautiful redbed exposures on the Pennsylvania Turnpike. I think these are in the Hampshire Formation, but I could easily be wrong about that, considering I've never been here before. Here's a few photos. First, some beautifully rhythmic alternations between sand and mud, now preserved as alternating layers of sandstone and mudstone:

Then, some nice "ball and pillow" structures, as heavy sand sank downward into squishy mud. In places, the mud skooshes upward in "flames":

And lastly and most amazingly (for me), some awesome exposures of flute casts. These are erosional scours into a layer of sediment by a current, which then fills in the scours (called "flutes") with sand, making these flute casts on the underside of the overlying layer of sand:

The flutes "point" upstream, and open up (and shallow) in the downstream direction. More later!

Labels: appalachians, meetings, nova, pennsylvania, primary structures, sediment, teaching

On one of my field trips last week, I collected this cobble of sandstone (penny for scale):

And here's the other side:

There's a delicate but telling geopetal indicator here in this sandstone: it shows us which way these sandstone layers were oriented in space when they were deposited as loose sand. Geopetal indicators give us "paleo-up," sometimes called the "younging direction." Classic examples include graded bedding, cross-bedding, and mudcracks. Here, it's a bit more subtle: small erosional "divots" in the layers of sand. These "divots" may be caused by something pushing down into the sand (the trace of an organism's trail), or may be caused by small amounts of scouring erosion. We only get to see them in two dimensions, so it's unknown whether they are simple pinpoints in three dimensions, or linear features -- perhaps even branching linear features. Reviewing the cobble's many layers, I've found three types of "divots":

Type 1 is a simple deflection of the the dark layers. It is more likely that the layer is deflected downward, but there is no guarantee: it could be a little lump of sand poking up from the bottom, too. In other words, Type 1 is not a completely compelling clue for paleo-up. Type 2 is more convincing as a geopetal indicator: here a lower layer or two has been actively scoured, and then an upper layer is draped over the scoured-out hole. Type 3 can also be seen though, and it's a weird one: I'm having a hard time coming up with a reason why two successive beds would both have a "divot" in the same location. Is this a squishing downward effect? For instance, were I to go stand on my bed, my weight would push downward on my comforter, but also the sheets underneath. They would both deflect from the bed's horizontal surface in the same downward direction. (Would this be a "duvet divot?")

See if you can find examples of all three in the photos above.

Labels: coastal plain, primary structures, sediment

Last week on one of the many field excursions, I found a nice cobble of amygdular basalt. Amygdules are vesicles (bubbles in degassing lava that didn't get the chance to pop before the lava solidified into igneous rock) that have been filled in with mineral deposits. In the mid-Atlantic, most amygdules are found in the Neoproterozoic lava flows of the Catoctin Formation, from which my cobble was presumably derived. The amygdules are typically filled in with zeolites, quartz, and jasper. This one doesn't show any jasper, but the basalt still appears to be basalt, too -- whereas the Catoctin typically is metamorphosed to greenstone / greenschist. I've noticed an association between jaspery amygdules and epidote formation in the metaingeous rock.

As with Skolithos-bearing Antietam Formation quartzite cobbles, clasts of the Catoctin deposited in the river gravels atop the Piedmont/Coastal Plain unconformity indicate a Blue Ridge provenance for the cobbles, and therefore a eastward-flowing river to deposit them 100 million years ago.

I took the cobble back to the lab and sliced it open on the rock saw. The brown circle in the background is a penny for scale.

Here's what the sawn surfaces look like after I sanded them down a bit and then scanned them:

Right purty, ain't it?

Labels: basalt, blue ridge, coastal plain, primary structures, sediment, virginia

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

Picking up with my series of posts introducing the work my Honors students are doing this semester: today we'll take a look at Spencer's project, which involves field work on a bedrock terrace (strath) of the Potomac River near Chain Bridge (which can be seen in the background of this photo). As before, ignore the datestamp in the lower-right of the photo. These pictures were taken last week, not in 2004.

This is in the westernmost corner of DC's "diamond" shape. The bridge leads across the river into Arlington, Virginia. As you can see, there's a lot of rock exposure here -- the sort of thing we go crazy over here in the east. As noted before, this is metagraywacke (sometimes metamorphosed to schist, sometimes to gneiss, sometimes just strongly foliated, and sometimes so lightly metamorphosed / deformed that it even preserves original sedimentary structures like graded bedding. The interesting thing about the Chain Bridge locality is that in amongst the metagraywacke are big chunks of other rock types. I'll refer to these as "clasts." Some geologists have interpreted them as sedimentary deposits; others as "olistoliths" (tectonically emplaced chunks in an accretionary wedge complex). Spencer is in charge of documenting the variety of these clasts, in hopes that it may tell us something about their ultimate source. Here's a big elongate clast of gneiss:

We had a good little field routine going: Victoria and Jason would go scout out clasts, and then mark their location with a chalk arrow. Then Spencer would document each clast's lithology and characteristics (e.g. foliation at an angle to regional foliation) and then photograph it. Once he'd photograph it, he "checked it off" with chalk. All of this chalk graffitti gets washed away with the next big rainstorm.

Some of the clasts are no longer in their original condition. The one below, for instance, bears a multitude of garnets, metamorphic minerals which reflect how the clast's original composition reacted to the higher temperatures and pressures of Appalachian mountain-building.

Another thing we saw a lot of in the Chain Bridge locality is erosional features related to the incision of the Potomac River into bedrock. Here's Jason showing off a pothole that drilled all the way through one outcrop:

Next time, we'll take a look at Jason's project.

Labels: dc, field trips, geology, nova, sediment, teaching

Here's a view in the creek bed of Spencer and Victoria looking for kink band outcrops. (Ignore the date stamp in the lower right: it is not accurate.)

A nice kink band. Width of photograph is ~25 cm.

Victoria takes the strike of the metagraywacke's foliation:

Here's a Z-fold in the foliation -- more of a kink "knot" than a kink band. The kinematic sense of motion in this photo is top-to-the-right (right-lateral):

Here, Jason and Spencer measure the orientation of a kink band:

A nice little outcrop of crenulation cleavage, showing porphyroblasts of chlorite (green/blue) and garnet (red/brown). The pencil is parallel to crenulation "wrinkles".

Next time, we'll take a look at the projects that Spencer and Jason are working on.

Labels: field trips, geology, nova, sediment, structure, teaching

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

BLDGBLOG has put up a post the week before last about an idea to amputate the Mississippi River's outer "bird's-foot" delta and create a dumping of sediment along the periphery of the main Mississippi Delta. A quick review of the populated places in Google Maps suggests that not too many towns downstream would be cut off. The potential storm protection benefits for a wider wetlands storm buffer along the delta would outweigh the loss of even a medium-sized population center, it seems to me. I'm all for ditching New Orleans to the elements, but if we're going to hang on to it, then this would be an inexpensive way to protect that investment.

Labels: mississippi river, rivers, sediment