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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

And a close-up of the same slab:

And lastly, a nice slab showing tool marks:

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

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