Returning now to some of the stuff I saw when I was out in Bishop, California for the GSA Field Forum I attended in September. One of the cool little spots we visited was "the flipping fault," a location on the Volcanic Tableland north of Bishop where an east-dipping fault scarp dies out and a west-dipping fault scarp starts up. Check it out:


Here, try one with annotations:


Here's a Google Map of the location, as seen from the perspective of a passing turkey vulture:

Notice how the road, Casa Diablo Road, goes right through the notch where the two meet. Complicating the picture a wee bit is a Pleistocene drainage channel which uses the same route between the two scarps (and diverges from the road in the lower-left).

Another view, further back and higher up:


And of course we must annotate that one too:


Recall that these are normal faults busting through the Bishop Tuff's upper welded layer, the "Ig2." In the annotations, I've sketched in the approximate position of the "hanging-wall cutoff" (lower boundary of each scarp) and the "foot-wall cutoff" (upper boundary of each scarp).

There are roughly equal numbers of east-dipping and west-dipping faults on the Volcanic Tableland. Originally, some creative structural geologists wanted to interpret this feature as an overall "propeller" shaped fracture: a so-called "flipping" fault (as in, it's one single fault that flips its dip direction in the middle). However, this was not the interpretation of our workshop leaders, who suggested that it was simply two faults that started independently and then propagated towards one another.

Taking a fresh look at these images now, almost a month after I visited the outcrop, I find that I agree with them. One thing that seems obvious to me now is how the east-dipping fault truncates on the face of the west-dipping fault scarp. My annotations reflect this interpretation. What do you think?

Labels: california, conferences, meetings, structure

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

I spent last week in the Owens Valley of California, attending a GSA field forum on the structure and neotectonics of the Owens Valley and the Volcanic Tableland north of Bishop. It was really cool, and I learned a lot. I'll be sharing images and ideas on the blog in days and weeks to come.

Twenty-four people attended, plus the three conveners: David Ferrill, Alan Morris, and Nancye Dawers. Here's the team getting an orientation on Monday morning, looking west towards the Sierra Nevada. Note the time-honored geology field tradition of using magnets to hold posters and maps to the side of the van:


David discusses the tectonics and geomorphology of the "Eastern California Shear Zone," a transtensional zone between the Sierra Nevada and the typical Basin & Range. This area ranges tremendously in elevation: from Mount Whitney in the Sierras (14,494' elevation) to Badwater in Death Valley (-282'). The lurid colors on this elevation map show that:


A Landsat photo comes out at the next stop, looking northeast towards the Volcanic Tableland:


And yet another image, this one a beautiful side-scanning radar image of the Volcanic Tableland, which David and Alan (here assisted by Wes Hildreth) pulled out at a stop overlooking the Owens River Gorge (a canyon which dissects the Volcanic Tableland):


This image shows east-dipping normal faults as white stripes, and west-dipping normal faults as dark stripes:


This is the main reason we're all here: the young welded ashflow deposits of the Bishop Tuff (760 ka) record brittle strain as a result of the past 760,000 years of extensional and strike-slip tectonics. Due to the low rainfall and this excellent marker unit, you can really get a sense of how such systems operate. The faults are expressed topographically: a lovely marriage of structure and geomorphology.

Our first overview of the Volcanic Tableland, looking northeast from the Sierra Nevada over the fractured Bishop Tuff, towards the White Mountains in the distance:


Here's a Google Map of the Volcanic Tableland, showing the orange upper ignimbrite layer of the Bishop Tuff, and the north-south trending faults which rupture it. Green stripes are the Round Valley (southwest), Owens River (southern border, trending east-west), and Fish Slough (far east, trending north-south):


Here's another Google Marp, zooming in on some of the faults. Conveniently, Google opted for morning sunlight in this image, so it's "color-coded" the same way as the side-scanning radar image I showed you earlier: east-dipping fault scarps are light-colored, while west-dipping fault scarps are in shadow:

(Another very cool thing about this image is the northwest-southeast trending Pleistocene drainage channel -- more on that later!)

Many of the faults in the Volcanic Tableland are arranged in en echelon arrays, reflecting a broader zone of deformation:


In en echelon arrays of these normal faults, we find the individual fault segments are linked up with intermediary flexures of the the ignimbrite layer, called "relay ramps." This was a new term to me, but once I learned it, I saw them everywhere. Here's one atop the Volcanic Tableland:

(It's the shallowly-sloped bit in the middle, dipping towards us, bounded by two west-dipping fault scarps: the intensely-shadowed areas.)

Here, in Fish Slough, we see a couple of 'relict' relay ramps that have gotten cut off as the small fault segments linked up into a larger through-going fault. Pretty cool!


The group descends a relay ramp on their way back from a field excursion to the vehicles:


Annotated version of the photo above:


Relay ramps occur on many scales. This 'scaling' of fault systems (and deformation in general) was a theme at the field forum. Here's a Google Terrain Map of the Owens Valley area. Notice how, just west of Bishop, the Sierra Nevada front jumps to the west? That's a much larger relay ramp, the Coyote Warp Relay Ramp:


Looking west from the first stop at the Sierras, with the Coyote Warp Relay Ramp descending from upper left towards lower right:


Annotated version of the photo above:

That's a little taste to get you started. More geology from the Owens Valley in future posts...

Labels: california, faults, structure, volcano

On our hike to Grinnell Glacier this past July in Glacier National Park, I found lots of cool cobbles of float, mainly of the Mesoproterozoic metasedimentary rocks that make up the bulk of the park: the Belt Supergroup. One of these formations, the Helena Formation, is intruded by a diorite sill known as the Purcell Sill. It's a prominent rock unit showing up as a black stripe within the lighter-colored Helena Formation, exposed high on the glaciated walls throughout the park. Occasionally, you'll find pieces of it as float, and I noticed that the higher we climbed up, the more of it we saw. Here's one of my favorites among these pieces of the Purcell Sill:


This cobble shows beautiful slickenlines, small gouges into the rock as neighboring rock ground across its surface, along a fault. These physical gouges are decorated with a chemical accoutrement: the metamorphic* mineral epidote, which is a gorgeous grassy green.

Labels: faults, igneous, metamorphism, minerals, montana, national parks, proterozoic, structure

Took all these images of joint surfaces this summer in Glacier National Park on my Rockies trip. Enjoy!

Appekunny Formation, with two concentric ribs:


Grinnell Formation, showing well-developed hackle fringe (rough area at bottom):


In the lovely fine-grained limestones of the Helena Formation:

Labels: montana, national parks, proterozoic, structure, weathering

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



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


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


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

Labels: maryland, metamorphism, piedmont, structure

(Parts 1, 2, 3, 4, & 5 of this series...)

As we were climbing up a steep snowfield, we saw something that made us rush up to the top:


Interpretive sketch:

At first, we thought this was a big isoclinal synform that was cross-cut by a ptygmatically*-folded granite dike, but closer inspection at the "axis" of the "fold" revealed that it was instead just the trailing edge of a big boudin. It pinched down and then swelled again in the downward direction, hidden in this photo by the snowpack. Not quite as cool... but still pretty cool. And I can never say no to ptygmatic* folding, regardless of the setting.

This is also kind of cool:

What you're looking at here is a gneiss, with alternating layers of coarse-grained mafic and felsic minerals. The view of the photo is orthogonal to the plane of foliation, but the boulder has been weathered so that in some places the uppermost mafic layers has been worn away. There's one spot where you can "see through" the mafic layer into the underlying felsic layer (upper right) and another spot where there's a little isolated scrap of the mafic layer where the surrounding material has been weathered away. This reminded me of a larger-scale phenomenon where the same thing happens to thrust sheets: an erosional hole through a thrust sheet into the rock beneath is a tectonic "window" or "fenster" (German for window). An erosional remnant of a thrust sheet is a "klippe." The Grandfather Mountain Window in North Carolina is an example of a fenster. Chief Mountain in Glacier National Park, Montana, is an example of a klippe. So this little boulder gives us a nice physical analogue for regional-scale tectonic/erosional features.

Ahh... what cool stuff to see and think about. But the sun was setting, and we had to head back to camp and the rest of our team... Tomorrow: the story of the long hike home.

________________________________
* Really, more of a "cuspate-lobate" fold, without the parallel limbs that make for a truely ptygmatic fold.

Labels: analogies, art, montana, national parks, north carolina, nova, structure, travel, wyoming

(Parts 1, 2, 3 & 4 of this series...)

Today we'll look at some of the structural geology photos I took in Hanging Canyon, Teton National Park, Wyoming. These are all rocks of the Archean-aged Wyoming Terrane (or "Wyoming Craton"), one of the most ancient pieces of crust that make up the quilt-like North American continent. They include both metamorphic and igneous rocks that have been suffered enjoyed being deformed by tectonic processes.

Labels: archean, igneous, metamorphism, national parks, nova, structure, travel, wyoming

Parts 1, 2, & 3 of this series are at these links.

Today and tomorrow, I'll share some of the gorgeous Archean rocks that are exposed in Hanging Canyon, Grand Teton National Park, Wyoming. Today: the igneous stuff. Tomorrow: the structural stuff.

There were many pegmatite dikes that we saw along the hike. Here's a lovely one cutting across the metamorphic host rock:


A close up of some big muscovite "books" in the pegmatite:


A couple of parallel pegmatite dikes cutting across granite:


Here's the largest single feldspar crystal I've ever seen in the wild. The crystal starts to the left of my boot and continues for over a foot to the left of that. Its color varies between bluish gray and whitish. Where the left-most and most prominent blue stripe is, that's the edge of this monster megacryst:


Huh... Only four "igneous" photos... I guess I'll make up for that with tomorrow's structural geology post about Hanging Canyon... I have about forty photos of folds and boudins and what-not to share...

Labels: archean, granite, igneous, minerals, national parks, nova, structure, travel, wyoming

One of the highlights of this past summer's Northern Rockies field course was an afternoon set aside as a "choose your own adventure" hike in Teton National Park. Some students opted for Cascade Canyon; others climbed Blacktail Butte. Four of us wanted something really challenging, so we chose Hanging Canyon at the recommendation of my friend Amy Manhart, who lives in Jackson and knows the Tetons like the back of her hand.

We took a ferry across Jenny Lake along with the Cascade Canyon Crew, and then started climbing up. A thunderstorm rolled up Jackson Hole, with much ominous booming and lightning, but we didn't get hit with the storm directly. The climb was very steep, but we entertained ourselves along the way with a geological conundrum: We discussed how best to interpret a hypothetical piece of float that is half granite and half diorite: Is it more parsimonious to guess that the granite represents an intrusion or an inclusion? The implications for the relative dates of the two units are huge: if the diorite is an intrusion, it's younger than the granite. If the diorite is a xenolith (an inclusion) within the granite, then it's older than the granite. Consider the possibilities:



Ultimately, there's no answer to this question without finding an outcrop of the rock in situ, which is why it's entertaining to consider when you're slogging up a 2000 foot hillside. My co-instructor Pete Berquist and I upped the ante by each doggedly defending one of the two indefensible interpretations and sticking to it for the sake of argument. Pete was the xenolith man, whereas I came down fully on the side of the dikes. Our students Joel and Ken were "fortunate" enough to listen to Pete and I bicker about the relative merits of our favored interpretations. Rest breaks came whenever either Pete or I found a boulder along the hillside that showed evidence to support our position. We would stop to consider it, catch our breath, and the resume the uphill climb and the argument. The bad weather passed and the day was beautiful. We were unencumbered by the need to reach a conclusion or acknowledge the obvious: the best interpretation is that such half-&-half clasts "cannot be interpreted."

Here's Pete posing with an obvious dike (I forced him! Ha!):


Here's me posing with an obvious xenolith (Oh well, fair's fair...):


We had a similar ongoing "argument" on the trip about the merits of "Tertiary" versus "Paleogene." I think it keeps students amused to see their professors going back and forth over geologic ideas -- surely if these fellows spend this much energy and thought discussing some geologic question, it must be valid and important... ...right?

More on the Hanging Canyon hike tomorrow...

Labels: archean, geologic time, igneous, national parks, nova, structure, travel, wyoming, xenoliths

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

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

Another in the photo/sketch series...

This is an outcrop I saw in the Teton range (Wyoming) this summer. It's a nice example of relative dating, I think...

Labels: art, geologic time, structure, wyoming

(deep voice) "Previously, on NOVA Geoblog..."

...We were looking at the structure of the Bridger Range in Montana, near Bozeman. We discussed the concept of Pumpelly's Rule, which suggests that outcrop-scale structures (meso-scale) can help us understand the regional structure (mega-scale), and that the asymmetry of certain kinds of folds can tell us where we are on that structure (vergence). [Link to post]



So the Bridgers are an anticline, overturned in the southern part of the range... but that's not the whole story!

Starting during the Miocene, the west began to widen. The Bridger anticline cracked in half along its axis and the western half slid down relative to the eastern half. The downdropped western half became buried in younger sediments, and that's the Gallatin Valley, where Bozeman is located. When the block of rock above a fault plane slides down relative to the block of rock below the fault plane, we call it a "normal fault." (It would be normal for a kindergardener to slide down a playground slide, but the reverse of normal for them to slide up it!)



A Google Map "terrain" view to show how this is expressed physiographically - Bridger Range on the east, downdropped Gallatin Valley on the west:



And, zoomed out a bit to get some more regional context on how Basin & Range extension has left its mark on the physiography of western Montana, eastern Idaho, and western Wyoming:



I've visited some of these normal faults myself (solid lines); the rest I'm just inferring from the landscape (dashed lines). Basin & Range extension is one of the main reasons the west is so beautiful: those wide open spaces with mountains rising to define the horizon...

(sigh) ...I'm glad I got to spend so much of my summer out there. I'm looking forward to it again next summer. But in the meantime, this is the first week of classes at NOVA, and I'd best get back to work!

Labels: faults, montana, structure

After a post the other day, Michael wrote in to ask for clarification of "Pumpelly's Rule."

AGI defines Pumpelly's Rule thusly*: "The generalization that the axes and axial surfaces of minor folds of an area are congruent with those of the major fold structures of the same phase of deformation."

We saw some of this same idea expressed in yesterday's annotated photo series featuring parasitic folds on larger folded (and boudinaged) quartz veins. There were bigger folds there, and then those bigger folds were decorated with little parasitic folds. The idea behind Pumpelly's Rule is that you could get a sense of what the big folds are doing by looking at the little folds. But even more revealing than parasitic folds at the hinge area of a larger fold are the little folds that you sometimes see on the limbs of bigger folds.

Depending on the sense of the asymmetry of these folds, we call them either "S" or "Z" folds. The parasitic folds are more symmetrical towards the apex of the fold, but more asymmetrial along the limbs. Check out this diagram to see how small S-folds and Z-folds relate to the larger structure of the main fold. Blue arrows indicate the relative sense of shear on each limb of the main fold:


Pumpelly's Rule suggests that we don't need to see the whole picture to understand what's going on. Simply seeing the areas of the diagram highlighted in red are enough to give a sense of the bigger picture.

So how does that relate to this photo, which prompted the question?


Behind me in the photo, you can see an outcrop of the Cretaceous-aged Thermopolis Shale, exposed on Bridger Canyon Road, in the southern part of the Bridger Range, Montana. It has some sandstone layers in it. These sandstone layers, with their high color contrast against the surrounding black shale, record a series of lovely S-folds. The strata here dip moderately to the west. The S-folds relate the sense of shear on the larger structure of the Bridgers: they suggest that the bedding here is overturned, and that you're looking at the eastern side of a big north-south-striking anticline. In the southern Bridgers, therefore, the overall structure is an overturned anticline. Hiking west & uphill confirms this interpretation stratigraphically: as you go up, you go "back in time," encountering older and older strata: from the Thermopolis into the Kootenai, into Jurassic formations like the Morrison, the Swift, and the Rierdon.



Moral of the story: small observations can have large implications.

Raphael Pumpelly made this observation in 1894, presumably during his tenure as the head of the USGS New England Branch. Pumpelly sounds like he was an interesting guy, leading expeditions in Asia when that was a seriously sketchy prospect. In addition to his Rule, he is honored with a mineral named after him, pumpellyite.

* If you don't have a copy of AGI's Dictionary of Geological Terms, a good resource for looking things up online is this Dictionary of Mining, Mineral, and Related Terms sponsored by Hacettepe University in Turkey.

Labels: cretaceous, history, jurassic, montana, mountains, structure, websites

Unannotated photo:


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


Photo with vein quartz boudins and folds highlighted in yellow:


Sketch interpreting stresses that produced these structures:


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

Labels: art, maryland, piedmont, structure

Imitating the detail of a tartan plaid, perhaps?



These perpendicular cross-cutting dikes were observed by NOVA associate professor of geology Victor Zabielski on a trip this summer to Scotland. Thanks for sharing, Victor!

Labels: igneous, nova, scotland, structure

First Geological Society of Washington meeting of the new academic year
September 23
http://www.gswweb.org/
Dupont Circle, Washington, DC

Virginia Region of the National Speleological Society caving convention (but without the caving)
September 25-27
http://www.varegion.org/var/events/FallVAR/FallVAR.shtml
Battle of Cedar Creek Campground (Route 11, between Strasburg and Middletown, Virginia)

New York State Geological Association meeting
September 25-27
http://www.newpaltz.edu/geology/nysga.html
New Paltz, NY

Virginia Geological Field Conference
October 2-4
http://web.wm.edu/geology/vgfc/2009.php
Big Meadows, Shenandoah Nat. Park, VA

Labels: caves, gsw, new york, shenandoah, structure, virginia

A quick sketch of a glacial boulder that I showed you two days ago...



Here's what caught my eye:



What else do you see here?

Labels: granite, metamorphism, new york, structure

Very cool!

Here's my favorite... Or maybe this one? ...Or potentially this one? No, no: definitely this one.
Actually, this is pretty cool too. Sigh... too many to choose from. Too hard to pick just one fave...

Hat tip to Lee Allison!

Labels: structure, tech, websites

Yesterday, I asked you to see what you see here:


And today, I shall tell you what I saw...

Here's what it reminds me of:

...An Olenellid trilobite (slightly deformed)!

Here, I'll sketch it for you:


Garry Hayes came closest to my vision by suggesting the foam pattern resembled Marella splendens, Walcott's "lace crab" of the Burgess Shale.

Labels: art, arthropods, contest, fossils, structure

Last week, I visited the Taconian Unconformity in the Catskills region of New York. I found out about the outcrop via the informative website the USGS put together in 2003 to explain southeastern New York's varied and interesting geology (Click here for a map).

Here's me at the angular unconformity, demonstrating the layering with my forearms:


Here's the same outcrop, sans goofball, avec annotations:


This is a classic angular unconformity. It even graced the cover of the (excellent) GSA publication Excursions in Geology and History: Field Trips in the Middle Atlantic States (Frank Pazzaglia, editor; cover photo by Marli Miller). Why should we care? Because like the "original" angular unconformity at Siccar Point in Scotland (described by James Hutton), this outcrop represents a lot of geologic time. First, during the Ordovician period, the Austin Glen formation had to be deposited as layers of clastic sediment in an ocean basin. Then, during the late Ordovician Taconian Orogeny, those layers had to be deformed: folded and buckled so they stood up on end, and then eroded down to their nubs. Then, on that newly-formed erosional surface, a fresh layer of sediment had to be laid down, in this case, the Rondout Formation was deposited as a layer of carbonate mud during the late Silurian period. Then, that too was deformed, during the Devonian period's Acadian Orogeny. Finally, the whole package had to be uplifted to the surface and exposed (in this case, when a highway roadcut was completed). That's a lot of time!

I'm delighted to have had the opportunity to visit it first-hand!

Labels: devonian, maps, mountains, new york, ordovician, silurian, structure, travel, unconformities

Here's a nice fold I saw the other day at the Smithsonian:

Labels: museums, smithsonian, structure

My second day in Montana this summer, Lily and I took a hike in the Gallatin Range, in Leverich Canyon. There, I turned over a boulder of Archean gneiss (bearing a sweet isoclinal fold) and found two little pseudoscorpions:





My apologies for the blurry, pixelated quality of the photos: these guys were small and they moved fast! Each pseudoscorpion was about 3 mm in length. These are the best two photos out of 20 or so that I shot: they were not easy to capture in digital form.

Pseudoscorpions are members of one of my favorite groups of animals: the non-spider arachnids. This is a surprisingly diverse group that includes (Wikipedia links) pseudoscorpions, tailless whip-scorpions, harvestmen, solpugids, and vinegarroons. (Mites and ticks are also arachnids, as well as a host of less common groups both extinct and extant.) I've seen examples of all of them in the wild except for the solpugids. They're really neat creatures, hints of the wide range of biodiversity in the arthropod phylum.

Labels: archean, arthropods, montana, structure

Two examples of plumose structure, beautifully expressed in a small boulder of the Helena Formation (Mesoproterozoic limestone from the Belt Supergroup) in Glacier National Park, Montana. Photo by Bob L'Hommedieu.

Labels: field trips, limestone, montana, national parks, proterozoic, structure

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

This semester, I employed a new tool in teaching structural geology. Built by NOVA's uber-clever engineering guru Rob Woodke, this is the Butter Buster. The idea came from Structural Geology of Rocks and Regions by Davis & Reynolds, the text I use for teaching structure, and was recommended as a crowd pleaser by Aaron Martin, the structural geologist at the University of Maryland.

So what's the deal? The deal is that materials like rocks behave differently if they are cold or if they are warm. (They also behave differently if they are under high or low pressure, and if strain is applied quickly or slowly, etc., but here our independent variable was temperature).

We can demonstrate this difference by creating an analogy between rocks and a more familar substance, butter. The butter buster creates a fault/shear zone of adjustable width, and displaces the two ends of the butter in opposite directions. If it's cold, it breaks. If it's warm, it flows. Ta-da!

Check it out...

Cold:


Room temperature:


Warm:

Labels: analogies, books, structure, teaching

A couple of days weeks ago, I posted three photographs (reproduced below) and asked you to explain them. My sincere apologies that I haven't gotten the answers up sooner... it's been a crazy time. I've been swamped. And the explanations are not brief. Anyhow...

Here are my explanations -- and the winners for the contest!

A

This is a kink band that got reactivated fault. The tectonic stresses acting on these rocks changed over time, and with them deformation took different paths.

This kink band occured in an area of highly foliated metavolcanic rocks, which developed their transposed foliation (running left to right across the photo) due to transpression in the late Mesozoic. The orientation of the kink bands suggests that the second generation of deformation (the kinking) was caused by a maximum stress oriented at an angle of ~30 degrees to foliation. (see Figure 56, page 100 of my geology master's thesis). Some of the resulting deformation was taken up by (See Figure 52, page 95 of my geology master's thesis) kinking. If the second generation of deformation (kinking) were directed parallel to the foliation, we would expect to see conjugate pairs of kink bands, both at the same angle to foliation. But that ain't what we see... we see kinks in only one angular relationship to foliation. This tells us that the maximum stress (sigma-1 in part C of the diagram below) must have been coming in at an angle of about ~30 degrees to the foliation:


Later, those kink bands/faults were reactivated under a third generation of deformation, which then allowed those fault surfaces to open "void spaces" which instantly filled with whatever fluids were available. In this case, that appears to have been a quartz-saturated water, which filled in the void space with a deposit of milky quartz.

Winner? Kim came closest -- and also pointed out that this story is reinforced by looking around the area at similar exposures which show the same story. Kim, you win a bumper sticker!
_________________________________________________________________

B

This is a strained metaconglomerate, and it provides a nice case-study in strain localization.

This photo speaks volumes to me, because my geology master's thesis was a "real life" check on the predictions of a forward numerical model. My advisor wanted to try and understand the development of lineation in shear zones (ductile faults) via computer modeling. So he came up with a cool model that made predictions about the orientation of lineation relative to foliation and relative to the shear zone's boundaries, and he sent me out into the real world to see if real shear zones played by those rules. And the two didn't match up perfectly.

One issue that may contribute to the lack of agreement between the Sierra Crest shear zone system and modeling predictions is that models distribute strain systematically across a shear zone, whereas it is instead localized in natural systems. The shear zone is itself a localization of strain, of course. The question is, 'how local?' In other words, at which scale(s) is strain being accommodated? Possible triggers for strain localization are many: rheological contrasts between lithologies (Nadin and Saleeby, 2004), variations in temperature or fluid flux (due perhaps to proximity to an intruding magma body) (McCaig, 1984; O'Hara, 1988; Tobisch et al., 1991), variations in stress (due perhaps to salients of wall rock which project into the shear zone or the presence of resistant blocks inside the shear zone), presence of fluids, and / or pre-existing structural heterogeneities. For whatever reason, certain areas within a shear zone may accommodate more strain than neighboring areas. Shear localization may occur on many scales.

Photo B above shows cm-scale localization of strain as small pebbles in a metaconglomerate wrap around a larger, central, less deformed clast. Pebbles immediately across strike from the large clast are more deformed than pebbles along strike from the large clast (i.e. those in the rigid clast's 'pressure shadow'). As a result, the orientations of the long axes of the surrounding pebbles (i.e. lineation) occur in a variety of orientations, a condition also seen in traces of the foliation. On a shear-zone-segment (km) scale, strain localization may be noted in the appearance of pods of relatively undeformed rock surrounded by well-foliated and lineated rock more typical of the shear zone. In the Gem Lake and Mono Pass segments of the Sierra Crest Shear Zone system, for instance, lozenge-shaped pods of clast-rich volcanic breccia (See thesis Figures 14, 15, and 21) were far less deformed than neighboring rock. The implication is that the deforming portions of the shear zone 'flowed' around these pods of more resistant material.

Winner? Growing Tedium came closest, though nobody wrote about the strain localization.
_________________________________________________________________

C


I took this last photograph because it demonstrates well the relationship between bedding and foliation in these rocks. Bedding runs from the lower-left of the outcrop towards the upper-right. But within those beds, you'll notice that all the clasts are elongated vertically into elliptical shapes (ellipsoidal in three dimensions). That's because these rocks got squeezed from the sides when they were hot enough and under enough pressure to flow into new shapes. At this location, deformation played a light enough touch that we can still see relict bedding, but in most of the Sierra Crest Shear Zone, the rocks are much more pervasively deformed: they exhibit a transposition foliation, where no traces of their primary structures can be still be seen. So in some ways, Photo C is the opposite of Photo B: it's a zone of lesser deformation surrounded by a zone of greater deformation: a less-disturbed pocket of rock in an area defined by its disturbed rocks.

Here's how I interpreted this outcrop in my thesis:



Winner? Again, Growing Tedium came closest, by referencing the long axes of these clasts and the "bands" (bedding planes) which run through the outcrop at a 60-degree angle to the long axes. GT, please send me an e-mail with your mailing address, and I'll put your bumper sticker(s) in the mail to you ASAP.

Thanks to everyone for playing, and my sincere apologies for taking this long to get the answer up. (Is it apparent why it took me a while, now that you've read through this whole thing?) I've got a new, simpler contest planned for later in the week.
__________________________________________________________________

McCaig, A.M., 1984. "Fluid rock interaction in some shear zones from the Pyrenees." Journal of Metamorphic Geology 2, 129-141.

Nadin, E.S., and Saleeby, J.B., 2004. "Localization of shear along a compositional discontinuity: the Proto-Kern Canyon Fault, Sierra Nevada, California." GSA Annual Meeting Abstracts: Denver 2004.

O'Hara, K., 1988. "Fluid flow and volume loss during mylonitization: An origin for phyllonite in an overthrust setting, North Carolina, U.S.A." Tectonophysics 156, 21-36.

Tobisch, O.T., Barton, M.D., Vernon, R.H., and Paterson, S.R., 1991. "Fluid-enhanced deformation: Transformation of granitoids to banded mylonites, western Sierra Nevada, California, and southeastern Australia." Journal of Structural Geology 13, 1137-1156.

Labels: california, contest, structure

Dear readers,

Here's a list of the samples I'd really like to have to show my students examples of the processes we discuss:

• A lava pillow (maybe a pillow basalt). Fresh would be best, so I could show the outer crust of obsidian, and the inner basalt. An ancient pillow would be second best.


• Boudinage. A nice hand sample of boudinage, maybe a granite dike in a shist? Or a sandstone stratum within a shale matrix? I feel like I should already have one of these, but I don't... All the good local examples are too big.


• Flame structures/ball-&-pillow soft sediment deformation.


• A komatiite sample.


• One of these. (Eubrontes track with radiating mudcracks, featured this morning on ReBecca's Dinochick Blogs)

I've been a good boy this year. Anybody got any spares they want to trade for some nice Skolithos-bearing cobbles? (...or something else we've got a local supply of?) I'll pay shipping!

If not, please alert Santa that I'd appreciate him filling my stocking with these goodies,

Callan

Labels: basalt, igneous, primary structures, structure, teaching

My MSSE advisor John Graves (previously mentioned here) went on a float down the Green River in Utah last weekend.

This appears to be a huge set of concentric ribs (a.k.a. "arrest lines") on the face of a big joint in massive quartz-rich sandstone. Bedding runs ~horizontally across the image, though not to be confused with the perfectly horizontal "bathtub ring" waterstains from the river. John says, "My best guess from the guide book is that it's Entrada Sandstone, Carmel Formation & Navajo Sandstone top to bottom." The fracture appears to have started in the middle of the cliff and propagated downward and outward. Note how the ribs "flare" out at the far edge. I guess an alternative hypothesis is that this is some weird kind of dune cross-bedding in the Navajo Sandstone: the inside of a barchan dune, perhaps? (though barchans wouldn't form in the "sand sea" situation in which the Navajo was deposited)

Anyone else want to offer another interpretation for this? I think that's what it is.

Labels: joints, msse, structure, utah

Answers tomorrow... Only three entries so far... There's still room for another winner or two...

Labels: california, contest, structure

Callan is a busy boy these days working on his science education master's. But... (mainly through discussions in my Structural Geology class at George Mason University) I've been reminded of some of the cool stuff I saw when I did my geology master's thesis in the high Sierra of California. Here's a couple of neat images from my field work that ought to convey some of the magic of doing structural geology in the "Range of Light."

The challenge I now put to you: explain what's going on in these images. I've labelled them "A," "B," and "C" for easy reference. Winners get a "GEOLOGY ROCKS" bumper sticker. One winner per photo -- whoever comes closest to describing the geology most completely & accurately.

A


B


C


Just a taste of the magic that a summer of field work imparts... :)

Answers in a couple of days...

Labels: california, contest, structure

After we had collectively collected a hundred pounds of samples from Mineral Hill, the final stop on the University of Maryland petrology trip was in scenic Ellicott City, Maryland, where we visited the Ellicott City Granodiorite (map to outcrops).

Like everything else on this trip, the ECGD is intimately tied in with the Taconian Orogeny (late Ordovician; caused by the collision of ancestral North America with a volcanic island arc in the Iapetus Ocean basin). However, unlike the Port Deposit Tonalite we looked at early in the trip, this one crystalized from magma at 435 +/- 15 Ma (U/Pb in zircon). It is not only much younger than the PDT, but it's also pretty young even for the Taconian Orogeny, which reached its peak around 460 Ma.

It's more potassic than the Port Deposit Tonalite, as these K-spar 'megacrysts' show:


This potassium feldspar 'megacryst' shows internal growth laminations, as small mafic bits got caught up in the growing feldspar crystal, which consumed and included them:

Not only does this help us see how the feldspar crystal's habit is a reflection of its internal structure, but it's also an example of the principle of relative dating by inclusions, expressed in a single mineral crystal! Pretty cool.

As with the PDT, xenoliths may be seen in the ECGD:


Parts of it are equigranular, and parts of it are highly foliated:


And of course my eye is always drawn to the structures, like these small faults offsetting dikes of granite which cross-cut the ECGD:




The real prize with the Ellicott City Granodiorite is to view first-hand the magmatic epidote it bears:


Most epidote is metamorphic. However, as Zen and Hammerstrom (1984) showed that epidote could also crystalize from a late-phase magma as the melt interacted with hornblende at high pressures (8 kbar; roughly 30 km depth). You'll note in the photo above the intimate association between the epidote and the hornblende. (I'm not super-confident on my titanite identification, by the way; this rock also bears similar-looking allanite. Please correct me if I'm clearly wrong.) E-an Zen has guest-posted to this blog before, and once upon a time he tasked me with searching for magmatic epidote near Haines, Alaska, in 2006. I didn't find any, but it did pique my interest. So it felt good to be able to finally see some of this rare beast. I was surprised to find it locally, considering the the original magmatic epidote paper referred mainly to west coast plutons from California to Alaska. I was also suprised because of the tremendous depth of crystallization it implied: 30 kilometers down? Wild! I collected a sample for the NOVA lab.

Thanks again to Rich Walker and Roberta Rudnick for graciously hosting me on this trip. I learned a lot, and I'm greatful for the opportunity to expand my local outcrop knowledge.

_________________________________________________________________

Reference:
E-an Zen and Jane M. Hammarstrom (1984). "Magmatic epidote and its petrologic significance." Geology, September 1984. Volume 12, no. 9, p. 515-518. DOI: 10.1130/0091-7613.

Labels: alaska, maryland, metamorphism, minerals, structure, xenoliths

We are now halfway through our documentation of the University of Maryland petrology field trip. As a reminder, we've already seen the Port Deposit Tonalite and the Setters Schist. Today, we meet the Cockeysville Marble.

The Cockeysville is famed in some quarters because of its role in the construction of the Washington Monument. The upper portion (most) of the monument is made of this rock, although it is a purer (higher CaCO3 content) marble than we see here at this outcrop near the Hunt Valley Shopping Mall. Really, this is more of a marble gneiss.

Rich and Roberta talk with the students about this new rock:


The Cockeysville Marble has a well-developed foliation at this outcrop. Impurities in the limestone protolith (probably clay) have metamorphosed into muscovite mica:


Whether these foliations reflect bedding is an open question in my mind. Here's a look at how the outcrop is weathering out. Lovely, just like a limestone in the way it's dissolving away:


It's reasonably coarse-grained:


Here's two close-ups of the stringers of muscovite mica:




Some structural geology was also apparent. Here for instance, is a fault/shear zone


...And here's a fold. It's an overturned fold; Note how the foliation dips at two different angles, though in the same direction:


Still can't see it? Okay, let me help:


The sharp-eyed among you probably noticed the boudinage on the left side of the fold. Here's a portrait of the most prominent boudin:

The most mica-rich domains acted relatively stiffly under deformation, while the calcite-rich domains flowed more easily.

I also found a surface decorated with slickenfibers (crystal fibers growing aligned in small spaces along the surface of a fault):


... and a close-up, so you can see that the opposite block of rock was moving from the bottom of the photo towards the top. Running your finger over this outcrop from bottom to top would feel relatively smooth, while running your finger over it from top to bottom would feel rough as your fingertip would catch on the little mineral "steps":


The Cockeysville is a lovely marble. But we needed lunch at the Twin Kiss drive-in. The day was advancing, and refueling became a serious issue. And then? On to Mineral Hill...

Labels: marble, maryland, metamorphism, structure

Yesterday, I was fortunate enough to be able to tag along on the University of Maryland's petrology field trip, to five locations in Maryland showcasing a variety of igneous and metamorphic rocks. I'd like to thank Rich Walker and Roberta Rudnick for allowing me to come along on the excursion, and UMD graduate student Ryan Kerrigan for alerting me to the trip's interesting rocks in the first place. They have a crew of enthusiastic students, and some cool outcrops!

Our first stop was in northern Maryland's Cecil County. Along the banks of the Susquehanna River, just upstream from the I-95 bridge, is an abandoned quarry of the Port Deposit Tonalite.

Here's Rich and Roberta leading us into the quarry:


UMD students examine the semi-overgrown outcrops of the tonalite:


Tonalites are kind of like granites, except they have only very low amounts of potassium feldspar. This particular tonalite has a magmatic crystallization age of 515 Ma (U/Pb in zircon) and a metamorphic age of 490-480 Ma (Rb/Sr in biotite). Close-up of the rock's texture:


ADDITION: Kim notes in the comments that I didn't draw an explicit connection between the metamorphism and the metamorphic foliation that is so prominent in this photo. She's right: The wavy linear pattern you see in this photo is produced by minerals aligned by differential pressure. Squeeze the rock "top to bottom" and you produce a foliation that runs "left to right."

On the basis of isotopic evidence, the Port Deposit Tonalite is interpreted to have formed as an igneous pluton offshore of ancestral North America, underneath an island arc in the Iapetus Ocean. Later, subduction brought the island arc into contact with North America, triggering the Taconian phase of Appalachian mountain-building.

Here's a closer look at the texture and mineralogy. You can see some k-spar present here, though this was not a common mineral to see at the outcrop...


There were some nice xenoliths present, indicative of the host rock into which the PDT intruded:


Here's a quartz vein cutting through the tonalite. You'll notice that the vein is emplaced approximately perpendicular to foliation, suggesting the same maximum stress which imparted the foliation also extended the rock parallel to the foliation place, opening up fractures that when then fill with the most mobilizable minerals available (in this case, quartz):


If you look closely, you'll see that the fracture which opened up in the tonalite to allow this vein to be emplaced has a ragged edge (not a clean break):


Next up: the Setters Schist...

Labels: appalachians, cambrian, granite, igneous, maryland, metamorphism, ordovician, structure, teaching, xenoliths

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

After the Garth Run high-strain zone and a night hanging out by the campfire at Heavenly Acres with the William and Mary Structural Geology class, the second stop on our Structural Geology trip was in Shenandoah National Park, looking at the deformed meta-basalt columns on the Limberlost Trail. Longtime readers of the blog have seen these unique (in my experience) columns before, in a post from last May.

This is an outcrop of the Catoctin Formation, a series of (mainly) basaltic lava flows that erupted sometime older than 565 Ma (only the youngest, rhyolitic layers have been dated, and evidence suggested that significant amounts of time may have passed between the eruption of each stratum of basalt deeper down in the stratigraphic stack). As the lava cooled, it developed cooling fractures that formed perpendicular to the isotherms. These fractures likely initiated at the top and the bottom of the flow, and propagated towards the middle over time.

Later, during Alleghenian mountain-building (~300 Ma to ~250 Ma, roughly), the rocks were subjected to greenschist-facies metamorphism, and were deformed. The basalt's consituent minerals re-equilibrated and reacted to become other minerals, most notably chlorite and epidote (both of which are green).

Here's John and Joe checking out the columns:


Exquisite! Even arrest lines on the side of each column are preserved. In an undeformed basalt column, these arrest lines would be perpendicular to the column edge. Here, they have a pronounced angular relationship, indicating the shearing of the overall column:


Bobby measures the angular shear along the length of the column:




Goofball professor poses with column:


Jay plays the column like an electric guitar:


We found some nice plumose structure too:


Finally, we evaluated the concentric rings of minerals filling amygdules (vesicles that had been infilled with mineral deposits after lithification) in an attempt to determine whether they could be used as strain markers, or whether they may have attained their ellipsoidal shapes due to stretching of the bubbles in the originial lava (i.e. like this) and then been infilled with minerals:




...and then we were off to Field Study Area #3...

Labels: appalachians, basalt, field trips, igneous, metamorphism, national parks, primary structures, proterozoic, shenandoah, structure, teaching

A little follow-up to this morning's post of pictures from the Garth Run high-strain zone. A short distance away is the Quaker Run high-strain zone, which I visited in spring of 2003 with my geology graduate advisor, Dazhi Jiang, and two other structure students from the University of Maryland, College Park. Here's a beautiful sample of the mylonite I collected then:

Labels: appalachians, blue ridge, metamorphism, structure

I took my Structural Geology students on a three-day field trip this weekend to examine outcrops in the Blue Ridge and Valley & Ridge geologic provinces. Here's a few photos of the team at our first (of four) field study areas, the Garth Run high-strain zone:

Examining the structure and taking strikes and dips:






Fabric elements cross-cutting one another:


Mylonitic fabric:


Foliation wrapping around a feldspar porphyroclast:


This is kind of interesting: a big pancake (oblate ellipsoid) of blue quartz, with a potassium feldspar in the middle:


And if you zoom in close, you can see that the feldspar porphyroclast is broken in the middle (along the plane of cleavage) with non-blue quartz filling in the gaps:


I think this blue quartz likely formed in the pressure shadow of the resistant feldspar porphyroclast during flattening strain, and eventually that feldspar began to brittlely deform, extending in the direction of minimum principal stress.

Quite a bit of variation across strike:


The students found some nice euhedral garnets too, though this was a block of float from upstream, and not intimately associated with the high-strain zone itself:


More tomorrow, from Field Study Area #2...

Labels: blue ridge, field trips, igneous, metamorphism, structure, teaching, valley and ridge

Yesterday, e-mailing the link to Cornell professor Rick Allmendinger's stereonet software to my Structural Geology students, I stumbled across Dr. Allmendinger's excellent collection of photos online. There are some spectacular shots there; worth spending a few minutes ogling-time. Here's my favorite.

Labels: art, structure, websites

Almost a week after the field trip to Old Rag Mountain, and the Facebook-hosted pictures keep trickling in. Here's some shots by NOVA student Eileen Lodovichetti, and an ensuing discussion of feeder dikes and supercontinent breakup.

Here's a shot of the upper reaches of Old Rag, showing the characteristic spheroidal weathering of the Old Rag Granite and the relative lack of trees on top:

photo by Eileen Lodovichetti

...And here's a shot that Eileen took which shows the interior of one of the weathered-out feeder dikes we had to hike through on our way to the summit. You can actually see the classic geoprofessorial arm-waving caught in blurry motion!

photo by Eileen Lodovichetti

This is one of the coolest things about hiking Old Rag: after scrambling up on top of spheroidally-weathered granite domes, you drop into these tabular "hallways." The astute observer will note that the floor is made of a fine-grained, dark-green-colored rock, quite distinct from the light-colored, coarse-grained granite that makes up most of the mountain. These are dikes of metamorphosed basalt that intruded the granite during the breakup of the supercontinent Rodinia in the Neoproterozoic era of geologic time.

Here's one of my former Field Studies in Geology students, Mike Nelson, pointing out a similar dike along Skyline Drive, in the main part of the park:


Basically, the story goes like this: Around 1.2 to 1.0 Ga, continental fragments amalgamated into a supercontinent called Rodinia. In Virginia, this is recorded in the rocks of the Blue Ridge province, where the basement consists of granitoids (granites and related rocks) and metamorphosed granitoids (gneisses, mylonites). Among the youngest of these is the Old Rag Granite, which intruded the Pedlar Formation granite gness around 1.0 Ga.

Later, Rodinia broke apart, resulting in an extensional tectonic regime and mafic volcanism. Fractures opened up in the Old Rag Granite and funneled mafic magma towards the surface. Massive eruptions of basalt blanketed the landscape. The resulting layers of basaltic lava are known as the Catoctin Formation. At Old Rag Mountain, we can see some of the plumbing that led to these flood basalt eruptions: these are feeder dikes, because they "fed" the eruption above them.

Because the dikes (which were metamorphosed to greenstone during ~300 Ma Appalachian mountain-building) weather more rapidly than the Old Rag Granite, they are typically recessed into the landscape. That's what makes the "hallways" in the photograph above. Here's two more images, showing these weathered-out feeder dikes:



Check out how there's moderately-developed columnar jointing extending across the dike. These columns form perpendicular to the cooling front, and the dikes would have lost their heat out the sides. In horizontal lava flows, the heat is lost from the top and bottom surfaces, so you get vertical columns. Here, a vertical dike produces horizontally-oriented columns. Hikers appreciate these "steps" as they squeeze through the dikes on their way up the mountain.

Here's a map of part of Shenandoah National Park:


Please ignore the "hover" instructions at the lower right. I've reproduced the "hoverable" image below. Key: the orange is the Pedlar Formation. The pink is the Old Rag Granite, and the green is the Catoctin Formation. Feeder dikes of the Catoctin are shown as green lines.

Now, let's take away the map, and just preserve the orientation of the feeder dikes. This will tell us the overall tectonic stretching direction:
Various plate reconstructions show either Amazonia or the Congo craton offboard of Virginia at the time Rodinia broke apart and the Iapetus Ocean began seafloor spreading. I've illustrated it here as the Congo, but that might be wrong.

So: the hike up Old Rag is great exercise, and offers scenic views, but for those willing to consider the rocks and how they got there, it's an insightful view into the tectonic past.

Lastly, here's a lovely, well-developed weathering rind on the Catoctin meta-basalt (greenstone). When the dark green rock adjusts to the conditions at the Earth's surface, it breaks down, resulting in the tan/"buff" color on the outside. You're watching the rock "rot" from the outside surface, working its way inward:


More on the geology of Shenandoah National Park can be seen at this page on my website.

Labels: appalachians, basalt, blue ridge, field trips, metamorphism, mountains, national parks, nova, plate tectonics, shenandoah, structure, weathering

Ptygmatic folding is a style of folding characterized by an "intestine-like" appearance. The folded item (in this case, a granite dike) folds back on itself, and the cross-cut material (in this case, a schist) flows out of the way. In other words, there's a viscosity contrast between the relatively-stiff granite dike and the relatively-weak schist.

This particular ptygmatically-folded dike is in a boulder outside of the National Museum of the American Indian, in Washington, DC. That's a quarter at the top of the photo, for scale.

Labels: dc, igneous, museums, structure

Last weekend, I took a group of students, mostly from NOVA but also 3 from GMU, up to hike Old Rag Mountain in Shenandoah National Park.

Here's a Google Map showing the terrain (and trails, which is a cool new addition to the already cool Google Maps):


The crew discusses debris flow deposits in the forest on the way up the mountain:

photograph by Charlie Corrick

The first spot where we get a nice view out over the valleys below:

photograph by Charlie Corrick

Spheroidal weathering in Catoctin Formation greenstone:

photograph by Jared Fortner

Spheroidal weathering in granite (the Old Rag Granite, 1.0 Ga):

photograph by Charlie Corrick


photograph by Charlie Corrick

Student Jared atop a spheroidally-weathered boulder of the Old Rag Granite:

photograph by me

Grain-size differences in the Old Rag Granite (balanced atop my leg):

photograph by me

Non-foliated Old Rag Granite (showing lovely "blue quartz"):

photograph by me

And the foliated version of the Old Rag Granite:

photograph by me

Labels: appalachians, basalt, blue ridge, field trips, geology, granite, igneous, mountains, national parks, nova, structure, weathering

I shot these two videos this weekend from the Billy Goat Trail. They both show the surface of the Potomac River, decorated with little blobs of foam. As the river flows, the blobs of foam record the flow and deform in distinct patterns. I am reminded of the processes that must have occurred in the very rocks I was standing on to take these videos. (See the previous posts on boudinage, folding, and texture in migmatites.) You can see foliation developing, shear zones, folding, and even boudinage. The blobs of foam are acting like more competent geological units (feldspar or garnet porphyroclasts, for instance), while the intervening water is less competent (easily flows out of the way, like quartz or calcite under sufficient pressure).



This one really shows boudinage well. Track the big blob that gets "fed" into the shear zone a few seconds into the video. As deformation proceeds, it separates into three augen-shaped chunks that then move apart along the plane of foliation (which is itself deformed).




A note of caution: these foam blobs are not perfect analogies for the flow of rocks at depth. The dynamics you're observing in these videos are playing out on a two-dimensional surface where water meets air. Because its density is intermediate between the water and the air, the foam stays at this surface, though the water in between the blobs is free to circulate downward into the river if conditions demand it. In real rocks, the deformation would be a three-dimensional phenomenon, and hence a bit more complicated.

Labels: analogies, rivers, structure

A couple of weeks ago, I did a fun experiment with my Structural Geology class at George Mason University. We injected plaster of Paris liquid into a 2-liter soda bottle full of solidified gelatin to induce a fracture by increasing the pore pressure. These experiments show that the fractures (which geologists call "joints") are elliptical in shape, and bear distinctive structures including hackle fringes which correspond to real structures seen in real rocks. (see previous NOVA Geoblog posts on joints and their distinctive structures)

Here's a video of our first joint, made with a horizontal maximum stress:


Here's a shot of the experimental set-up for the second round, in a bottle with a vertical maximum stress:


Lousy, blurry video of the second experiment:


Low-res shot of the resulting joint surface. Note how it flares to parallel with the side of the bottle due to variations in the stress field, and also the lovely hackle fringe:


A close-up of the hackle fringe where the "joint" (white plaster of Paris) stops and the "rock" (transparent gelatin) begins:


As you can tell from the audio in the YouTube clips, we all found this pretty exciting!

For those who teach geology, this is a relatively simple experimental set-up (although I hate the smell of unsugared gelatin cooking) that is a great visual demo of the relationship between stress orientations and joint orientation, pore pressure, and joint/ vein structures. A big thumbs up from the Bentley classroom!

Labels: structure, teaching

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

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

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




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







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

Tiny folds:


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


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


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

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

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

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

Boudinage of granite in metagreywacke:


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


Felsite boudins in amphibolitic gneiss:


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



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



...and in vertical cross-section:


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


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



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



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


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


...And with annotations:


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

Labels: geology, igneous, metamorphism, piedmont, structure

Labels: maryland, structure

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

Check out these amazing structural photographs, available in high resolution too!

Labels: england, northern ireland, republic of ireland, scotland, structure

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

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

Today, I would like to share with you some spectacular images that I took using a new toy, a Nikon binocular dissecting microscope with digital camera mount. These photos show a limestone in which you can find a large number of the single-celled foraminifera called "fusilinids." These benthic forams are about the size and shape of grains of rice, and here you will be looking at them in cross-section, seeing the spiral shape with numerous internal chambers that helps support their cytoplasmic bulk. Remember that these are macroscopic, not microscopic: each fusilinid is a single cell the size of a rice grain!




Now, you might be wondering why I'm so keen on showing off fossils. After all, this isn't a paleo blog... But there's more than just fossilization going on here. These fusilinids have also been squeezed. The weight of overlying sedimentary strata has compressed this rock perpendicular to the bedding plane, (top to bottom, in all these photos) and some of the fusilinids got crammed against their neighbors. Now, fusilinids make their skeletal material from the mineral calcite, and calcite can go into solution when the pressure is high enough. In places, you can see where one fusilinid has penetrated into its neighbor, dissolving the neighbor away as it intrudes. The following two images are close ups of the upper image. Photo #2 is from the lower left of the first image; Photo #3 is from the upper right of the first image:






In both, you can see where the edge of one of these internally-spiraled, ellipsoid-shaped fusilinids has dissolved its way into a neighboring fusilinid, disrupting the neighbor's internal architecture and symmetry. Insoluble minerals like dark-colored clays build up along this dissolution horizon. Here's one more photo for good measure:




Pretty cool, eh? The fossils serve as strain markers, hinting to us about how much of the rock's calcitic volume has been lost.


I would like to thank the Nikon representative who demonstrated the camera for me, Stanley M., for taking the time to show how the device works, and for allowing me to make some images before I had officially bought the thing.

Labels: fossils, limestone, structure

That is the Richat Structure in Mauritania. It's wild looking. Dave has posted on it previously, so I won't add to his ample discussion here, except to say, "Whoa. That is seriously cool."

Labels: africa, deserts, structure

I found this image the other day on Geoff Lloyd's research homepage:



A couple of weeks back, I showed you another image depicting structural geology in the British Isles: the gorgeous hand-drawn diagram by Voll (1960). There are some differences between these two similar diagrams. As an exercise in thinking about how to depict rock structures on the two-dimensional space of paper or computer screens, I think they are worth taking a few moments to examine. Let's compare and contrast...

Similarities:

• Similar perspective (block diagram with the "front" at lower-left).
• Diagram is drawn with the short end along the strike of the structures, and the bulk of the diagram across strike.
• Both depict structurally complex rocks that vary across strike.
• Both use landmarks to give the reader perspective on where on the land's surface these subterranean structures are changing from one motif to another.
• Both are isometric, with the horizontal scale of the block being equal to the vertical scale.

Differences:

• This one was drawn by computer; Voll's was by hand.
• This one is in color; Voll's was in black and white.
• Voll's was generalized to show variations in rock fabric over a large distance; this one is reflective of specific localized data. (I like how it even side-steps a short distance where it apparently wasn't physically possible to go completely perpendicular to strike; see for instance the short jump at the Maer Anticline, and another larger jump at marker 0740 on the scale.) Voll's diagram, in contrast, smooths out those particular rough spots in the data to produce a seamless "summary."
• Voll's was one long wedge; the one is even longer, and as a result has been split into three separate views that are graphically stacked but connected with dotted line, so you can display them in a square- or retangular-shaped space, but can follow along with the overall story from "front" to "back." I think this is a good compromise, graphically speaking.
• Voll's showed the upper and side-facing-us views of the rock units; while this one shows the lower and side-facing-away-from-us views of the rock units, with occasional structures projected out into space between them to show their three-dimensional shapes.

Other thoughts? Observations about these two gorgeous depictions summarizing countless hours of field work? I like rock art; and thinking about rock art -- If you have thoughts, please share them in the comments area below.

I'd like to point out that some other informative sketches have been popping up elseswhere in the geoblogosphere lately: See (in chronological order): here, here, here, here, here and here.

Labels: art, england, structure

I've discussed the phenomenon of jointing on this blog before, and how when rocks fracture, sometimes they leave behind structures we can see that tell us something about the jointing process. Where did it start? Where did it stop? To answer these questions, we turn to structures like plumose structure, arrest lines (concentric ribs), and hackle fringes.

On this past Sunday's field excursion out to the Massanutten Synclinorium (Shenandoah Valley), MSSE John Graves and I saw some more nice examples of these phenomena, and as usual, I took some photos of them.

Let's start with this one, which shows plumose structure (and thus joint propagation) starting at the right and heading to the left.



A closer-up shot of this same fracture surface (in the Ordovician Martinsburg Formation):



Here's another one (in the Devonian Needmore Formation):



Sorry -- no sense of scale in that (above) one -- it was a few feet above my head. Total width of the photo is about two feet (call it half a meter).

This one (also in the Needmore) shows some really wavy plumes:



At the end of joint surfaces, we find hackle fringes, these "rough edges" where the little ridges and valleys of the plumose "topography" flare up and out in a spiralling kind of shape. When you slice through this spiral shape, it appears as a series of little itty-bitty joints at an angle to the main joint. Here's some hackle fringes on a joint surface from the Martinsburg Formation:



Each of these represents the edge of the fracture at one point. But then stresses built up again past the rock's strength, and it cracked anew, extending the fracture and producing a new hackle fringe. A closer-up shot (rotated) of the above fringes:



And back to the Needmore again, for a lovely series of hackle fringes that I've shown you before, but I couldn't resist photographing again. But to mix it up a bit, this time I used a penny instead of a quarter for scale...



Contrastified version of the above, with annotations:



Lastly, remember that I showed you this photo on Monday, from the Billy Goat Trail?



Well, I think you can see some hackles there, too. Take a closer look...

Below, I've zoomed in on the far upper right of the previous photo, and rotated it 90 degrees. I've also transplanted the penny from another part of the photo to maintain a sense of scale, and drawn a quick sketch of the fractures:



I think the little itty-bitty fractures (again, infused with quartz, making them weather out in high relief) traversing the main left-right joint trace are hackle fringes associated with that joint. Anyone care to differ?

Labels: devonian, geology, joints, ordovician, quartz, structure, virginia

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

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

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

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





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





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

Labels: metamorphism, msse, piedmont, quartz, structure

For the second time, I'm distributing paper block models by Martin Schopfer at the Fault Analysis Group at University College Dublin. My experience has been that these little models are really useful for helping non-3D-minded students to visualize the subterranean form of geologic structures.

Labels: art, structure, teaching

They don't make 'em like this any more. Check out how much information is infused into this one diagram, by Voll (1960) in a paper on the structural geology of the Scottish highlands:


Multiple generations of deformation overprint one another in a diagram that is at once informative and beautiful. It's a work of art. My geology M.S. advisor, Dazhi Jiang, shared this image with me once as an example of an elegant structural illustration. I think that it's just so good, that I have to share it at a larger size (rotated, so it will fit in the blog's space):

Voll, G. , "New Work on Petrofabrics," Liverpool Manchester Geol. J. Vol. 2, 1960, pp. 503-567.

Labels: art, scotland, structure

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

The day before Inauguration, I decided to celebrate George Bush's last day in charge of my country by taking a walk in the woods. Okay... that wasn't really my motivation. I was just procrastinating writing up my Structural Geology labs for the coming semester. Anyhow, for one reason or another, I took a stroll in the woods.

I brought my rootin' tootin' new camera with me, and took a few photos. I've got four things to show you: (1) some differential weathering, (2) some kink banding, (3) some cool effects in frozen soil, and (4) a critter.

(1) To start, check out this close-up photo of a stone bridge where the Klingle Valley merges with the Rock Creek Valley:


Several of the (local) stones used in the bridge are weathering at a faster rate than the mortar (cement) that holds them together. As a result of this differential erosion, the less-stable rocks are recessed into the face of the bridge:



Here's another one, where you can see that not all minerals are equally stable at Earth surface conditions. The large central quartz augen stands out in high relief as the micaceous & feldspathic schist around it weathers away.



Yet another: recessed about an inch into the bridge:



That's not all I saw. I also re-discovered the location of some kink bands along the Rock Creek Park bike path:



These kink bands are similar to the ones that Spring 2008 Honors student Victoria measured and analyzed in Broad Branch (also in Rock Creek Park), but these ones are in a different location, further south in the park.



What's worth noting about these kink bands is that they overprint the regional foliation of these schistose rocks. In order to do that, the force that generated the foliation must have been coming from one direction (call it east-west), causing the mineral grains to line up at right-angles to that stress. That allignment is what we call foliation. Later, a new generation of deformation came in from a different direction (call it north-south, approximately parallel to the foliation), kinking the pre-existing foliation. For more on kink bands in DC, see my "DC rocks" page.



(4) One of the disadvantages to hiking in Rock Creek Park in the winter is that it's pretty monochromatic. One of the advantages is that with all the leaves off the trees, it's a lot easier to see new stuff. It's great woodpecker-watching weather, for instance. I saw five woodpeckers of two species that day. Also, it makes it a lot easier to see where the trails are. I saw a new trail that I had never walked before, and so I decided to check it out. I'm glad I did. One thing that I saw that is pretty cool is this effect in frozen soil:



When water freezes, it expands in volume by about 9%, and that shows up here as the upper layer of wet soil froze, it expanded in all directions, pulling away uniformly from two large cobbles of quartzite. It almost makes it look like the quartzite cobbles shrunk in their "sockets," but really it's the "sockets" that got larger.

(4) Lastly, I was doubly glad to have taken the new trail because it was a "road less travelled" kind of deal. I was the only one there. As I trod along, suddenly I heard a scampering noise. It was a critter! It ran up a little gully and then paused as still as a stump, looking at me:



Can't see it? Try this zoomed-in shot:



It's a red fox! Vulpes vulpes, one of two wild canids we have in Rock Creek Park. Pretty good sighting -- only the fourth time I've seen one here (and I spend a lot of time in this park). And every one of those times was in the winter. Again, it's having those clean leaf-less views that allows hikers to see stuff like this.

Labels: critters, dc, ice, mammals, national parks, structure, weathering

Kilauea Iki is the name given to a lava lake that formed in Hawai'i Volcanoes National Park in 1959. It erupted from Pu'u Pua'i, the mound you see in the middle distance of this photograph:

The lava pooled in a pre-existing crater below to a maximum depth of about 400 feet, and has been solidifying ever since. Researchers have drilled though the cooling crust of Kilauea Iki to determine how fast the lava cools. By 1981, a good 200 feet of solid rock had formed at the top of the lava lake.

Here's a view into Kilauea Iki from a different angle, with me rotated about 90 degrees along the crater rim relative to the first photograph:



As you look down there, you'll see that Kilauea Iki does not display a nice smooth surface. Instead, it's fractured, and those fractures have a familiar shape: polygonal and relatively regularly-spaced. They look kinda like the tops of ginormous columns...


When you get down inside, it's pretty flat. You really get the feeling you're walking on a giant layer of soup scum:


...But it's not completely flat. There are cracks and crevices, buckles and upwarps:


Dynamics playing out in this mega-scum layer atop a roiling lava lake are thought to be human-scale analogues of the motion and dynamics of tectonic plates. Here, for instance, two "plates" of cooled lava have drifted towards one another. This meso-scale "convergent boundary" has raised up a mountain range fit for Lilliputians:


Elsewhere, "plates" of lava scum have drifted apart, opening up a "rift" between them. Here, I lie down to bridge the rift:


These cracks are utilized by plants because they offer a shaded nook where moisture isn't immediately evaporated by the sun:


Lastly, I thought I'd point out some neat mass wasting and structural geology I saw there. Here's a shot looking roughly westward across Kilauea Iki, towards the cinder cone of Pu'u Pua'i:

I know it's kind of washed out, but in this photo, you can see a big solidified lava flow that came over the lip of the crater, and then solidified, and then partially collapsed downward.

This sequence resulted in the big talus pile you can see at center-right, but there are remnants of the original sheet (or "tongue") of basalt there.





















Zooming in and cranking up the contrast, let's label a few things:
Up at the top, we can see some fault scarps that have developed as the massive tongue of basalt pulled downward.

A major scarp marks the edge of the cliff, and then below it you see a big slab of basalt with an edge that's just barely in the sunshine, and a bunch of more fragmented pieces below that (marked "breakdown"). Another big slab is seen alongside the breakdown.

What really caught my eye, though, was the en echelon array of pull-apart fractures seen in between the arrows. Here, the stress of the main tongue of basalt sliding downhill sheared this slab of rock, causing it to develop fractures at a ~40 degree angle to the shearing direction. These pull-aparts therefore represent a big surface-condition analogue for tension gashes that can form in subterranean rocks experiencing shear stress.

Labels: analogies, basalt, hawaii, landslide, mass wasting, plate tectonics, structure, travel

It's the 50th anniversary of Chinua Achebe's Things Fall Apart, a reminder that things continue to fall apart. Like... rocks. ...and steel. Today, I'd like to share a "compare & contrast" of two kinds of fractures I saw on my Thanksgiving trip to Hawai'i. One is caused by a decrease in volume; the other is caused by an increase in volume.


Type 1: Columnar jointing (shrinkage fractures)









Columnar jointing results from the decrease in volume as hot lava crystallizes into cool rock. The overall shrinkage in the rock's volume is accomodated by fractures that (all else being equal) are oriented at 120-degree angles on the surface of the flow, and then propagate downward into the flow, perpendicular to the cooling front (isotherm of the critical fracturing temperature, which here is subparallel to the surface of the lava flow). Similar fractures form in drying mud, where the volume loss is due not to cooling but to the evaporation of water. Generally, these mud contraction fractures (a) don't go as deep, and (b) experience more volume loss, resulting in wider fractures. These are in the Mauna Lani resort area, on the western shore of the big island of Hawai'i.
_________________________________________________________________


Type 2: Rust blisters (expansion fractures)








Here, we see fractures forming not due to a loss of volume, but the opposite: an increase in volume! Here the metal (steel, presumably?) in the pole is oxidizing, and in completing that reaction, rust is forming. The layer of paint probably got nicked, water (probably saltwater?) got under it, and then the paint kept the water down there, facilitating the rusting reaction. As the rust formed, it swelled relative to the volume of the original metal. It expanded in the direction that offered the least resisting stress (out away from the surface of the pole). As the rust bumps grow, they impart a new stress on the metal/rust, and this causes fractures to form subparallel to the pole's surface. These are near Ka Lae ("South Point"), near the start of the hike to Green Sands Beach.

Labels: basalt, hawaii, structure, travel, weathering

Who needs a Brunton compass when you've got your iPhone?

I had a few beers earlier in the week with geoblogger-home-for-the-holidays Jess Ball. I was telling her how I was going to be teaching structural geology next semester at George Mason, which prompted Jess to show me a very cool application on her iPod Touch that also works on the iPhone: it's a clinometer!

It is very cool. Twist and turn the thing, and there in two confident digits, is the angle of inclination for the device's straight edge. I was impressed. Future structure students, take note: you need this thing ($1). But first, you need an iPhone (>$1). Or I can just loan you one of GMU's Brunton compasses ($0). Your choice.

Image from John Naughton, showing that there is a margin of error associated with this cool toy.

Labels: structure, tech

What's the tallest mountain on Earth?

Everest, right? Well, yeah: if you're measuring from sea level. If you're measuring from the top of the crust the mountain rises from though, it's Mauna Kea, Hawai'i. It's about ~13,800 feet above sea level, but it rises ~33,500 feet from the oceanic crust to the peak (that's compared to Everest's mere ~29,000 feet from base to peak. So... you could say that Mauna Kea is the tallest mountain on our planet... (you could!)

On Thanksgiving day, my friend Lily and I took a drive up to the top of Mauna Kea, and did a little hike up there at high elevation. Today, I'd like to share some photographs of that excursion. We saw some pretty cool geology.

On the drive up the mountain, we saw an animal which was apropos, considering the day:

Gobble, gobble, gobble. Watch out turkeys, we'll be back after we work up an appetite...

Here's Lily's jeep in the "saddle" between Mauna Kea and Mauna Loa, looking north (with Mauna Kea in the background and basaltic lava flows from Mauna Loa in the foreground):


Some cider cones (the Hawai'ian word for cinder cone is pu'u) in the saddle:


Turning the other way (looking south), you can see the bulky form of "the long mountain," Mauna Loa. What a classic shield volcano shape! I love the fact that it's so dang wide it makes a lousy photograph. You just can't capture its spread-out bulk in a photo; it's too massive:


This was the spot where I pretended to have my toes overrun by a pahoehoe flow:


As we drove up the road to the top of the mountain, I was amazed at the raw volcanic landscape, decorated with cinder cones like this one:


At one point, we passed a neat little angular unconformity on the roadside. Here it is, with a nickel (white dot left of center) for scale:


Here's a closer-shot of this small angular unconformity. Earlier layers of ash and lapilli were deposited at a steep angle, and then eroded (perhaps by glaciation? pure speculation there) before more ash and lapilli were deposited atop it, at a lower angle. There's not likely to be much time missing here, and so perhaps it's better to think of this as the top of a cross-bed, an advancing front of pyroclastic deposition moving down the mountainside, overrun by later eruptions, which may have scoured off the upper few inches (??? pure speculation) or so before deposition.

Really, the truncated tops of cross-beds are mini-angular-unconformities, when you think about it; just not with the same amount of time missing at a "real" angular unconformity (with millions of years missing) due to mountain building like the one at Siccar Point. (Video of cross-beds forming)

Here's something else which the clueless geologist might mistake for a sign of mountain building:
No, those aren't originally-horizontal strata that have later been folded. They're layers (again of ash and lapilli) deposited on the originally-rough topography of the mountainside, covering small ridges and filling small valleys. Where a given layer is exposed at higher elevation, I interpret to be a paleo-topographic high; where that same stratum is exposed at lower elevation, that's a paleo-topographic low. The roadcut reveals these layers have undulating shapes, but this is unlikely to be folding that results from tectonic compression: instead, I think it's showing us the lay of the ancient land surface.

Looking south, we could see past Mauna Loa to the actively erupting steam vent coming out of Halemaumau Crater at Kilauea Caldera (source of the vog!):


Near the summit of Mauna Kea, there are a bunch of astronomical observatories:






On the summit is where you find those examples I mentioned the other day of hawaiite, a rock of basaltic composition that is very dense (ostensibly due to erupting beneath the extra pressures of a Pleistocene ice cap):


Here's me on the summit:


View to the north from the summit: More cinder cones...


Here's a YouTube video of me pointing stuff out from the summit (Kilauea, Hualalai, Mauna Loa, observatories, hikers, etc.). Unfortunately the wind makes it all but unintelligable, but I filmed it, doggone it, so I'm going to post it:



I found a beautiful example of a volcanic bomb up there:


After the visit to the summit, we went for a hike to a small supposedly-glacially-gouged-out lake below the summit (Lake Waiau):


Here's a Google Map, showing the lake's location:


I was surprised to see a thick biofilm on the bottom of the lake:


Encrusting the pebbles and cobbles there, it reminded me of Nora Noffke's modern and Archean biofilm photos in the recent GSA Today, as well as my "Life in Extreme Environments" class this past summer at Montana State University.


We saw some nice examples of structural geology on this hike. Previously, I've mentioned plumose structure, a branching pattern on the topography of fracture surfaces in fine-grained rocks. We saw some of that on blocks of basalt atop Mauna Kea, as in this example (again a repeat photo, but the other day I showed it to you for the vesicle; today I'm showing it to you for the plumose structure.)


A similar feature are arrest lines, which again are minute variations in the surface of a fracture. Like plumose structure, which branches from a source point (where the fracture initiated) and branches out in the direction of propagation, arrest lines tell us about the development of a joint. Unlike plumose structure, though, they are not parallel to the propagating fracture front. Instead, they form perpendicular to it, and record how the fracture propagates in small "steps." Each of these arrest lines is interpreted as being a spot where the fracture grew a little bit, then stopped ("arrested") and then grew some more. In this case, the fracture face we're looking at started at the bottom of the picture and grew towards the top of the photo. You can even see some less-discernible plumose structure backing this up:

Similar arrest lines can be seen in basalt images here and here...

We also saw some pretty spectacular xenoliths. Here's one of gabbro in basalt:


Here's one of peridotite in basalt:


And a few more:



My boots, with another volcanic bomb:


Driving back down the mountain afterwards, we got this nice view of the cinder cones (pu'us!) in the eastern part of the "saddle" between Maunas Kea and Loa:


This Mauna Kea excursion was one of my favorite things that I did on my all-too-brief trip to Hawaii. It was great to get up in the high country, where the air is thin (and vog free!) and the skies are deep blue, and the geology is surprisingly varied (at least it was surprising to me, and pleasantly so). The hike let us work up a good appetite, so we headed back down the mountain and straight to Thanksgiving dinner!

Labels: basalt, birdies, geology, glacial landforms, hawaii, igneous, mountains, msse, primary structures, structure, travel, unconformities, volcano, xenoliths

Dave at the Geology News blog is hosting this month's Accretionary Wedge on the topic of "favorite places to do field work."

My favorite place to do field work is in California's "range of light," the Sierra Nevada.

I did my geology master's field work in the eastern Sierra, along the Sierra Crest Shear Zone, a major high-strain zone which parallels the eastern edge of the Sierra Nevada Batholith through older meta-sedimentary and meta-volcanic host rocks.

In 2003, I spent the summer out there, starting with my first field area at lovely Gem Lake:

An angular unconformity can be seen in this image as the tilted (close to vertical) metasedimentary and metavolcanic rocks (orange and gray) are overlain by dark colored "Tertiary" basalt flows. A big talus slope of basalt chunks makes a black triangular shape that heads downhill toward the lake. In the distance, where the land rises appreciably, the granites (and granodiorites) of the batholilth begin.

We camped on this peninsula sticking out into Gem Lake:


Dazhi Jiang (Then of UMD-College Park; now at the University of Western Ontario) and USC's Geoff Pignotta examine strained metavolcanics near Gem Lake:


Here's me with the Ritter Range in the background:


Glacial striations sculpting my strained metavolcanics:



Field gear:

Labels: california, field trips, glacial landforms, granite, igneous, metamorphism, structure, travel

Second in my "ductile flow in everyday objects" series... Ultimately the goal of posting these photos is to develop a repository of teaching images for familiar substances which flow when conditions of temperature and pressure are sufficient.

Here's a plastic cat-food dish (originally square) which deformed in a ductile fashion after going through the heat-dry cycle on a kitchen dishwasher:


Note how the dish has "sagged" around one of the dish rack's supporting bars, like a damp cloth draped over a stick.

Now that it has cooled, it can be removed and show how much it has deviated from its original shape (how much it has strained):

Labels: analogies, structure, teaching

Rocks flow when conditions are right. At the introductory level, many students exhibit an initial tendency to resist the idea of something they "know" is hard and brittle acting in any other way. Faulting, they get. Shear zones... not so much. I find analogies useful in communicating the behavior of rocks at depth, like mylonites. Often I invoke wax, which can be cold & brittle, hot & ductile, or molten.

But I reckon it's instructive to have other clear indications of ductile flow: everyday objects that have flowed under stress.

Today, I offer the first in what I hope will eventually build into a longer series: everyday examples of ductile flow. We begin with a cassette tape left in a hot car (viewed through the back window, which is why the photo is so lousy):

Even the relative moderate stress of leaning on the seat cushion was sufficient to bend this cassette tape, provided it had attained the right temperature (which it's easy to do in the Virginia summer time in a closed automobile).

Anyone else have examples of everyday examples of ductile flow?

Labels: analogies, structure, teaching

Browsing through the October issue of EARTH magazine, I noticed an advertisement (p. 62) for a service offered by AGI (the nonprofit which publishes EARTH): they maintain an online image bank with 6000 images of earth science stuff. Pretty cool. While the website interface is a bit clunky, there are some real gems there. In the structure category, here's a few that caught my eye (all three by Marli Miller at the University of Oregon):

Labels: art, geology, structure, websites

Yesterday, I mentioned that the main point of this weekend's field trip was to attend the Virginia Geological Field Conference in Marion, Virginia.

We arrived on Friday night at Hungry Mother State Park, and got some background information and logistical direction from the trip's leaders and the various officers of the VGFC. We also got some sobering news about how Virginia budget cuts will affect the Division of Geology and Mineral Resources... but more on that tomorrow.

On Saturday morning, we headed out to examine the geology of the Pulaski and Saltville thrust blocks, two of the slices of Paleozoic sediments that got shoved bodily northwestward during the Alleghenian phase of Appalachian mountain-building. The point of the trip was to examine the structure and stratigraphy of these two thrust sheets, in an attempt to compare and contrast them. Both are an example of "thin-skinned" tectonics, where sedimentary strata are deformed (folded/faulted), but they are disconnected from the tougher underlying "basement" rocks (the crystalline rocks of the North American continent beneath). Sliding along a big basal fault called a decollement, these sheets of sedimentary rocks created the northwestern fringe of the Appalachian mountain belt; a zone called the "fold and thrust belt." (This is in contrast to the "thick-skinned" style of deformation exemplified by the Blue Ridge province immediately to the east, in which the basement rock is itself deformed, and shoved up on top of these younger sedimentary strata.)

Here's two of the three field trip leaders: Loren Raymond (holding map) and Bill Whitlock (talking into the microphone), giving us relevant details for our first field stop:


Fred Webb (the third trip leader) used the same technique of large graphics as an aid in explaining the local geology. Here, he explores the geology of Saltville, VA, from a scenic overlook:


Here's Fred and Loren using another visual prop to illuminate the distribution of sediment types (Knox dolomite versus Moshiem limestone) on a farm in the Rich Valley:

Does anyone else out there use large visual aides like these on field trips? I think it's a pretty good idea.

There were a lot of people who attended the conference: over 120! Here's the crowd at the Saltville Overlook stop:


...and the throngs of geologists shutting down traffic on the way to another stop:


...and still more geologists all over the right-of-way at our final stop of the day:

Kudos to the trip organizers for coming up with a coherent way of running the trip with so many participants!

So why were we there? ...To look at these deformed sedimentary strata, and increase our understanding of the deformation mechanisms that accomodated strain during Appalachian mountain-building. Here's a look at the Max Meadows tectonic breccia, a zone of crumbled rock at the base of the Pulaski Fault:


Just above the breccia, the rock is still pretty deformed. Here's some intense folding and boudinage in dolostone & shale layers:


At another location, Honors student Hope W. shows a fault in the Nolichucky limestone:


In other places, folds were the main variety of strain observed in the rocks. Here, we see this in the Honaker dolomite (with elbow for scale):


Ditto for this exposure of the (Cambrian) Nolichucky limestone (enthusiastic caver for scale):


After a superb lunch put on by a church group, we strolled out in some karstic fields in the Rich Valley. Here, several field trip participants drop down into a sinkhole:


I was interested to see that there were a lot of Mississippian-aged evaporite deposits in this corner of Virginia. Saltville's salt was from the Maccrady Formation, as is this gypsum (note fingernail scratch mark):


Here's the spectacular final outcrop of the day, where we looked at deformation within the Cambrian-aged Nolichucky and Honaker Formations, as well as the Mississippian-aged Maccrady Formation they override at this location on the Saltville Thrust Fault:


Of note to you environmental types out there: Saltville was not only the "salt capital of the Conferderacy," but it was also the site of the very first Superfund site (due to dumping of mercury as a byproduct of soda ash + chlorine production).


And I'll just conclude the photo section of the post with a couple of photos of cool spiders we saw. Each of these arachnids is a good three inches in length (including legs):

I think the upper one is a 'garden spider.' The bottom one is silver! I've never seen a silver spider before...


All in all, it was a good day in the field. We returned pleasantly tired and hungry, and had dinner at the Hungry Mother State Park "The Restaurant". Over food, we discussed the pros and cons of field trips like this, and slept well that night.

I was particularly pleased to meet up with and hang out with folks like Cy Galvin (part of my pre-GSW dinner group), Jon Tso (Radford University), Pete Berquist (Thomas Nelson Community College), Amy Gilmer (Virginia Division of Geology and Mineral Resources), and Chuck Bailey (College of William and Mary). Pete, Amy, Chuck, and I are all W&M geology department alumni. Chuck mentioned the good news that he will soon be joining the geoblogosphere too -- watch this site for an announcement of his (surely to be excellent) geology blog as soon as it goes live.

Labels: arthropods, conferences, field trips, nova, stratigraphy, structure, valley and ridge, virginia

Here's a quick recap of my cross-country journey, for those who are interested in such things.

I left Bozeman on Saturday morning, July 26, and drove east on the Interstate to Billings, then diverged southeast towards Little Bighorn. There, I verified a comment from a Lakota friend at MSU that with my new bushy mustache (see change in icon above), I look a wee big like George Armstrong Custer (Custer & his men were killed by Lakota and/or Cheyenne warriors). After a short picnic there, I kept driving across southeast Montana, and into northeast Wyoming. My goal for the night was Devils Tower, where I have positive memories from my "North by Northwesty" roadtrip two years ago. I got to Devils Tower in mid-afternoon, just in time for a wicked-looking thunderstorm to roll in. Pendulous looking mammatus clouds were hanging down, and the skies turned a darker grey than Lola. Rain and wind came through, and a big dead branch from one of the cottonwoods in the campground came crashing down, but not on anyone's car or tent. When the skies cleared up, I drove up to the visitor's center and took a walk around the tower. It's awesome: massive columns, some of them twenty feet across. The rock is a porphyritic phonolite, and it's quite pretty to look at: big feldspars (5mm) set in a fine-grained grey matrix. Lovely.

The next morning (Sunday), I headed for Red Bird, Wyoming (along Wyoming's eastern border), where Cruisin' the Fossil Freeway suggested there would be oodles of ammonites in concretions in the Pierre Shale, some a foot across. When I visited the Denver Museum of Nature and Science earlier this summer, Kirk Johnson reiterated to me that Red Bird was the place to go for ammonites. But once I got to where Red Bird should be (according to my road atlas), there were no highway signs indicating that the town existed. Worse, there were no outcrops, and no sign of public land. (And one thing that an amateur fossil collector does not want to do in Wyoming is trespass on a rancher's land.) So, no Red Bird ammonites for me. Oh well, no worries: I had collected ammonites from a tongue of the Pierre Shale (the Bearpaw Shale) earlier in the summer on BLM land near Glendive, Montana, and scored some good specimens there. I cruised south, stopping at the Sierra Trading Post outlet in Cheyenne, Wyoming, and dropping some cash on some new duds (STP is mainly a catalog business, famous ten years ago for their amazing deals, but the company seems to be shifting to more mainstream business nowadays, including multiple brick-and-mortar locations). Then another hour on the road brought me to Fort Collins, to the house of Larry Wiseman, where I stayed earlier in my trip. He and I got some pizza and 90-Shilling Ale (Odell's) and traded tales about our summers.


The next morning, we had coffee on Larry's front porch and watch the sun rise. I packed up and hit the road, heading for Kansas. In my rear-view mirror, the Rockies shrank and vanished from sight, a melancholy fade. Out into the plains... In mid-afternoon, I rolled into Oakley, Kansas, where I headed for the Fick Museum. The Fick Museum is interesting on multiple levels: it's got some stellar fossils from Kansas's Smoky Hill Chalk (member of the Niobrara Formation), like a Xiphactinus (massive fish) and a Tylosaurus skull (even more massive mosasaur). But it's also got some whacked-out art: the founder, Vi Fick, was into making art with local "art supplies," and so the walls show his portraits of eagles rendered entirely in rattlesnake tails (see image at right, from this online gallery), or his geometric arrangements of thousands of fossil shark teeth. There's even an oil painting Fick did of "God making the Cretaceous seas," which shows a bearded diety surrounded by flames (it kind of reminded me of Hindu art) making pleisiosaurs and pterosaurs. Not the usual way you see fossils displayed, or paleontology depicted!

At the Fick Museum, I met up with Ron Schott, doyen of the geoblogosphere, who graciously agreed to show me some cool Kansas geology. Ron and I headed south from Oakley towards Monument Rocks, an outcrop of the Smoky Hill Chalk. Ron was eager to gigapan the outcrop, and he set up the little device: essentially a robot that directs his camera to take high-resolution photos in a systematic grid. Pretty cool, really -- I guess I hadn't realized what a Gigapan really was before seeing it in action. I got to meet Ron's two little plastic elves that he uses for scale, and personally placed them on a ledge of chalk for the photograph. The grid of pictures eventually gets digitally stitched together by software, and available for sharing online.

From there, Ron and I headed back up to Oakley, stopping en route so I could collect a couple samples of the aquiferiferous Ogallala Formation, and then headed east, then south again, towards Castle Rock, another chalk outcrop. Here, we tested out my Prius' shocks on the dirt tracks, and checked out the largest cliff in Kansas (nearly getting blown off it by the intense wind), and then prospected for fossils below. I found some fish scales, and a shark tooth! Also inoceramid clam fragments, encrusted with oysters (apparently a common feature of the bottom of the Western Interior Seaway). No mosasaurs, though... Back to the road, and into Hays, Kansas, where Ron put me up in his guest room. We had dinner and a few beers at the Lb. Brewing Company, and thought about recording a PodClast, but then it slipped our minds. We discussed field trips, tenure, publications, and related topics. A good time! Thanks again to Ron for being such an excellent host.

The next two days (Tuesday and Wednesday) were essentially just driving. On Tuesday, I made it to Indianapolis, Indiana, and spent the night in a hotel there. On Wednesday, I turned north, and drove up into Michigan, and crossed into Ontario at Port Huron / Sarnia. Why go to Canada on my way from Montana to DC? Well, I'm teaching my Snowball Earth class this week at NOVA, and some of the rock samples I needed were stuck at Brock University in St. Catherines, Ontario. Usually they get shipped to educators who want to use them, but because of alleged border complications, I had to go get them myself; a five hundred mile detour! Fortunately, I have good friends who leave in Waterloo, Ontario, so I went and stayed with them. Mike and Natalie Leuty have been friends since 1996, and we had a good evening catching up. They have a sweet house in a suburb full of professorial types who teach at one of the several universities in town.

On Thursday morning, Mike and I had coffee on his front porch while his kids played in the yard, and then I packed up my kit and got rolling. I made it to Brock by 11am, and got the Snowball Suite. Because it's in a giant black case that looks suspiciously like a rifle case, I packed it under a pile of other gear in my car. At any rate, I crossed back into the United States without any static from customs officials, and rolled through Buffalo, New York (twice in one year!) I made my destination for the night Ithaca, New York, where I have a friend who's going to grad school at Cornell. I've never been to Ithaca, but I hear that it's "gorges" from many people. So I called my friend, Kathryn Werntz, and she was indeed around and accepting visitors, so I drove through the finger lakes region (five subparallel glacial troughs now filled with water), and found my way to her bungalow. Kathryn and I took a walk through Cornell's campus (two amazing gorges cutting through it), had some Indian food, and went to get dessert at Purity Ice Cream.

In the morning (Friday), I got up and we went to Gimme! Coffee for some caffeine. Thus fortified, I hit the road for my final day of driving. East to I-81, then south through Pennsylvania. At Harrisburg, I turned onto I-83, which took me to Baltimore, and from there it was a familiar zoom down the B-W Parkway into northeast DC. The dome of the Captiol was visible to my left, and then the comfortable sights of Florida Avenue and U Street. Up the hill, and a left on Harvard Street, and I was back in Adams-Morgan. Home! Finally!

Labels: appalachians, canada, chalk, colorado, field trips, glacial landforms, kansas, montana, national parks, new york, snowball earth, structure, travel, wyoming

Picking up where I left off yesterday, in describing Saturday's field trip out to western Maryland:

2:20pm: We exit Interstate 68 and go south on a dirt road for about ten or twelve miles. This road takes us through the Green Ridge State Forest, and I can tell the students are wary of it. I love a good dirt road, and this one even shows outcrops in the road surface -- resistant sedimentary layers tracing across its rutted, potholed surface. The sun comes out, and I roll down the window, relieved that the weather has finally broken.

3:00pm: We arrive at the C&O Canal's Paw Paw Tunnel, in Maryland just north of the Potomac River and the town of Paw Paw, West Virginia. ("Paw paw" is a native tree in the custard apple family with a lovely fruit also called a paw paw. They're delicious, if you can find one the raccoons haven't already claimed.) Paw Paw is the site of the most pronounced entrenched meanders seen along the length of the Potomac River. These exaggerated loops suggest an old age river system, but they are "locked" at the bottom of deep canyons, which suggests a young river system. The usual interpretation is that the Potomac is a rejuvenated river system: it was "old age," equilibrated to base level and meandering actively, but then base level dropped and it incised to a deeper level, maintaining the meandering shape even though the meanders no longer actively squiggle from side to side.

3:10pm: At the upstream end of the tunnel, we discuss the Brallier Shale (Devonian), and note the angle of the bedding here, which is tipped into the Canal's valley: ideal for landslides. When C&O Canal engineers came to the Paw Paw Bends, they faced a tough choice: construct the canal to parallel the river around its multiple entrenched meanders, or carve a tunnel through a mountain made of this stuff. They opted for the tunnel, saving 6.5 miles of Canal length, but the digging of the tunnel took 14 years!



Because the weather is good, we decide to hike over the mountain first and then walk through the tunnel on the return trip. The hike gives us views of some of the meanders' loopy shapes:



We don't see a whole lot else on the hike, but it feels good to stretch the legs.

4:oopm: We reach the Tunnel Hollow, a long linear valley on the downstream side of the tunnel. Signs of the morning's torrential rains are everywhere in the form of increased runoff. For instance, we see a large stream emerging from the base of a talus slope, flowing across the path and into the canal:



Heading up the Tunnel Hollow, we are greeted with the sight of numerous waterfalls arcing down into the valley:





Here, the layers of the Brallier Formation dip into the Tunnel Hollow, again presenting the potential for slip between the layers, and suddenly big slabs of rock dropping down into the valley. We note the "pins" holding these unstable sheets of rock in place:



4:20pm: My favorite thing about the Tunnel Hollow is the world class exposures of slickensides there. During Alleghenian mountain-building, these sheets of shale slid over one another, as a deck of cards will buckle when squeezed. Sliding between the layers ground grooves into the rock face, and also deposited mineral fibers alligned in the direction of sliding.





4:40pm: Lastly, we got to the downstream end of the Paw Paw Tunnel itself, where multiple waterfalls were cascading down onto the towpath. A fine mist fills the air, and catches the beams of sunlight. There's a nice anticline exposed just above the tunnel archway, and usually I have students climb up the stairs (on the left) to check it out up close. However, today a waterfall was landing on the stairs!







Four of us decided to go for it anyhow, just for the thrill of passing through a waterfall. Several (smarter) students who chose to stay down below pulled out their video cameras and recorded parts of our folly. Here's one showing the climb: (Unfortunately it's both silent and taken "sideways" and I'm not video-savvy enough to know how to fix it in either regard.)



Here's another video of the four of us (Nicole, Jan, Dave, and me) up on top:



4:35pm: Time to enter the tunnel. Flashlights come out, and we begin to walk through the Paw Paw Tunnel. It's a remarkable feat of engineering. It's 3/5 of a mile long, and pitch black. We walk along the towpath, where mules once pulled barges up and down the C&O Canal. It's nice and cool in there, like a cave.

5:10pm: We load up in the vans and depart the Paw Paw Tunnel. It takes a full two hours to drive back to Annandale, so we get rolling. We cross West Virginia, and then work our way east across Virginia. Several students nod off, while others discuss geology and travel along the way.

7:12pm: We return to the Annandale campus. Adios, estudiantes! The NSF crowd (Michelle and Nicole) and I retire to the Auld Shebeen in Fairfax for some Boddington's and Gaelic tunes. It's been a long day; we've covered a lot of ground and seen some cool stuff. Time for a pint!

As with yesterday's post, all photos are by Nicole LaDue, NSF. Thanks, Nicole!
Videos are courtesy of Amy Bertsch and Dean Kauffmann.

Labels: field trips, maryland, stratigraphy, structure, teaching, valley and ridge

Saturday morning, 6am: I roll out of bed and check the weather. Storms forecast for Hancock, Maryland, where I'm due to be leading a field trip that day. Hmmm. But based on the radar animation, it looks like they're going to hit hard from 10-11am or so, and then ease off for a bit before hitting hard again later in the day.

6:15am: Making coffee, with Lola the cat underfoot. I check the weather again, and convince myself that the timing of the rain will work for our trip's timetable. I decide to go for it.

7:00am: I call Dale Shelton (of the Maryland Geologic Survey) at home and confirm that it's okay if we go out on the outcrop if it's merely wet, but we can't go out if it's actively raining.

7:15am: I e-mail the students, confirming that the trip is a 'go.'

8:15am: "Bye, Lola!" I leave DC and drive out to Annandale. Once on campus, I gather up a few items (first aid kit, whiteboard, topographic maps), and then go out to the parking lot where students are gathering.

9:00am: We depart campus and head northwest.

9:45am: We leave the Piedmont and cross into the Blue Ridge province.

10:03am: We leave the Blue Ridge province and cross into the Valley and Ridge province (though there are a number of Marylanders who persist in calling it the "Ridge and Valley").

10:15am: The rain hits, hard. Windshield wipers on. Behind the wheel, I grimace. Hope it passes...

10:56am: We pull in to the Sideling Hill Visitor Center. Other cars containing other students are there already. We meet up and head indoors.

Sideling Hill is a massive roadcut in western Maryland. If you've ever seen it, you'd remember it. I won't go into all the geological details here, because (due to the rain) we didn't see them all. But if you're interested, you can read in more detail about Sideling Hill on my website. Long story short: We've got some early Mississippian strata here, derived from the weathering of the Acadian highlands to the east, deposited at the edge of the Kaskaskia epeiric sea. Then they were folded up during Alleghenian mountain-building.

12:00pm: After reviewing some of the salient details inside the Visitors Center where it was warm and dry, we ventured out into the rain and wind. Fortunately, a pedestrian walkway over the highway gave us a decent vantage:





Even from this limited vantage, we are able to observe and interpret some interesting features. For instance, check out the differential weathering of the shale vs. sandstone layers here on the eastern side of the outcrop. We likened this to other examples of differential weathering, like at Monument Valley, Arizona.



We also got a good view of what an oxbow lake looks like when viewed in cross-section. Note how this paleo-channel cuts into the layers beneath it, and is filled with a plug of dark shale, indicating low-energy, low-oxygen conditions.



2:00pm: After giving up on our chances to get out on the first berm of the outcrop, we depart the Sideling Hill Visitor Center, but pull over a short distance down the road to examine the diamictite on the western side of the roadcut. In drizzle, and shouting over the traffic, we discuss the multiple origins of diamictites:



2:10pm: On the road again, headed for our second destination, the Paw Paw Bends...

(More on that tomorrow)

All photos by Nicole LaDue, NSF. Thanks, Nicole!

Labels: field trips, maryland, stratigraphy, structure, valley and ridge

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

A couple days ago, I showed a photo of plumose structure here, a feature that sometimes forms when rocks fracture (i.e. a joint is formed). I invoked the image below to show the relationship between the plumose structures and the concentric "ribs" that sometimes show up on a joint (here labeled as "arrest lines"). The point was to show how they were mututally perpendicular.

But the diagram shows something else, too: that the delicate topography of the plumes becomes more exaggerated away from the main surface of the joint, and they grow into twisted "hackles" along the edge of the joint. Joints have ruffled edges! These hackle fringes can also be spotted on many rock surfaces, if you're looking for them.

Here's a photo I took a couple of weeks ago, in the Silurian Needmore Formation (exposed in the Massanutten Synclinorium between Waterlick, VA and Seven Fountains, VA). It shows a series of hackle fringes parallel to one another, showing the growth of the fracture surface over time.

Here it is again, with the Photoshop "contrast" dial turned up to 11:

The high-contrast view helps bring the hackles into high-relief, and also illuminates the subtle plumose structure. Looks like this surface formed from the top, down. As I read it, this joint started on the right side of the image and propagated leftwards as time went by.

(The hematite nodule at left is a bonus feature.)

Labels: concretions, joints, 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

Here's a photo one of my Audubon students (Albert) took this past Saturday on the Berma Road, in C&O Canal National Historical Park. The lighting was just right, so that when we passed by this outcrop of metagraywacke, we saw an illuminated example of plumose structure:

Plumose structure is something that forms when rock breaks. The fracture starts at one point, and then grows, propagating thorough the rock and leaving behind a telling signature of its growth. In this case, the fracture (also known as a joint) started at point A and propagated through the rock to point B (central 'shaft'), expanding laterally (feathery 'plumes') at the same time.

Sometimes, concentric 'ribs' form, perpendicular to all these feathery plumes, showing the actual leading edge of the growing fracture surface. An example most people are probably familiar with is the "clamshell" shape of a classic conchoidal fracture. Check out this image to see how the two relate to one another.

When we saw this lovely example, I pointed out to the students that if we had been there fifteen minutes earlier or later, this subtle topography would either have been obscured totally in shadow, or washed out in full light. It was only because the light was at juuuuuust the right angle relative to these mm-scale variations that we noticed it.

Labels: joints, 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

Ahhhh.... the semester's just about over. Yesterday, I gave my last lecture and delivered two lab practical exams, and now all that's left to do is give the final exams on Tuesday. Not a moment too soon! It's been a very busy time over the past couple of months. What with my regular teaching duties, my Audubon class, my online MSSE class, GSW, various talks (like Wednesday's "Geology along the C&O Canal" at NSF), supervising homeschoolers visiting the NOVA chemistry lab, grant finagling, writing projects, and just life, I'm dog tired. I'm seriously ready for a nice break.

This ought to mean I'll have more time for posting on this blog, and hopefully that the posts will be richer and more thoughtfully composed.

Anyhow, let's share some pictures today. These are photos I took last summer on Dave Lageson's "Geology of Glacier National Park and Surrounding Areas" course at Montana State University - Bozeman. Dave is a great field trip leader, and I'm looking forward to another of his courses this summer: "Northern Rocky Mountain Geology."

For the Glacier course, we loaded up the vans in Bozeman and drove northwest through Helena and up to Sun River Canyon, one of the best areas in the world to look at multiple imbricated thrust sheets. Dave's been taking students here for a long time, and in fact "wrote the book" on it as a field trip location. In the photo below, the prominent cliff is Paleozoic limestone. The gently-sloping hill in the foreground, however, is Cretaceous shale. As is often the case, tectonics trumps superposition. Compressional tectonic forces have shoved the older rocks up on top of the younger rocks. (An analogous situation in the east is the Blue Ridge's Grenvillian rocks thrust up and to the west over Cambrian and Ordovician carbonates of the Shenandoah Valley.)


Here's a map showing how the Canyon trends east-west across the north-south strike of these mutliple thrust sheets:

Next up: Waterton Lakes Park, Alberta. We slipped over the border and spent an evening drinking beer in the southernmost of the Canadian Rockies. ...Purty.


Here's us looking at the next day's field stops.


Still life with fun stuff:


The next day, crossing back into the U.S., we stopped to get a good look at Chief Mountain, another scene of thrusting older rocks on top of younger rocks. Again, the lower unit is Cretaceous, but this time the upper rocks are older, much older. They're Mesoproterozoic rocks of the Belt Supergroup, thrust eastward along the Lewis Thrust, which underlies the base of this mountain. Chief Mountain is an erosional remnant of the Lewis Thrust sheet: that is to say, erosion has cut into the thrust sheet and left behind this one isolated outpost of what was once a continuous sheet of allochthonous rock. (It's a klippe!) The thrust sheet picks up again in the mountains of Glacier National Park.


Next day: a hike up to Grinnell Glacier, a classic glacier in a park named for classic glaciers. Like all of Glacier's glaciers, however, Grinnell is melting. It's receded quite a lot, as repeat photography shows:


Here's a view looking down the Grinnell Valley at a string of pater noster lakes blue with "glacial flour."


Here's what's left of Grinnell Glacier:


Where the glacier once stood, there's now a new lake. Several of my classmates decided that they would go for a dip. Note: all these guys are from Montana...


As for myself, I stayed out of the water, amusing myself with the amazing sedimentary structures displayed by the Belt rocks. Here's an outcrop of the Grinnell Formation, showing amazing Mesoproterozoic mudcracks. (As David Byrne said, "Same as it ever was, same as it ever was...")


Glacier's Belt Supergroup rocks are reknowned for their stromatolites, fossilized cyanobacterial mats. Here, a stromatolitic layer in the Helena Formation was exposed in cross-section by glacial erosion. Penny for scale (atop middle stromatolite).


And here's another view of the same stromatolitic layer, exposed in map-view section (a horizontal slice, as opposed to the vertical outcrop above). Enthusiastic geologist for scale, imagining doing the backstroke through the Proterozoic Belt Sea.


And... that's it for today. I'm off to the Blue Ridge this weekend, so I won't be posting again until Monday or so. But hopefully I'll have some cool new images from Virginia's oldest rocks to share at that time. Be good.

Labels: canada, montana, msse, primary structures, proterozoic, structure

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

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

This past week, I stayed in Arlington, Virginia. My dad and stepmom were in London, and I was looking after my youngest siblings (both teenagers) by staying at dad's house and serving as the Responsible Adult. It's the same house I grew up in, and it has a lot of nice memories associated with it. At the end of the street, there's a little trail which leads off into the woods and downhill towards the Potomac River along a little creek called Windy Run. ("Windy" as in the weather, not as in sinuous, though it is that, too.) At the bottom, Windy Run launches off a waterfall and tumbles down into the Potomac Gorge. On Saturday morning, I decided to go take a hike down to Windy Run and reacquaint myself with the landscape and its rocks. Here's the view from the top of the waterfall looking across the river into D.C.


Here's a view of the waterfall from the side. The big ice-rimed log at the base is about a foot and a half in diameter, to give a sense of scale:

On the way down the trail, there lies a big boulder of quartzite. This is my first rock. By that, I mean that this specific boulder is the first time I learned to put a name to a chunk of the Earth: my dad taught me that it was quartz, and I committed the name to memory. Today I would note that it's milky quartz, indicating hydrothermal deposition. (Tiny inclusions of water in the crystal lattice scatter incoming light and make it appear white.) Its upper surface is covered in black lichen. Pondering it anew on Saturday, I wondered if learning the name of this boulder in the late 1970s was the first step leading to me towards my ultimate career as a geologist. Lens cap is 5 cm in diameter.


My "first rock" lies at the base of a hill, below a linear trail of other quartz boulders. This array likely represents a subterranean vein of hydrothermal quartz, a common feature in the Virginia Piedmont.


For instance, here's a big vein of hydrothermal quartz (center) cutting across the metagraywacke host rocks at the top of the Windy Run waterfall. It's about a foot wide, and emplaced at a ~20 degree angle to the regional foliation (which strikes ~N25E). The quartz vein is oriented approximately vertically, just east of true north.


Here's some more vein quartz in the metagraywacke matrix. Foliation runs approximately left-right across this image. Note how there are large bodies of milky quartz arrayed semi-parallel to foliation: these are probably best interpreted as boudins: the results when a tabular vein of quartz was broken into chunks, and these chunks were smeared out along along the foliation during mountain-building. Boudinage (the process of producing boudins) is a somewhat brittle behavior (breaking) and somewhat ductile (smearing): under the proper combination of high temperature and directed pressure, quartz can act like pizza dough. It's capable of being molded, but also capable of separating into coherent pieces. We call these "boudins" because they resemble sausages strung out in a row ("boudin" is French for sausage). Here, only one boudin is shown, but click here for some other examples. The boudin is about 3 cm in thickness, to give a sense of scale.

There are also smaller quartz-imbued veins (white arrows, extended with dashed lines) in this rock, cutting across foliation at nearly right angles. Note how the "infusion" of quartz along these thin fractures makes them more resistant to weathering (they stand up in high relief, as seen in the lower left). This set of small quartz veins was likely emplaced at the same time the rock was being squeezed during mountain building, for reasons I explain in the next photograph.

So here's my stress interpretation of this rock. The big blue arrows represent the principal stress direction. To simplify, you could think of one blue arrow as representing Africa and the other as North America, pushing on these poor oceanic sediments caught in the middle. The yellow arrows represent extension. As the rock gets compressed in from "top" to "bottom," it gets squished outwards left to right. This deforms pre-existing quartz veins by rotating them into parallelism with foliation, and also potentially boudinaging them into chunks like the big one. The green ellipse demonstrates this overall process. One way to accommodate the rock's stretching in the yellow-arrow direction is by opening up small fractures (like the ones on the left) which get infilled with quartz.


On my walk, I saw a couple of exposures of hydrothermal quartz that strained the definition: that is, they weren't all quartz. Instead, parts of them (~5%) appeared to be granite pegmatite. In this shot, you can see several large crystals of potassium feldspar set in the quartz. Large flakes of muscovite were also semi-common. Lens cap is 5 cm in diameter.


Here's another shot of the same phenomenon seen elsewhere on the trail: large crystals of potassium feldspar and muscovite set in the "quartz vein." At what point do we stop calling these quartz veins and start calling them pegmatite dikes? Is a single crystal of non-quartz enough to change our perception of the fluid from hot mineral-rich water to wet magma? Like many things in geology, these features indicate that phenomena like dikes and veins are on a spectrum between end-members. In other words, there are shades of grey in how these things form (in addition to how we interpret them). By the way, the greenish hue is algae, not epidote. Lens cap is 5 cm in diameter.


Granite dikes (including pegmatitic ones) are reasonably common in the Virginia Piedmont. Here, as a Windy Run example, is a small granite dike I saw in a boulder on my Saturday walk. Lens cap is 5 cm in diameter.


Tomorrow, I'll explore a rockslide I saw on Windy Run, as well as the nature of the metagreywacke itself. Stay tuned, rockhounds...

Labels: geology, granite, structure, virginia

We've had a cold week in the mid-Atlantic this week, and increasingly my thoughts turn to warmer conditions and the summer. Last year, this year, and next year, I'm scheduling time in Bozeman, Montana, to take classes at Montana State University. I'm working on a second master's degree in science education. It's a pretty cool program which mixes educational practice and "action research" with science elective courses, including plenty of geology offerings.

Today in the blog, I thought I would begin the process of share some images from my time out west last summer. I'll start with the Bridger Range, north of Bozeman. Here's a meadow where we parked the vans before hiking up into the hills on Dave Lageson's excellent Alpine Field Studies seminar:


Once we had huffed and puffed up about tree line, we started to see some pretty cool geology. Here for instance, you can see tilted, folded, faulted Mississippian-aged strata that have been carved into by a glacier. A few minutes after this photo was taken, the class walked straight down into this cirque and climbed up the other side: there's some serious gravity-fighting going on with a route like that. We had lunch on the other side at the top of that orange-colored chute in the upper left:


In the photo below, my hands bracket a tilted zone of paleo-karst in the Mississippian-aged Madison Limestone. With massive limestone above and below, this orangey zone speaks of a time when the limestone deposits of this area were exposed at the surface. Caves and sinkholes developed, as did an iron-rich paleo-soil. It probably looked a lot like modern-day Florida, without the strip malls and retirees. Later, the sea returned and deposited more limestone on top. The paleo-karst is obvious because it contains big blocks of limestone from cave-roof collapse, and is stained by hematite and limonite:


Fellow DC resident and geology educator Nez Nesbitt follows Dave Lageson (the instructor) south along the crest of the range. The drop to either side was substantial, including the headwall of a cirque to the left (east). The loose scree we were walking over added an additional challenge:

In all that scree on the slope we're walking over, there were some cool fossils, including this awesome crinoid calyx ("head" region) - front and back views:


Atop a peak, we paused for a break, and Dave unfurled his Tibetan prayer flags to flap in the wind. I was struck by how a simple little string of cloth imparted a really cool aesthetic to the mountain-top:


This is the trail leading down Sacagawea Cirque. There's some substantial switchbacking going on here:


Here's me atop the highest peak in the Bridger Range, Sacagawea Peak. The views are pretty good from up there:


The class spent the next day mapping glacial landforms in Sacagawea Cirque: it was fun, but I didn't take as many pictures then. When the mapping was over, I prowled through the lateral moraines for fossiliferous chunks of limestone, and found some awesome rugose corals and other treasures. These samples now reside in the NOVA Historical Geology teaching collection.

Labels: fossils, geology, glacial landforms, montana, msse, structure

Outside of Shamokin, Pennsylvania, is a coal strip mine that has had the coal stripped away. Under the coal was a Pennsylvanian (in the time sense of the word) carbonaceous shale (the Llewellyn Formation), which is now preserved in lovely undulating Appalachian folds. Thanks to the removal of the coal, these fold surfaces appear in three dimensions -- a rarity for structural geologists like myself. The area is known as "The Whaleback" because of one anticline (center) with a shape that evokes a surfacing cetacean:

I went to the Whaleback last fall on a fossil-hunting trip with the The Calvert Marine Museum Fossil Club. In today's post, I'll take a look at the structure, and in a later post, I'll show you some photos of the fossils themselves. Here's some of the guys on the trip:

At the north end of the excavation, a cross-sectional view of the absent upper levels is preserved, showing this syncline. It once continued towards the camera's perspective in the air, a downflung fold between the Whaleback anticline and the neighboring anticline which made up the background "wall" in the first photo.
Okay, that's it for today. Tune in soon for the fossiliferous sequel.

Labels: appalachians, concretions, pennsylvania, structure

Geological travels in Northern Ireland, Part VI:

The word "joint" in geology refers to any fracture in a rock unit along which movement has NOT occurred. (If movement DOES occur along a fracture, that makes it a fault.)

The Giant's Causeway in County Antrim, Northern Ireland, shows jointing of a particular pattern: the intersection of the joints divide the rock into column-shaped pieces, shaped roughly like an un-sharpened pencil.

This is an image of two of the "Causeway basalt" layers exposed in a gorge east of the Giant's Causeway itself. Note their difference in size: slower cooling produces larger columns. Faster cooling produces smaller columns. Therefore the lower flow cooled off more rapidly than the upper flow.



Lava, when hot, takes up more volume than cold igneous rock. As it cools, the solidifying lava contracts. Because the whole volume of rock is contracting, evenly-spaced centers