Archive for May, 2010

In part two of my Henson Creek fossil series on this blog I will be discussing Ostrea compressirosta. This bivalve is a member of the subclass Pteriomorpha like the Cucullaea Gigantea mentioned in my previous post, though this one adhered to the surface of the seafloor as opposed to burrowing into the sediment. Both were filter feeders that strained their nourishment from the water using a siphon.

O. Compressirosta ranges in age from the late Paleocene to the late Pliocene, but is found in Maryland only from 58.7 – 55.8 million years ago. Younger fossils have been found in Virginia, North Carolina, and Florida (I’ve heard a lot of people like to retire there, too).

This guy is also a member of the family Ostreidae, which are known as “true oysters” and the kind that we, as humans, like to dip in cocktail sauce. Unfortunately for us, we will never know what this particular oyster tastes like. Like all oysters, O. Compressirosta is monomyarian, meaning it has only one abductor scar from the muscle used to close the two shells. As you can see from the pictures below it also had irregular shell shape. The shell of this particular fossil would have been composed of calcite that weathered away leaving a sandstone cast like the C. Gigantea found in the same location. Oysters like these reproduce hermaphroditically, possessing both the larva and egg producing abilities sometimes switching between the two based on circumstance.

Try not to think about this post next time you are at Red Lobster. Sorry.


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A few years ago I took a Geology Field Studies class through NVCC. One of the trips we took led us to a creek in Maryland just off the Capital Beltway. I believe it was Henson Creek, but it has been awhile and the memory is a bit foggy. There was no path so we rolled up our pant legs and tromped through the creek, where the glauconitic quartz sandstone, Aquia Greenstone Formation, is exposed nicely. It wasn’t until we had walked in about 150 meters that it was brought to our attention we were walking all over these guys:

That is Cucullaea Gigantea, a now extinct bivalve from the Paleocene ranging in age from 58.7 – 55.8 million years old. These guys were infaunal suspension feeders, meaning they burrowed into the seafloor and strained their food from the water. By now the shell had completely eroded away, but what was left is a perfect mold of the interior composed of the greenstone. When this creature died a small opening was left so that sediments could fill in and later harden into rock. As time passed the more easily weatherable shell went away and all that remained was the more durable greenstone. C. Gigantea is about twice the size of its relatives such as Cucullaea recendens averaging about 8 – 14 cm in length, while C. Recendens averages 5 – 8 cm. Finding marine fossils in a Maryland creek bed also shows that it was once a marine environment. Along with a plethora of C. Gigantea fossils I also found some Ostrea compressirosta fossils, which I will talk about in a future post.

So, if you ever feel like going fossil hunting in the Washington DC area, Henson Creek in MD is bound to get you a handful of these guys

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For the people (both of you) who requested the video:

At 0:17 the video switches to a point when the temperature was hotter. Notice how the movement becomes more intense.


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Route 55

Mummy bags are great. I have one that keeps me warm in cold temperatures as low as 0 °C. Problem is, the temp dropped below freezing our last night of camping. We know this because our water was partially frozen the next morning. As the night progressed, the further I slunk into the footing of my mummy bag. Despite my desire to get there I am apparently larger than the space allots. The point is I was cold, and everyone else was too. But a nice cup of coffee in the morning did the trick in warming me up on the last day of our trip. Today we were venturing into West Virginia to take a look at the newly constructed Route 55 and the fresh outcrops that were made in its path.

We made two major stops; the first of which (Google Earth file) included a large anticline exposed on both sides of the road. I climbed to the top of the southern end and Nik climbed to the top of the northern end so we could determine the axial hinge’s trend of 204° from the north and 026° from the south. While up top it was noticeable that the mountain to the north of the one I was on exhibited the same sense of folding.

The outcrops are composed of the Tuscarora Sandstone which is the same lithology as the Massanutten Quartzite, just further west. Within this sandstone were other features such as: cross bedding, faults, ripple marks, and more mud rip-up clasts like seen in the Swift Run Formation. The cross bedding shows that the current direction during deposition was to the west; which is attributed to an ocean basin being located on the western edge of these mountains during the Silurian.

While the faulting seen exhibits dextral behavior on the northern side and sinistral on the southern side, both are evidence of West over East thrusting. On the northern fault we also found the presence of slickenlines and a brecciated layer.

The Tuscarora would have been deposited after the previous Appalachian Orogenies, the Taconian and the Acadian, and then folded broadly during the Alleghenian. Similar to that of the Massanutten Sandstone, but since it was further from the collision center not as severe deformation.

The second stop we made (Google Earth file) was further west than the first, and now the depositional environment was different. At this outcrop instead of sandstone we found river / floodplain mudstone deposits of the Hampshire Formation put in place after the Acadian Orogeny. At this outcrop there are two more folds, but less intense than the more eastern examples seen on this trip. On the northern end a very broad anticline, and a monocline on the southern end.

Here we found a plethora of smaller scale structures such as: plumose (lots of plumose), flame structures, drag folds, and ripple marks though not in citu. The most impressive to me being the flame structures which are examples of soft sediment deformation, and my first seeing them in outcrop. They are formed when the overlying heavier sediment pushes down onto the softer sediments which then propagate up giving the look of a flame.

Piling back into the vans the structural students and professor travelled the few hours back to campus. Though tired, spirits were high.  The unlucky ones that gave in to slumber’s temptation found themselves bearing the brunt of Brunton compasses across their face. Once back at the school, bags and rock samples were collected into individual cars and the remaining snacks were divvied and devoured. Farewells were said, and all made way back to their homes.

I had a great time, learned / saw a lot and was too worked up to fall asleep before a reasonable hour that evening. Work was terrible the next day.

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Veach Gap

The rest of the drive up Skyline was enjoyable as always, and there were a few extra stops along the way. One stop was at the Little Stony Man parking lot, where just off the trail a bit we were privy to a 400 Ma unconformity of the Catoctin lying directly on top of the basement complex. I have always enjoyed unconformities and been awe-struck by the incredible span of time represented on such a small scale. In the image below Danny’s finger bridges the nearly half a billion year gap.

Another stop along the “Drive” was to indentify numerous basalt feeder dikes along an outcrop. If you look real close you can see the one in between Alan and Nik below.

After leaving the park we headed westward into the Fort Valley of Massanutten Mountain. Here is where we would make camp for the evening at a nice spot in the woods next to a creek bed. We staked claim to the site and setup our tents before climbing back into the van with one more stop planned for the evening.

Back in the valley we drove eastward this time cutting through farm land with a watchful eye on the mischievous wildlife. Once parked, we set foot into the Veach Gap (Google Earth file), originally commissioned by George Washington as passage for Structural Geology students. After a long hike through the shade we came to a point where our fearless leader guided our eyes to a cliff on the southern side, oh and some amazing anticlines behind us, but that talus slope…man! (We all took the bait…hard).

After wiping some egg of our faces we giddily skipped through the trees to the many well displayed anticlines. In total we found about five all composed of the Massanutten Sandstone. Below Alan and Danny do little to contain their excitement next the tectonic feature.

Using Jeremy’s GPS we measured the approximate distance between each axial hinge and then calculated a wavelength of 12.8 meters. The trend of the axial hinges were within a range of 220°- 250°, and plunged within a range of 10° – 21. Though the shapes varied, all of the anticlines were asymmetrical and similar.

These are parasitic folds located on the larger Massanutten anticlinorium which were deformed during the Alleghenian Orogeny (again), and are all composed of quartzite. Confirming that these are in fact anticlines and not overturned synclines, further up the hillside we found cross bedding in another outcrop indicating these are upright.

After a full day of geologic excitement the group made our way back to camp, stumbling across some free firewood along the way. Once back it was a hijinx-filled evening including undercooked veggie dogs and stories of unintentional self-harm to quell the fear of ghosts (had to be there).

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After spending an evening around a campfire spinning some good ole’ Ben Roethlisberger yarns, and good night sleep was needed. Unfortunately, we were in for a night of hurricane force winds. Not really, but it was pretty windy and the tent kept slapping me in the face all night. It must not be a fan of my low-brow satirical sense of humor.

Today we hopped back in the van and headed out towards Skyline Drive; which if you don’t live in Virginia, is where city folk bring traffic to rural folk. It is also a great place for some pretty spectacular views of the Commonwealth and West Virginia, and some pretty decent hiking trails.

After making a few stops along the road at some beautiful, if not incredible windy, overlooks, we parked and set out down the Limberlost Trail (Google Earth file). What the Limberlost trail lacks in strenuous hiking it makes up in a two geologically exciting outcrops of columnar jointing in basalt flows. This basalt is part of the Catoctin Formation which flowed to the surface during the rifting of Rodinia. The Catoctin is younger slightly as the Swift Run in found beneath it. As the basalt cooled, it contracted and cracked, leaving (for the most part) hexagonal columns with arrest lines running perpendicular to the joint surface, for every period of jointing.

The columns would have sat there looking like the Giant’s Causeway of Ireland for a few hundred million years until the Alleghenian Orogeny (AO) knocked them around. Taking the strike and dip of the columns, we were able to determine another instance of East over West deformation; further evidence of the AO coming from the east.

The typical case of columnar jointing exhibits six sides with 120° angles between. After deformation the “tops” (since we were unable to determine paleo-up) of the columns were sheared and stretched to create larger angles or in some cases shortened (100° and 90°). The arrest lines which were originally perpendicular to the edges are now at angles of ~106°.

After getting our gentile sufficiency of columnar jointing we headed back down the fire road to our vehicles, and were off to our campsite and then one final stop for the day (possibly my favorite of the trip).

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After leaving the Garth Run outcrop we drove collectively in our vans (one being far superior to the others) to an area along Route 33 just near the entrance to Skyline Drive. Here we pulled over, and piled out of the vans and into the woods. There was no trail for us to follow here, just a creek bed and an intrepid guide/professor. But, even in the rough overgrowth each member of the excursion made sure to mind their footing and the branches of troublesome trees… well, some of us anyways (not naming names). Even so, all of us were troopers and muscled on motivated by the promise of pizza.

At the outcrop itself we saw sedimentary rocks instead of igneous/metamorphic this time. The Swift Run Formation (Google Earth File) is comprised of arkose sandstone and some finer sedimentary rocks including mud rip-up clasts that were not easily recognizable at first glance. Originally thought to be some type of xenoliths, these mud clasts were put in place by a large increase in depositional energy that tore these mud chips from the bottom and then displaced them amongst the sandstone. Further along the outcrop there was a rock that exhibited the transition from coarser to finer grain sizes drastically.

There was also some interesting tiger-stripe-like foliation seen.

There was also evidence for tectonic effects seen in some folding. The first fold found was an overturned syncline that was only about 10 cm in length and found at the base of the outcrop. It was noticeable because the darker mud layers contrast nicely against the lighter grained arkose. Interesting about this fold and the second set found were smaller, in this case parasitic, S-folds found. In both cases the s-fold has an East over West, or sinistral orientation. This is because the orogeny was coming from the east, and would have pushed the bedding layers in such a direction.

The Swift Run Formation was deposited during rifting of the super continent Rodinia. The sediments would have been deposited in an environment similar to modern day the Culpeper Basin. It overlays the basement complex seen at the first outcrop, but is underneath the Catoctin basalts that were deposited over top as rifting of the continent progressed. The folding witnessed here would be attributed to the Alleghenian Orogeny, which will be a common theme through the next couple of posts.

After about an hour of pressing our faces up against the rock we made the trek back through the brush to the vans.  We pressed onward to find the elusive 10-topping pizza.

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