My final day at GSA was fruitful. I started off in the "Earth, et al." session hosted by ODU's Nora Noffke. It was devoted to the Precambrian, and had some interesting talks about fluctuating oxygen levels, mineral evolution, microbially-induced sedimentary structures, and Neoproterozoic glaciation. This last one was most interesting to me: UMD's Jay Kaufman talked about field work he conducted in Siberia last summer, documenting a diamictite unit between Ediacaran strata and Cambrian strata. There's even a carbon isotope excursion to match up with it! Cool... literally.

I had lunch with my friend David Dantzler, who I hadn't even realized was at the conference, until I saw him come in to one of the Darwin-focused sessions. In the afternoon, I attended another eight talks, including some on greenstone belts in South Africa, some on geological education, and a couple about the evolution of orogens, with an emphasis on South America. (One of these was an excellent talk by Brian Romans about his field area in Patagonia.) I finished up with Kim Hannula's talk about the geoblogosphere's role in supporting women geoscientists. Then it was time to bug out: back to the hotel, then to the airport, then to Los Angeles, then to Dulles, where I arrived this morning at 6:30am. On the flight, I took an Advil PM, put in earplugs and wore one of those little eye-masks so I could get some decent amount of sleep... Mixed success on that front. Once I got to Dulles, I got some coffee, and headed straight to work! It's good to be back in the familiar environs of my office and lab again. Thanks for a great conference, everyone!

Also: GSA is maintaining a webpage summarizing the various posts from registered geobloggers. It's incomplete, but a useful idea: a repository for all the stuff being said about the conference from the various attending geobloggers.

Labels: archean, blogs, geology, hadean, meetings, oregon, patagonia, proterozoic, snowball earth

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

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

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

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

All photos in this post by Rockies student Charlie Corrick.

Obstacle in the road...


Tetons...


Charlie and Jared on Blacktail Butte:


Luke on Blacktail Butte:


Charlie, Luke, and Jared on Blacktail Butte:


Checking out the fault scarp of the Hebgen Lake Fault, north of Hebgen Lake:


Examining the Grinnell Formation for the first time:


Looking down the St. Mary Valley, Glacier National Park:


Stromatolites in the Helena Formation, Glacier National Park:


Victoria points out the contact of the Purcell Sill with the surrounding Helena Formation (limestone), Glacier National Park:


Callan points out the contact of the Purcell Sill with the surrounding Helena Formation (limestone), Glacier National Park:


Pete and Joel point out the contact of an apophysis of the Purcell Sill with the surrounding Helena Formation (limestone), Glacier National Park. Notice that the sill cuts across stratification down by Joel's legs.


At the Bozeman Airport on the way home, John entertains us with geology songs he composed, which cracked up the instructors:

Labels: glacial landforms, montana, national parks, nova, proterozoic, travel, wyoming

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



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



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

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


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


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

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

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

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

A funny coincidence transpired a couple of weeks ago. I posted about "Amygdules!" and so did Andrew. We were both so excited by these cool primary igneous structures that we added an exclamation point to our post titles. Over the weekend, I found some more. These are in Dark Hollow, in Shenandoah National Park, above the falls. Pretty sweet, eh?



I hereby give them two exclamation points. Let's see if anyone else can come up with two-exclamation-point-worthy examples of amygdules...

Labels: basalt, blogs, blue ridge, falls, igneous, primary structures, proterozoic, shenandoah

Our third Rockies training hike took place Saturday. Six of us hiked White Oak Canyon, in Shenandoah National Park. It was about an eight mile loop, with six big waterfalls on it. There were a lot of plungepools where other hikers were swimming.

There wasn't an astounding amount of geology on the trail: it was mostly Catoctin Formation, with a few outcrops of underlying Grenvillian granitoids. A few nice amygdules; no columns.

The waterfalls sure were purty, though. Here's Jason at the uppermost falls (86 feet tall):


Me departing from one of the lower falls:

Photo by Chris McMahon

I got home tired and sore from this hike -- it was a good time, but I slept well last night as a result!

Labels: blue ridge, falls, geomorphology, national parks, proterozoic, shenandoah, virginia

Amygdules (mineral deposits filling extrusive vesicles) in the Neoproterozoic-aged Catoctin Formation meta-basalt, Shenandoah National Park, Virginia.

Labels: basalt, blue ridge, igneous, primary structures, proterozoic

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

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 graduate of my summer Snowball Earth course just forwarded me a link to a news item about a recent discovery which finds biomarkers exclusive to the sponges during Marinoan-glaciation-aged sediments on the Arabian Peninsula. The strata are "at least 635 million years old."

This is significant because the usual line about Snowball Earth is that multicellular animals show up after the glaciations end, not during. So what's going on here? Looks like we didn't understand the thing as well as we thought. For many people, the co-incidence (in the most literal sense) in timing between the end of the glaciations and the first multicellular animal fossils was one of the most intriguing things about the Snowball hypothesis -- we all want to know where we came from, after all -- and this may take some of the wind out of those sails. As humans, we like a good story, and this may be one reason the idea of a Snowball Earth is such a popular notion: it's a dramatic story about where we came from, and one that stretches our conception of the limits of change on our planet. But now that story exhibits a flaw upon closer scrutiny, and it makes it less satisfying. The consolation prize is that event though the story isn't as neat, it's closer to the truth. That's the way science works -- especially earth science, which isn't often as tidy as a fairy tale.

Hat tip to Christina T. for passing this on!

UPDATES: (1) Chuck read the paper and wrote it up at his blog. (2) WIRED magazine is also reporting on this, calling the discovery the world's "oldest animal fossils." I'm not sure I agree with that phrasing -- but that probably stems from my lack of familiarity with the reliability of biochemical signatures over traditional body or trace fossils.

Labels: chemistry, fossils, geology, porifera, proterozoic, snowball earth

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

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


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


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


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


Same thing:


Same thing again...


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

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

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

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

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

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

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

This summer, I saw "the Great Unconformity" in a couple of locations.

An unconformity is a break in the local geologic record -- a period of time which elapsed without being recorded by the deposition of rock units. Often unconformities mark places where erosion has erased part of the local rock record, but sometimes they just mark periods of non-deposition. (Analogy: You can get a blank page in your diary two ways. You can either take a day off from writing, or you can write that day's entry and then later go back and erase it. Either way, you end up with a day going by and no journal entry.) People call the major break between metamorphic and igneous "basement" rocks and overlying sedimentary layers the "Great" Unconformity, though it's not the same age everywhere. It's just shorthand, really.

Anyhow, here it is in the Grand Canyon (photos provided below are both unadorned and annotated versions):





Give or take, there's about 1.2 billion years missing along this ancient erosional surface. Intuitively, this probably makes sense, since metamorphic rocks like schist and 'distilled' intrusive rocks like granite are characteristics of mountain belts, where they form at depth. In order to get those interior-mountain-belt rocks to the surface takes lots of erosion over lots of time (though not necessarily that long -- in DC, for instance, we have interior-mountain-belt rocks exposed that 'only' took 360 million years to make it to the surface). In the above photos, the metamorphic rocks and granites below the unconformity formed about 1.7 billion years ago, during the Mazatzal Orogeny, and the sedimentary layers on top (both quartz sandstones) were deposited in the Cambrian period, about 543-488 million years ago. They represent passive margin sedimentation along an ancient transgressive seashore, something like modern day beach sands along the east coast of North America. So, to get something like the Great Unconformity, take something like coastal Maine (Acadia National Park, say), and bury it beneath something like Virginia Beach.

And here "it" is again, in Wyoming's Wind River Canyon (between Thermopolis and Shoshoni):





A zoomed-in look at this same outcrop:





This time, however, the rocks below the unconformity are much older* metamorphics (schist & amphibolite) and granite. According to Maughan (1987), these are the oldest rocks exposed in Wyoming, having formed about 2.9 billion years ago. They were then metamorphosed at 2.75 billion years ago. These truely ancient rocks (Archean) were then eroded and exposed at the surface, where quartz-rich sand was laid down atop their burnished roots. Aside from the difference in the age of the underlying basement rocks, the story is very similar to the one at the Grand Canyon.

* Thanks very much to Kim, who pointed out my error in under-stating their age in an earlier, more-poorly-researched version of this post.

Reference:
Maughan, E.K. (1987) "Wind River Canyon, Wyoming." In: Geological Society of America Centennial Field Guide - Rocky Mountain Section. S.S. Buess, ed. p. 191196.

Labels: archean, cambrian, dc, grand canyon, primary structures, proterozoic, unconformities, wyoming

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

Here's Chief Mountain:


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


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


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


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


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


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


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


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


That's it for today... Enjoy!

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

This weekend, I took a backpacking trip in Shenandoah National Park. Thought I would share a few photos today: scenery first, geology second...

Here's the view looking east from Skyline Drive:


The temperature difference due to elevation was striking. It was still early spring up on the top of the mountains, on Skyline Drive:


...But down below, it was green and lush (and sodden with pollen!):


I camped out for two nights near Corbin Cabin, and did a day-hike around Thorofare Mountain on Saturday, visiting this waterfall at lunchtime:


The geology of Shenandoah National Park is interesting: it records the assembly of the early supercontinent Rodinia at about a billion years ago, and then the breakup of Rodinia about 600 million years ago. The first event recorded is the generation of granite gneisses and granites due to the Grenville Orogeny. The oldest unit in the park is the 1.1 Ga Pedlar Formation, a granite gneiss. There's a slightly younger granite which intrudes it called the Old Rag Granite (~1.0 Ga), but I didn't see any outcrops (or float blocks) of it, so I'll not mention it further. There's a thin, patchy sedimentary cover called the Swift Run Formation deposited directly atop the granite gneiss and granite, providing a nonconformity surface. Atop that is a series of volumnious tholeiitic basalt flows: these mafic extrusions record the breakup of Rodinia and the opening of a new ocean basin: the Iapetus. In many places in the park, you can see "feeder dikes" of the Catoctin cutting through the older plutonic and metaplutonic rocks (see image below). There are also some sedimentary rocks layered atop the Catoctin (the Chilhowee Group), recording the transgression of the Sauk Sea on the North American platform. But I didn't encounter any good outcrops (or float blocks) of them on this trip, so I'll stick to the tectonic story: the Pedlar Formation shows us Rodinia getting put together, and the Catoctin Formation shows us Rodinia breaking apart. Later metamorphism due to Appalachian mountain-building resulted in changes in both of these rocks (development of "blue quartz" in the Pedlar, and the Catoctin metamorphosed to greenstone).

Here's a massive dike (possibly a "feeder dike" feeding surface lava flows) of the Catoctin basalt cutting through the Pedlar Formation granite gneiss, just north of the Marys Rock Tunnel. Note the columnar jointing extending perpendicular to the walls of the dike:


Having covered all that, I now propose to spend the rest of this blog post showing you the variety of cobbles and boulders in my campsite. I camped at the little wedge of land above the confluence of two streams. One stream's catchment basin was Catoctin, and the other drained outcrops of Pedlar. As a result, the "float" in my camp was all either Pedlar Formation or Catoctin Formation. I'll just run through them one after another so you get a sense of the range of variety in each formation.

You'll notice that the Pedlar is sometimes coarse, sometimes fine, sometimes well foliated, sometimes not so much. You'll also notice that the Catoctin varies a lot in terms of its extrusive texture: sometimes aphanitic (fine-grained), sometimes amygdular (formerly vesicular), sometimes it even runs to volcanic breccia. All of these original lithologies have been metamorphosed to various degrees in the Catoctin, which here can be seen by comparing the amount of green in the rock. This green comes from two metamorphic minerals: chlorite and epidote. Enjoy!

Pedlar Formation:



















Catoctin Formation:

Labels: appalachians, basalt, blue ridge, granite, iapetus, metamorphism, national parks, primary structures, proterozoic, shenandoah, virginia

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

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

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

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


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

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


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


Still life with fun stuff:


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


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


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


Here's what's left of Grinnell Glacier:


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


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


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


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


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

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