You may have heard that D.C. got some snow this weekend. (It's true.) We went for a walk this morning to check out what the snowed-in city looked like. Here are a few photos...





This is fun:


K Street, home of the lobbyists:


Group of robins hanging out at National Geographic HQ:


The White House gets whiter:


A magnolia tree in Jackson Square, not doing so well:

(Magnolias seem particularly susceptible to losing limbs via heavy snow...)

Photogenic trees:




Washington Monument:


Up-side-down Diplocraterion? Or just where someone sat in the snow?


This trace fossil is more obvious; Bicyclus, clearly:


The National Mall (Smithsonian's Natural History Museum at left, Capitol Building at right):


Doppelganger week for the Capitol:


Cold Triceratops:


Snow decorates the trees in front of the FBI building:


Pennsylvania Avenue:


Callan checks on the snow depth:


Guess this roof isn't very well insulated...


Some structures... Here's a set of two normal faults in a snow stratum atop a hedge:


(Glove for scale, of course.) Here's a different angle on these extensional structures:


(Because GMU classes were canceled on Friday, I assigned my structural geology students to make some structures in the snow -- like Kim's example, perhaps, or perhaps like this hedge, but really limited only by their own imaginations...)

Here's a different one:

That's a sheet of snow being driven downward by gravity, sliding over a roof (fault-like) but then arching up at the tip (this would look 'antiform' if it were rotated 90 degrees...). Kind of like a compressional antiform transitioning into a thrust fault, a common 'structural ingredient' in fold and thrust belts the world over.

Some more normal faults, including en echelon arrays like we saw last September in the volcanic tableland north of Bishop, California... These are viewed from the bottom -- they are forming in snow atop the glass roof of the pagoda-thingy that covers the Columbia Heights metro escalators. Notice too the color difference (due to more or less snow) from the peak of the pagoda (where the faults are -- an area of "crustal" thinning) to the bottom (where the snow is thickest).


Finally, if you haven't already seen it, check out this time-lapse image of the snow accumulating! And here's one from Greg Willis, who has shared videos on this blog before... Enjoy!

Stay warm out there, everyone...

Labels: dc, dinosaurs, fossils, humor, smithsonian, snow, structure

Well... after a week in Parque Nacional Torres del Paine, it was time to head out of the wilderness and back to the relative civilization of Puerto Natales. We woke on the seventh day, and were pleased to see that the sun was hitting the Cuernos del Paine in a pleasing fashion:


Our tent in the foreground of the Cuernos del Paine:


We made our last batch of camp coffee and our last batch of oatmeal, and then started hiking out. As we walked along, we saw some interesting geology.

Here's a decent little weathering rind. Notice how the initially rectangular profile of this clast is being weathered towards a progressively more bread-loafy shape:


A small dextral fault offsetting turbidite layers:


Looking up into one of the valleys we passed on our way east, we saw an intact glacial end moraine sealing the valley shut.

I'm used to seeing these depositional features bisected by streams, but this one looked just like a wall built perpendicular to the valley trend. Erosion hasn't yet undermined it.

We arrived at the Torres area and fortified ourselves with Snickers bars dipped in peanut butter, then strolled on. There were a great many people there: somewhat shocking to the dirty backpackers...

As we hiked out from the Torres campground/village/tourist extravaganza to the entrance station at Laguna Amarga, we turned around and saw the Torres themselves, namesakes of the park, faintly through the misty distance:


Several kilometers on, we approached the Laguna Amarga Ranger Station, which is situated next to a lovely syncline in the Cerro Toro conglomerate:


Another view, from lower elevation, and closer to the axis of the fold:


Our time in Torres del Paine was unforgettable. Backpacking the Grand Circuit was a travel experience I would recommend to anyone with the ability and temperment to camp and hike in such gorgeous surroundings. It had been the primary goal of our trip, but we weren't done travelling yet. We headed back to Puerto Natales on the bus, and gorged ourselves on pizza that evening. We did laundry, got showered up, and slept like hibernating bears. In the morning, we boarded another bus, one that would take us across the border into Argentina...

Labels: chile, glacial landforms, national parks, patagonia, south america, structure, travel, weathering

You will recall that the first photo I showed you from Patagonia was this one:


That's from just outside the Paine Grande Lodge, where we stayed for our fifth night in the park. I rose at dawn and was fortunate to have the camera handy for a few minutes of good low-angle pink/orange light. By the time the coffee was finished, the sun had risen higher, and the "golden hour" had finished. The mountain now looked like this:


We began the day's hike, headed east along the southern face of the Paine Massif, aiming for the legendary Cuernos ("Horns") del Paine:


A last look back at aquamarine Lago Pehoe, with a Nothofagus tree in the foreground:


I saw a nice example of plumose structure in this boulder (fingertip for scale, far lower left):


After not seeing any conglomerate since the first day of hiking, we started encountering it again, meaning that we had hiked back sufficiently to the east to re-enter the Cerro Toro formation. The conglomerate was varied, and so in one ravine, I took the opportunity to photograph its many guises...












Nice mudstone rip-up clasts in this one:


One final sedimentary shot for this post: another graded bed, as viewed in cross-section:

I love graded beds. They're a key part of the geologic saga at my favorite DC-area locale (the Billy Goat Trail), and the ones in Torres del Paine were just classic: light-colored sand transitioning gradually into darker-colored mud, with a crisp boundary between each graded bed and its neighbors above and below. As noted before, these primary sedimentary structures are formed when a cascading turbidity current slows down and starts dumping its particles. The heaviest drop out first, the lightest in weight drop out last. Each graded bed = 1 turbidity current.

I've got a lot of other shots from Day 6, but I think I'll save them for a second post.

Labels: chile, national parks, patagonia, plants, primary structures, sediment, south america, structure, travel

On Boxing Day morn (Dec. 26), we woke at Refugio Grey, and took our camp stove outside. Just for a lark, we walked over to the shore where an iceberg had beached itself, and popped off a chunk to melt and make coffee:

You haven't really had coffee until you've had coffee made with water that's been locked out of the hydrologic cycle for 14,000 years!

With warm coffee and a granola bar apiece, we walked over the small peninsula where Regugio Grey is located to the bay on the other side. There, a flotilla of icebergs had rafted up against the peninsula. We decided to spend a little bit checking them out, before heading out on the day's (short) hike to the next refugio. We had it all to ourselves:


The icebergs varied tremendously in size, shape, color, and texture.


So, presented with a wealth of icebergs like this, what would you do? If you answered "put one on my head!" then apparently you think the same way that we do:


Silliness expended and coffee consumed, we grabbed our packs and hit the trail again. Today's destination was the Paine Grande Lodge. It wasn't an especially long hike, and it was essentially parallel to Lago Grey for most of the distance. Here's some more icebergs, further down the lake:


One geological site that really caught my attention was this sweet outcrop showing gorgeously folded turbidite layers. To give a sense of scale, each of those green bushes is about 1 meter in diameter:


Here's (white arrow) the Paine Grande Lodge, on the shore of a new lake (you can tell by the color), Lago Pehoe.


Closer in shows the detail of this nice, modern facility:



It was our second night indoors, and while it was a bummer that we had to share our room (6 bunks) with 4 other people, one 'up' side was that the Paine Grande took credit cards, which mean that the pisco sours were on Callan and Lily!

This is the lounge, with a nice woodstove and great views of the landscape for birdwatching or just sitting back and feeling satisfied. We spend a while hanging out here, particularly as a few rain squalls moved through.

The usual routine followed: dinner, bed, dawn, coffee, hiking... on to Day 6!

Labels: chile, glaciation, ice, patagonia, south america, structure, travel

Christmas day in Torres del Paine National Park: We packed up our gear at Paso Campground, and hit the trail in the rain. It rained on us for about an hour as we walked south, parallel to the downstream flow of the Grey Glacier, a huge gleaming presence to our right. Occasionally, the trail exited the forest as we had to cross deep ravines, like this one:


Snowmelt coming off the Paine Massif carved these ravines, and the park service had placed ladders in a few key locations, like this one:


Some people didn't like the ladders very much:


Once we got far enough along, we could see the terminus of the Grey Glacier:

Umm, wow.

Slightly different photo composition, with a tree in the foreground:


Nothing but terminus:


Looking south-ish, down the axis of Lago Grey:

Our destination for the evening was Refugio Grey, located on the far side of that first little hook-shaped peninsula.

Iceberg in Lago Grey:


Refugio Grey:

This was our first night spent under a roof on this trip. After three nights in a tent (me with a flat Therm-a-rest), it was quite luxurious to indulge in hot showers and a mattress! We also had a superb Christmas dinner behind those plate-glass windows, eating pork loin and drinking Gato and watching icebergs float by. It was pretty freaking cool.

That afternoon, we went for a walk down the beach, checking out the rocks. There were nice sedimentary structures and nice tectonic structures. Here's some trace fossils seen on one of the bedding planes of the turbidite strata:

I saw a fair amount of bioturbation in the turbidites, but this was without question the best exposure I saw.

Here's a tight little anticline:


Callan takes a nap in a little synclinal bed:


Flame structures with palimpsest glacial striations:


And another set, a few feet over to the right (same bed):

There appear to be some burrows here, too (the little circles of sandstone in the mudstone below the main sandstone contact).

We slept well that night. I was especially pleased by the fact that it rained for half the night (since I was sleeping indoors).

Labels: chile, fossils, glaciation, ice, patagonia, primary structures, south america, structure, travel

After our explorations of the Los Perros Glacier, its moraine, and the bedrock it has scraped so deliciously clean, we headed on up the trail, towards the highest point on Torres del Paine's Grand Circuit: Paso John Gardner. Here's a look back at the valley we've been hiking up from Refugio Dickson... Note the Los Perros moraine and the edge of the lake:


First graded bed of the day. I photographed this one for the lovely scours into the underlying muddy (dark) layer:


Another turbidite clast. Is that a clastic dike on the left?


I thought this was really cool, too. It's a vein: a fracture filled in with a mineral deposit. I really like here how you can see little shreddy flakes of the mudrock (dark) peeling back and flexing in the fracture's void space (prior to being locked in place by mineral deposits):

I interpret this to indicate that the fracture opened in a transtensional fashion, with the top to the right.

A ravine revealed this blind thrust:


Can't see it? Here's an annotated version. The thrust fault below morphs into a fold further up:


A sand-dominated series of graded beds:


Annotated below. Some of the turbidites I saw were a meter thick!

...and what's up with those rotty-appearing rusty spheres? (like the one left of my boot) I saw them several places... hematite concretions? (???)

Brace yourself. Here is possibly the most spectacular boulder I've ever seen:


Annotated version below. This boulder shows a series of graded beds (sand = light colored; mud = dark colored). The direction of gradation shows us that the boulder is upside-down relative to original depositional orientation. A couple of small flame structures reinforce this interpretation. It has been gently folded into a broad anticline (remember, it's upside-down!) and there appear to be some small "parasitic folds" superimposed on the broader fold (at boulder-bottom; depositional-top). Additionally, the turbidites are cross-cut by a small fault which has offset the layers. If I could choose just one boulder to be airlifted from Patagonia to the front of the Science Building at NOVA, this would be the one I would choose.


We keep hiking. We cross several snowfields and other bouldery alluvial aprons, interspersed with fingers of forest reaching up towards the hills. Looking up at the peaks, we can see turbidite layers intensely folded. Check out the straight-limbed anticline (left) and syncline (center) on this mountainside:


Looking up ahead -- there at the left center (between the two peaks) is John Gardner Pass:


We cross through a few more stretches of forest. This one really struck me: "Creep much?"

Besides the freeze-thaw soil-shoving action of creep, I think another factor for the J-shaped (or even L-shaped) tree trunks in this forest is the thick blanket of snow they get each winter: this tamps down the whole forest in a downhill direction.

Look! On the left! Another glacier!


...Shift the perspective a bit, and something else pops out. Once the hammy glacier is off-screen, you can see the wallflower in the background: A mountain composed of pink granite rather than black turbidites.


We keep climbing. Higher up, another opportunity for gazing down the valley we have climbed. The Los Perros Glacier moraine and lake are readily distinguishable even from this distance:


Up in the snow, we trudge higher and higher, and eventually reach the Pass. But for that, and for what we saw on the other side... I'm going to make you wait for Part III.

Labels: chile, glaciation, mass wasting, patagonia, primary structures, south america, structure, travel

Day 3 of our backpacking tour in Torres del Paine National Park (Chile) was an especially rich one. There's so much material to share that I'm going to divide the day up into three chunks: (I) the area around Los Perros Glacier, (II) the area on the east side of John Gardner Pass, and (III) the area west of John Gardner Pass, including the Grey Glacier.

We begin in Los Perros. As you will recall from our last installment, Lily and I had put in a long day of hiking, essentially pulling double duty by hiking all the way from Seron to Los Perros, and skipping Dickson in between. Because the park only allows camping in certain designated areas, trekkers are often put in the position of either hiking less than they want on a certain day, or more than they want. Day 2 was more than we wanted. We slept heavily, and woke to a drier world. We made coffee (we tried out those new Starbucks "Via" instant coffee packets on this trip and found them reasonably acceptable) and decided that before the day's slog, we should backtrack a bit to the Los Perros Glacier and check it out in more detail. As we were hiking in to camp the previous evening, we only had a 5 minute window of decent weather to view the glacier and its surroundings, so we wanted to see what we had missed.

It was a good call. I really enjoyed poking around there. To start with, check out this perspective view down the valley we had hiked up the previous day, the horseshoe-shaped glacial moraine, and the gray-colored glacial lake backed up behind the moraine:


A closer look at the till making up the moraine:


...And in another direction, too:


Composite stitched together of the moraine-dammed lake, using both of the previous two photos plus three others:


Looking down the axis of the lateral part of the moraine (perspective is towards the glacier, though the ice itself is hidden by the bedrock ridge):


"Lil on till":


We walked towards the glacier, checking out the accumulation of icebergs up against the moraine:


A closer look at the terminus of the glacier:


It was a cool place to look at rocks, too. Everything was so fresh, since the glacier had so recently scraped them clean. In this photo, looking across the lake, you can see the line where the vegetation abruptly stops, showing where the glacier was until relatively recently, when it receded to its present position.

A little lateral moraine clings to the walls of the valley. Where it has been eroded away, you can see details of the bedrock, like the granite dike visible on the left.

If you look carefully in this photo, you will see a large vertical granite dike. Follow along in the direction I am pointing. Hopefully you will be able to find it:


This dike continued out into the space where the glacier eventually carved the valley where the lake now sits. But in the middle of the lake is an island, and right along strike from the big granite dike, you can see a granite dike cutting across the rock of the island:

I'll bet it's the same one.

These dikes had some cool details revealed in the area around Los Perros Glacier. Here's an explosion of dark xenoliths in one intrusion. This is clearly intrusive, because it cuts across several turbidite layers, but I was confused about the texture. I expected granite, but it really looked kind of like... sandstone.

I know there are some clastic dikes in the area, and this may be one of them. I've never seen clastic dikes before, but I guess this is what I would imagine they would look like.

...and how about this???

I think that's two clastic dikes cross-cutting a turbidite bed. A nice relative dating exercise, eh?

Some Z-folds reconfigure quartz-filled tension gashes:


I also found a cool little chunk which showed a nice set of concentric ribs:


Classic slickenlines, lacking those gaudy crystal fiber lineations you usually see on fault surfaces. This is gouging, pure and unadulterated and simple:


Such geological goodies! It's better than instant coffee for perking a fellow up in the morning hours. Energized and invigorated, we headed back to Los Perros Camp for our packs, and hit the trail, heading further up the valley. More on that in part II!

Labels: chile, glacial landforms, glaciation, granite, igneous, patagonia, primary structures, sediment, south america, structure, travel, xenoliths

Rested up from Day 1 in Torres del Paine, we were pleased to see that day 2 dawned bright and sunny.

Lily takes a morning break to shed layers:

The weather in Patagonia was really variable, as was the trail. This meant that all day long, we were stopping to put on layers or take off layers as we got cold or hot. It was kind of a pain. Whine whine whine.

All day, clouds scudded along, but we didn't get any rain until late in the day. The main part of the Paine massif was coming into view. Here's a shot from noon-ish:


Here's a shocker: ...We saw more rocks!

Here's another graded bed:


And a little plumose structure, showing a nice twisty hackle fringe:


When I first saw this outcrop, my brain's pattern-recognition center peeped: "CRINOID STEMS!"

...But upon closer examination, they lacked pentameral symmetry, and were some were kind of lumpy. And considering the main rock here is Cretaceous-aged, crinoids could be present, but they aren't as likely a candidate for fossilization as they would have been if these rocks were Paleozoic. So I think these were concretions of some kind. Chert? I shared this image with Patagonia geology expert Brian Romans, and he pointed out something I hadn't noticed in this image: the flame structure in the lower left. That indicates this boulder is upside-down, relative to its original depositional position.

Here's another concentrically-zoned jobbie, which I interpret as a concretion. Overall, this thing was like a pig-in-a-blanket, but on steroids:

I think it's a flint nodule. Brian hasn't seen any crinoids or any concretions in these rocks, so I'm at a loss to offer further explanation.

I was flummoxed by this one, too:

This time, the pattern-recognition center wanted it to be a trilobite, but that's impossible (or, strictly speaking, not impossible but history-re-writing-able) in these aged rocks. Brian tells me it's almost certainly an inoceramid bivalve. That works for me.
(...Or could it be... pseudosegments???)

We walked on, through fields of little white flowers:


Angling towards the main massif, more gnarly peaks came into view...


One thing you can see well in this shot is the contrast between the color of the darker Cretaceous-aged sedimentary host rocks (turbidites) and the light-pink-colored granite which intruded them around 12 million years ago (Miocene).


A bit further on, we could get a decent look at the intrusive relations (through binoculars):

(In this annotated photo, "T" is for "turbidite," "Gr" is for "granite.")

We dropped down off a moraine towards Refugio Dickson, where we made tea, rested a bit, and pushed on again...


At the head of Lago Dickson was an impressive looking glacier, dropped down out of the South Patagonian Ice Field and into the lake:


It was around 1pm when we got to Dickson. We were tired, but the day was only half over. We decided to push on, and essentially do two days' hiking in one. Next stop: Refugio Los Perros!

We hiked on through PRIME Magellanic woodpecker habitat, and it just KILLS me that I didn't see one there, though I did see a few other new birds. Then the rain started, and we started to get tired. But we were committed at this point... We pushed on, and on, and on, and on, climbing up through a forested valley, until finally we popped out on fresh glacial moraine, and saw this:

That's the Los Perros Glacier! A short distance further up the valley was the campground. At this point, the rain had morphed into snow, blasting us in the face as we slogged along, really looking forward to dinner and sleep. Maybe not in that precise order. Eventually, we got there.

Fortunately, the clouds parted for literally 5 minutes, and we were able to have our portrait taken by a doctor from Santiago, who was hiking there for Christmas with his family. They were literally the only Chileans we met who were in the park as tourists (i.e., not park employees or concessionaires) our entire trip. I think we look happy to be in such a special place, don't you?


Next up: Day 3, when we cross John Gardner Pass and see the Grey Glacier for the first time!

Labels: chile, cretaceous, fossils, glaciation, granite, miocene, mountains, patagonia, primary structures, sediment, structure, travel

From Puerto Natales, we took a bus with other backpackery types north to Torres del Paine National Park, the main object of our trip. It was a bit rainy when we left the bus and walked off into the park, aiming to complete the Grand Circuit, a 7-day, 100-km hike around the Paine Massif. Heading down the trail:


Google Map of the Paine Massif, the main focus of the national park:

It's a little offshoot of the main Andean cordillera, a relatively isolated block of mountains rising from the Patagonian steppe. The park is named for some towers ("torres" in Spanish) in the eastern part of the massif. The word "paine" ("pie-nay") apparently means "light blue" in the pre-Spanish native language. This is apparently because many of the lakes (so prominent in these maps) are light blue in color due to the large influx of suspended glacial "milk."

Here's the specific route we took (approximately) in blue:

We hiked in a counter-clockwise direction.

So we started off over in the eastern part of the park, headed north by northwest. We were hiking through steppe, with the snow-covered mountains rising to the west:


...But wait, what's that on the horizon?

Guanacos! These are camellids -- related to the better-known llamas of Peru.

Thumbs-up for guanacos!


The rocks of Torres del Paine are mainly Cretaceous-aged turbidites (shale, graywacke, and conglomerate), intruded by granitic magma in the Eocene. All along the whole trip, I was drooling over the many beautiful graded beds I saw. Here's the first photogenic graded bed I found, with the paleo-top of the bed at the top of the photograph:


These rocks of the Cerro Toro Formation were deposited in the Magallanes Basin, a Cretaceous to Paleogene retroarc foreland basin. Brian Romans of Clastic Detritus shared some information with me before I went down to Patagonia, and I am indebted to him for the insights I gleaned from that sharing. However, any errors in identification or interpretation are my own. According to the model of Romans, et al. (2009)*, a fold and thrust belt was operating to the west, and an elongate north-south oriented submarine trough flexed downward east of that during the Cretaceous. Mud and sand and gravel flowed into this sedimentary basin mainly from the north in three phases which can be contrasted readily with one another in terms of depositional style and confinement of depositional area. These three phases of deposition correlate to different facies, and are exposed well in the area north of Puerto Natales due to subsequent deformation and uplift (not to mention recent deglaciation).

I'm a structural geologist, and deformation is what I am all about, but I honestly didn't expect to see much structure when in Torres del Paine. (I was eagerly anticipating the graded bedding, though!) So it was somewhat shocking to see some very deformed turbidites on that first day of hiking. Here's me standing on the edge of the Paine River, surrounded by tilted turbidite strata:


...And this stuff was really messed up:


Zoomed-in, you can see some severe folding and faulting having shuffled up these rocks:


We hiked about six or seven miles that first day, and camped at a place called Seron. The park has set up these dozen or so campgrounds where you are allowed legally to camp. Some are free, some cost a few bucks. Seron cost about $8 per person to camp there, but we got hot showers with that cost: Nice! The sun set on our first day, and we slept well.


More soon, on our second day of hiking...
____________________________________________
* Romans, B.W., Fildani, A., and Hubbard, S.M., 2009. "Controls on Deep-Water Stratigraphic Architecture," in Stratigraphic Evolution of Deep-Water Architecture: Examples of controls and depositional styles from the Magallanes Basin, southern Chile, SEPM Field Guide No. 10, p. 7.

Labels: chile, cretaceous, eocene, mammals, maps, national parks, paleocene, patagonia, primary structures, rain, sediment, snow, south america, structure, travel

OK, time to start showing some photos from this winter's trip down to Patagonia. Today, I'll talk about our journey south from Puerto Montt, Chile, to Puerto Natales, Chile. We took a ferry, the M.V. Evangelistas, operated by Navigaciones Magallanes, better known as Navimag. We flew through Santiago, and had to spend a couple hours laying over in that airport. During that time, we checked out this tower of luggage that had been set up in an otherwise-unused space:



Black-fronted ibis (see full bird list here) in Puerto Montt:


The Evangelistas in port, prior to our departure:


Steaming out of Puerto Montt, we got good looks at two volcanoes. The smooth white one on the left (north) is Volcan Osorno, and the craggier one on the right (south) is Calbuco:

Heavy cloud cover prevented us from seeing Chaiten the next day, which was a bummer considering all the press it got for its eruption in 2008.

A few shots to show the scenery typical of the next three days as we sailed south towards Puerto Natales:










A ship that ran aground in the 1960s:


We passed a lot of the time in birdwatching. Peering over the deck with binoculars pressed to your eyesockets is a good way to attract other birders. So we made friends with Rory and Leann, a South African couple on a month-long tour of South America. That's Rory in the red jacket:


Doing this, I saw my first penguin, dozens (hundreds?) of albatrosses, and the flightless steamer duck, which is, as Rory enthusiastically pointed out, "a f#%king flightless duck!"




When I see a new species, I note the date and location in my bird guide:


One day, we made a detour to go check out the "Pio XI" or Bruggen Glacier draining into the ocean from the South Patagonian Ice Field (fourth largest ice sheet in the world, after Antarctica, Greenland, and the Elias-Kluane ice field in Alaska and Canada). The Bruggen Glacier is the longest in the southern hemisphere, outside of Antarctica. It is the largest glacier in South America. And it is named for a Chilean geologist!


Here's a satellite view of the area, courtesy of NASA's Earth Observatory:

On the way over to the glacier, we saw the first iceberg of the trip:

Note all the sediment in that ice: it's dirty stuff!

Getting closer:


Closer still, and a medial moraine becomes visible as a dirty stripe running through the middle of the glacier:


Happy tourists:


Continuing south, we encountered more and more islands, and in many places the channel through which the Evangelistas sailed was quite narrow.




At one point, we squeezed through this NARROW gap:


Finally, we approached Puerto Natales, a small town that serves as the main access point for Torres del Paine National Park:


Looking in the opposite direction, I was pleased to see a broad syncline screaming out from the mountainside:


More on Puerto Natales this weekend...

Labels: birdies, chile, glacial landforms, glaciation, mountains, oceans, patagonia, satellite imagery, south america, structure, travel, volcano

Last new year's, I stated some of my goals for the year. Here they are again; I'll bold the ones I actually accomplished, and strike through the ones that I didn't.

1. visiting the Galapagos Islands
2. visiting the high Andes (Cotopaxi, Chimborazo), Ecuador
3. finding a cool outcrop of graded beds in the Martinsburg Formation (late Ordovician turbidites in the Shenandoah Valley of Virginia) that Rick Diecchio told me about last week
4. "walking on the Moho" in Gros Morne National Park, Newfoundland (late summer)
5. seeing Snowball rocks and Ediacarans on the Avalon Peninsula, Newfoundland (late summer)
6. visiting Egg Mountain paleontological site, Montana
7. joining my colleague Ken Rasmussen's field trip to the Culpeper Basin, a Triassic rift valley in northern Virginia
8. some cool trip next winter break (2009-10): perhaps Patagonia? Or Antarctica?

I've also got some big teaching resolutions:

1. Running a successful and robust Structural Geology course for George Mason University (spring semester).
2. Running a successful and innovation Environmental Geology course for NOVA (spring semester).
3. Running a successful and safe Regional Field Geology of the Northern Rocky Mountains course for NOVA (summer semester).
4. Preparing and running a successful and groundbreaking Honors Historical Geology course linked with English Literature 242 at NOVA, where the English professor and I will bridge the two subjects with readings of Lyell, Darwin, "A Pair of Blue Eyes," and others (fall semester). (didn't get sufficient enrollment for this one; bummer!)

On other topics:

1. Finish my M.S.S.E. degree (July)
2. Buy a house
3. Put together a series of geology 'vodcasts' on local geology (though I've gathered a lot of footage that will eventually go into this series)
4. Write a few freelance articles [link]
5. Publish one cartoon per month in EARTH
6. Prepping (cutting and polishing) a backlog of rock samples from all over the place (I'll have to share the story soon of how our "Campus Safety Officer" emasculated my rock saw out of a fear of accidents and ensuing litigation: it's an absolute abomination...)
7. Successfully moving the geology department into our new building

So what's up for the coming year, 2010?

1. More structure at GMU! Bigger, better, tweaked towards greater learning.
2. Hire and train a new member of the NOVA geology team to take on some of the tasks that my colleagues and I can't currently keep up with.
3. Actually get up to Newfoundland this year. I've got a family reunion up there in early August, so hopefully that will be the catalyst. (My mom's side of the family are Newfies.)
4. Run my Rockies course (with co-instructor Pete Berquist) again.
5. Update my website's numerous mentions of "greywacke" (English spelling) to "graywacke" (American spelling).
6. Get my geoblogging under control. I'm clearly devoting too much time to this for too little recompense. Maybe an alternate would be: find a grant or some such to fund the time I spend writing this blog.
7. Continue my cartooning for EARTH. Also occasional freelance writing pieces.
8. Scan my Cartoon Guide to Geology and post it for download/printing on Lulu.
9. Take meaningful action as a "citizen scientist" to combat climate change misinformation, creationism and Intelligent Design mumbo-jumbo, and other forms of pseudoscience pertinent to my expertise as a geologist.
10. Get those geology vodcasts going.
11. Go to Antarctica. (fingers crossed)
12. Work less. Relax more. Be creative. Enjoy life.

Labels: antarctica, canada, climate change, evolution, montana, newfoundland, pseudoscience, structure, teaching

The other day, I mentioned the lineated granite gneiss we saw when hiking in Hickory Nut Gorge State Park on Thanksgiving Day (hey, maybe it's not quite up to the standards of last Thanksgiving's hike, but I'm cool with that). The next day, we headed to Chimney Rock to check out the scene there.

Here's a Google Map of the site (satellite view):


...and a zoomed-in view where you can see the tan ellipse of Chimney Rock itself:


Chimney Rock is located right at the Blue Ridge mountain front, where the mountainous terrain underlain by Grenvillian basement complex (Mesoproterozoic) gives way to the multiple metamorphosed oceanic terranes of the Piedmont province (like the metavolcanics mentioned earlier this week). Here's a view east across this physiographic (and geologic) boundary:

This boundary is called the Brevard Zone, a fault/mylonite zone of complicated structure. I don't know much about the Brevard Zone, but those Carolinian geologists are all over it. It's something I'd like to learn more about. If you have any particular expertise to contribute, please leave a comment telling us more (and giving us outcrop recommendations!).

Here's the star attraction of Chimney Rock Park:

You can see from the bridge and the flag that it has been developed, and much of the park exhibits "improvements" from the natural state.

Before climbing up to the Rock, we decided to hike out to the waterfall upstream. Chimney Rock projects from the wall of a deeply incised canyon carved by the Broad River. A tributary of the Broad flows over the lip of the canyon, providing a lovely waterfall. This scenic location was the spot where they filmed the final scene in the movie version of Last of the Mohicans:


Here's a view across the gorge (from underneath an overhang), looking towards the north:


At the site of the waterfall, I was intrigued to note that the rocky walls were exhibiting "onion skin" weathering (exfoliation jointing) that in my experience is more typical of granites (say, like those in the Sierra Nevada):


Here's a smaller version of the same phenomenon: a flake parts with its source rock, leaving the source rock more spheroidal than it was before. Oak leaves provide a sense of scale:


The rock exposed in Chimney Rock Park is a gneiss. I didn't see any here that was noticably lineated, but it had a pronounced horizontal foliation. The rock varies quite a lot in its texture and degree of deformation. Here's some photos:


Penny for scale:


Penny for scale:


In places, the metamorphic foliation has been deformed. Mainly this is evidenced in charismatic, high-contrast folds, but there is also some small-scale faulting visible, and some boudinage. Here are some images of the folds:


Isoclinal fold. Penny for scale:


Penny for scale (bottom):








Lily points above her head at some parasitic folding:


Here, Lily appears to hold up a big ellipse with an axial ratio of 6 or 7:

This is a section through an isoclinal fold, so that the fold axis is transected once on the left and once (on a differently oriented surface) on the right.

Finally we approached Chimney Rock itself, a looming monolith whose presence was indicating by a loudly flapping flagpole. When a gust of wind came along, the sudden clatter of the flag whipping in the wind was quite disconcerting. From below, it sounded a lot like a rockfall had initiated somewhere up above us. Note how the shape of Chimney Rock appears to be a compromise between the ~horizontal fissility of the gneiss and the spheroidal weathering associated with exfoliation:


Little wooden walkways and staircases are draped all over the face of the mountain, including a catwalk out to Chimney Rock itself:


Atop Chimney Rock, we found these little holes which were filled with water. I forget the name of these things - can someone remind me in the comments section below?

Essentially what's going on here is a self-perpetuating focusing of weathering. A small initial divot in the rock face allows water to accumulate. That water facilitates additional weathering through freeze-thaw action and chemical breakdown of the minerals in the gneiss. This weathering enlarges the size of the depression, which allows more water to accumulate, which triggers more weathering. It's a nice example of a positive feedback loop: a small initial perturbation auto-catalyzes itself into a much larger final effect. I've seen similar structures atop many mountaintops.

Labels: blue ridge, metamorphism, movies, north carolina, piedmont, structure, travel, weathering

A special seminar coming up at the University of Maryland, College Park:

Wednesday December 9, 11:00am-12noon
CHEM 0115 (next to the Chemistry office)

John Suppe, Department of Geosciences, National Taiwan University
"Deep and shallow structure of the Taiwan arc-continent collision"

Abstract: The on-going oblique arc-continent collision in Taiwan between the Luzon arc and the Eurasian continental margin provides a classic spatial view of the temporal evolution of the collision. In addition Taiwan is a well-known site of critical-taper wedge mechanics and erosional forcing of deformation, leading to a mountain belt that is approximately in topographic steady state, with erosion balancing the compressive flux. This classic picture, based largely on surface and upper-crustal data, is now being illuminated at lower crustal and upper mantle levels with highresolution local seismic tomography and earthquake locations. These new data document in 3D that the deep structure is remarkably independent of the shallow thinned-skinned mountain belt. Here we show that the lower crust and upper mantle of the Eurasian plate undergoes a transition from normal subduction and accretionarywedge tectonics south of Taiwan to a strongly localized progressively bent geometry with a vertical to overturned plate interface. The lower crust and Moho of the Eurasian plate is vertical to overturned to depths of 70-80 km under central and northern Taiwan. In this region the deep plate shortening is accomplished by folding of both the Eurasian and Philippine-Sea lower lithospheres without an active subduction zone, whereas the crust of both plates above the main detachment is mechanically and kinematically separated from this deep shortening.

I'll be there, for sure!

Labels: maryland, meetings, plate tectonics, structure, taiwan

Thanksgiving Day, Lily and I took a hike in Hickory Nut Gorge State Park, North Carolina, just south of the much-better-known Chimney Rock, which was closed for the holiday.



Just outside the park, on the public right-of-way, I collected this lovely chunk of granite gneiss which shows both foliation and lineation:


This is classic Blue Ridge province basement rock; it formed ~1.1 or 1.2 billion years ago during the episode of mountain building known as the Grenvillian Orogeny. We've got many of the same sorts of rocks (though slightly younger) up in Shenandoah National Park in Virginia. However, the thing that caught my eye about this one is the fact that it has such well-developed lineation. You're probably already familiar with foliation, the planar alignment of mineral grains in many metamorphic rocks. Lineation, a linear alignment of mineral grains, is somewhat less common, as it requires a different sort of stress field to form. In the scanned image above, you're looking straight at the plane of foliation. Within that plane of foliation is the pronounced lineation, which indicates that the maximum principle stress was directed perpendicular to foliation (plane of the screen), an intermediate principle stress was directed left to right, and the minimum principle stress was directed top to bottom, which is why the gneiss squootched out in that direction*. Hemmed in from the sides, smushed from the front and back, it had nowhere to go but "up." The strain ellipsoid here would be shaped something like a flatworm, or a baguette that had been run over by a steamroller.

* I'm assuming a monoclinic stress field.

Labels: blue ridge, granite, igneous, metamorphism, north carolina, structure

I have shown you tafoni/metate photos from this site, and I have shown you petroglyph/meander photos from this site. But I have not yet shown you the real reason that field forum leader David Ferrill brought us out to the site. Hopefully this picture will convey some of the structural wonder of this location:


You're looking down the axis of a horst, a normal-fault-bounded block of rock that has had normal-fault-bounded blocks of rock slide downward on either side of it (called grabens, from the German for "grave"). The collection of automobiles provides a sense of scale: this is a pretty skinny little horst.

In miniature, this site shows the processes that are playing out on a larger scale in the Owens Valley (itself a graben), the White Mountains (a horst), and the Basin & Range province in general.

I used Photoshop to stitch together a panorama looking along the graben marked "#1" in the annotated photo above:

(Click here for a BIGGER version.) That's the Sierra Nevada in the distance, and the Owens River at far left, with the skinny horst at the far right. I like how you can see some of the welded "Ig2" layer right where the graben meets Chalk Bluff (far left), while it's buried beneath sediment in the rest of the valley. That same layer is the resistant one that caps the horst (atop which I took the photo).

I'm not sure I had ever consciously stood atop a horst before. Yeehaw. Giddyap!

Labels: california, faults, rift valleys, structure, travel

Shall we return now to the volcanic tableland north of Bishop, California?

Yes, let's shall.

Today's topic: cooling columns in the Bishop Tuff. Like all volcanic rocks, the Bishop Tuff erupted hot and cooled off as it "set." This change in temperature led to a change in volume, and the upper welded layer, known affectionally to its friends as "Ig2," lost enough volume and was stiff enough that it developed a set of polygonal fractures, which propagated downward, mostly vertically. This divided up the Ig2 into blocky columns, which now topple over where exposed along normal fault scarps:

View is to the south/southeast. White Mountains in the distance. Simon Kattenhorn for scale.

Here's a Google Map of this locality. It's just north of where that syn-deformational drainage channel flows down a relay ramp into a graben.


Up atop the footwall block, you can see some of these columns separating from one another, opening up zig-zag-shaped fractures as the columns nearest the scarp rotate outward into the graben. The resulting gape gets filled in with sediment, like a rift valley in miniature:


A close look at the columns themselves (next three images along the fault scarp) reveals some of the lovely smaller structures that serve as decoration and fodder for the structural geologist. Consider, for instance, this delectable "crack panel" showing the arrest lines as the fractures which define the column propagated downward a few inches at a time:


A closer look: the arrest lines are ~horizontal on a ~vertical column:


Also, this caught my eye:

I think what we're seeing here is two intersection "hackle fringes" on the corner of a block of Bishop Tuff Ig2. I don't think they're arrest lines, so they don't appear to have anything to do with columnar jointing. The prominent "ruffley" set of hackles on the left appear to have all formed as part of a single episode of jointing. That joint apparently cross-cut an earlier joint which left the less-prominent hackle fringe on the right (hackles at a ~10 degree angle to the first set of hackles). At least I think that's what's going on here... Would anyone else care to offer another "read" on this outcrop?

Labels: california, faults, joints, meetings, structure, travel, volcano, weathering

A nice example of plumose structure, enhanced by a pronounced joint set which cross-cuts the be-plumed surface. Hammer for scale. White Mountain front, California, September 2009.

Labels: california, joints, structure

On Sunday morning, I mentioned a frozen pizza, and my interpretation of its geologic history. Afterward, Elli from UPJ wrote me a note asking if my post didn't actually quite closely resemble "Figure 1.42 in Davis and Reynolds"? (Davis and Reynolds' Structural Geology of Rocks and Regions is a popular structural geology textbook.)

Well, yep.... Yep, it does. That's the way uniformitarianism works. The same physical laws and less-than-fully-frozen pizza delivery methods that mildly deformed George Davis' pizza in in early 1980s still apply in late 2009. I'd like to point out (as a proud structural geologist) that my pizza was more deformed than Davis'. (It also had more ingredients.) Of course, he did a better job than I did, in describing and mapping that deformation:


(The full diagram also included a cross-section and a kinematic reconstruction. Used without permission, but with a sense of what I hope is 'fair use.')

In the interest of fully citing my sources, I'd like to explain my relationship to the pizza/structure analogy. Part I: In the spring of 1995, I attended one of the fun, rollicking pizza parties that Dr. J held for the William & Mary geology department. Dr. J provided blank pies and a slew of ingredients, and we hungry students could load them up as we saw fit. It was very generous of him, quite tasty, and a lot of fun. Lubricated by a goodly amount of Dr. J's red wine (which was present in gallon jugs), I (hazily) recall a fun discussion with some of my fellow geology majors. We were congratulating ourselves on having picked the coolest major around, and full of geo-ego*, our conversation focused on the fact that we could see geological principles everywhere!

Our pizza came out of the oven, and sure enough: look there! It was full of beautiful examples of (munch munch) stratigraphy! And structure! (chew chew, bite) And the ingredients were like minerals! (slosh, gulp) Wow! This is great!

Part II: The following fall, I took structural geology with Bruce Goodwin, and the assigned textbook was George Davis' Structural Geology of Rocks and Regions, first edition. And there, I found in the first chapter the pizza analogy illustrated above. With a frisson of personal recognition, I thought back to the pizza party. And I said to myself, "I like the way this author thinks! I'm going to like this class."

Indeed I did, and many years later, when it came time for me to teach structure, I turned to Davis' book, now in its second edition and co-authored with Steve Reynolds. It's still a great text, and full of good analogies and a sense of fun. I wonder how much that one diagram turned me on to structural geology: that single pizza sketch may have influenced the course of my life!

Part III: I made a pizza, and had a digital camera handy. Now you know the full story.

___________________________________________
* I hereby lay claim to coining what I'm sure will be a very useful term!

Labels: analogies, books, humor, structure

This pizza-ite clearly passed into the brittle-ductile transition while it was vertically-oriented. This event translated coherent ingredients downward, presumably along less competent flowing cheese/dough/slush surfaces. High-contrast olives serve as good marker units for detecting the overall kinematics: note their greater concentration at the paleo-bottom of the pie (also bottom of this photo). It is inferred that these olives were originally dispersed across the face of the pizza-ite at approximately equal distances. The overall pie has strained from an original circular shape to an elliptical one, and detached from the basement cardboard along a major fault. The "top" of the pizza-ite may therefore be regarded as an overall extensional regime, while the "base" of the pie is compressional. The highest-pressure zone at the base appears to have metamorphosed up some new substances, including an ice/cheese amalgam (darker yellow).

Subsequent to this photograph being captured, the pizza experienced a high-temperature, low-pressure event which has been theoretically located to the second rack of my oven, and then was broken into eight ~equal area terranes separated along a radial series of fractures. An episode of physical weathering pulverized the pizza-ite, followed by chemical disaggregation in a low-pH medium. The energy released by this process was sufficient to power the typing of keys on a keyboard, and ultimately generated a new entity: a blog post. Through the twin miracles of digital technology and structural interpretation, we can work out that the protolith of the blog post was the deformed pizza. Careful dating of this blog post reveals it passed through closure status at 7:45am on November 15, 2009.

Labels: analogies, food, humor, structure

These days, I'm engaged in the lovely process of rediscovering the geologic record of Shenandoah National Park. This 'rediscovery' was prompted by the recent Virginia Geological Field Conference based at Big Meadows. While I wasn't able to attend in person (I was in Yosemite that weekend!), colleagues like Pete Berquist and John Weidner were there, as well as three of my Rockies students from last summer. They've all shared their perspectives on the conference with me, and John loaned me a copy of the field guide to the conference. This guide, authored by other colleagues like Chuck Bailey of William & Mary and Scott Southworth and Bill Burton of the USGS in Reston, makes for great reading. I'd link to it so you can read it too, but it's not online.

The guide led me to the revelation that there is a new geologic map of the park and the surrounding area that was published earlier this year by the survey. This map* is authored by Chuck, Scott, and Bill, along with their peers at the survey and other institutions. Why wasn't I informed? (Just kidding) It's a beautiful work of art and science. I'm having the NOVA duplicating services team print me out a copy this week.

The new insights offered by the map (and the VGFC field guide) include the fact that the oldest rocks in Shenandoah National Park are diverse and complicated. It used to be that geologists considered these rocks to be a granite gneiss called "the Pedlar Formation," which was intruded in places by younger granitoid plutons. Modern work in the park has revealed that it's more complicated than that. There are a dozen or more separate rock units comprising what the pros are now calling "the basement complex." These rocks are distinguishable based on texture, mineralogy, and age. (These newer, more precise ages are one of the key advances of recent work by John Aleinikoff of the USGS: the granitoids and their metamorphic successors have crystallization ages ranging from 1,183 Ma (+/-11 Ma) to 1,028 Ma (+/- 9 Ma).

I've updated my Shenandoah web page to reflect the new preferred terminology plus these new dates. More updates to come -- I've got many new tidbits of inspiration from reading the 100+ page write-up that accompanies the new map. The web page, like all of my web pages, is a work in progress. Nothing makes that clearer to me than a steaming helping of fresh science!

When I was out in the park last weekend, I found this new outcrop of the basement complex, which shows some of this intriguing diversity:


Annotated version:


The outcrop is on the hike over Bearfence Mountain, described (and mapped) in the new VGFC field guide. It's a granite gneiss, partially altered to unakite (the plagioclase and pyroxene in the graniotid reacted in the presence of water to generate epidote. A pronounced foliation is cut by no less than 3 separate sets of fractures, two of which are filled in with fibrous quartz, and another by something dark. The granitoid formed during the Mesoproterozoic Grenvillian Orogeny, and was deformed later in that same episode of mountain building. The fractures formed at some point after that: just when, I can't say. Vein sets 1 and 2 are infilled with apparently identical compositions, which would be consistent with them being contemporaries. Vein set 3 has something else lining its fractures. At first I thought it was just mildew, but Elli suggested some mineralogical possibilities. Vein set 3 does not show the same amount of dilation as the other two sets. Cross-cutting relationships show vein sets 1 and 2 cross-cutting vein set 3, which suggests I was too hasty in labelling them in my photo. "3" is the oldest; "1" and "2," despite their names, are younger. Maybe they're related to Neoproterozoic breakup of Rodinia, or Alleghanian mountain-building, or uplift? So many mysteries...

More to come on this topic, surely, as I get re-introduced to my local national park.
__________________________________________
* Southworth, Scott, Aleinikoff, J.N., Bailey, C.M., Burton, W.C., Crider, E.A., Hackley, P.C., Smoot, J.P., and Tollo, R.P., 2009, Geologic map of the Shenandoah National Park region Virginia: U.S. Geological Survey Open-File Report 2009–1153, 96 p., 1 plate, scale 1:100,000.

Labels: blue ridge, conferences, granite, igneous, metamorphism, national parks, proterozoic, shenandoah, structure

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, fluid dynamics, 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.
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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: