28 May 2014
Last week, the “Border to Beltway” field exchange team went to Dora Kelly Park in Alexandria, Virginia. There, a ravine reveals the boundary between the Coastal Plain and the underlying metamorphic rocks of the Piedmont…
My NOVA colleague Ken Rasmussen joined us for the day, leading us deep below the soil profile to examine the basal nonconformity beneath our youngest geologic province:
The ravine was dark, so the next few photos were taken with a flash, and thus don’t have color saturations that match reality…
Here are Cretaceous Potomac Formation gravel deposits, with differentially weathered cobbles. The 100% quartz cobbles stick out as three-dimensional features, while the granitoid cobbles have been thoroughly chemically weathered, and are planed off to a two-dimensional representation on the saprolitic outcrop surface:
Hark! What new feature is that, below the gravels?
Why, it’s the Piedmont! Specifically, the Indian Run Formation, a schisty unit:
Within the Indian Run Formation are exotic clasts, including this one with a pre-existing foliation (at an angle to the foliation of the surrounding schist):
Are these clasts dropstones? Are they blocks within a tectonic melange? Such mysteries!
Downstream, at Holmes Run, Ernie examined a clean, non-saprolitic schist sample with his hand lens:
Next up: Henson Creek, in Oxon Hill, Maryland. There, we visited the Aquia Sandstone (Paleocene), a glauconitic sandstone that was chock full of fossils:
Here’s a nice Turitella in cross-section:
The big ones are Cucullaea gigantea, a massive bivalve.
Here’s a Cucullaea in cross section:
And here is one weathering out as a steinkern (internal mold) in the stream bed, catching organic debris in its umbo:
Two more, with deer tracks and a rock hammer for scale:
The students had a blast collecting these massive clam fossils:
We also saw another unconformity at this site – here’s the Aquia sandstone overlain by Pleistocene “Wicomico” gravels (reworked Potomac Formation cobbles):
And here, another layer appears in between them – this is the Marlboro Clay, a thin gray unit of probable late Paleocene or early Eocene age (this contact is therefore approximately centered on the PETM). It too has the Wicomico gravels on top of it, and some are pressed down into the mud:
Our final site was Scientists Cliffs, Maryland, on the shore of the Chesapeake Bay, where we saw Miocene Calvert Formation, which was also full of fossils, like this oddly asbestiform Pinna.
We spent several pleasant hours there, searching the beach for shark’s teeth, discussing coastal erosion, and imagining the violence of the Chesapeake Bay bolide’s impact 35 million years ago. As we wandered back toward the vans, I was struck by the verdant bluffs above the brackish water…
… A great day in the field, indeed. Thanks to Ken for leading us!