A few photos from last May’s spring field trip with the Geological Society of Washington… Here’s the group at Chain Bridge Flats (far westernmost-Washington, D.C.), looked at the metamorphic rocks there — a metagraywacke melange known as the Sykesville Formation.
Another group shot, with field trip leaders Tony (khaki shirt) and Gary (red jacket) Fleming in the foreground:
Euhedral metamorphic pyrite crystals (porphyroblasts):
An elusive bedding plane in the Sykesville Formation (a rare thing to see, as the rock has been pervasively metamorphosed and deformed):
Annotated version of the same, highlighting the grain size change that defines the bedding plane:
Boulder of Cambrian-aged Antietam Formation quartzite, washed ~25 miles downstream by the Potomac River, bearing characteristic Skolithos trace fossils.
A close-up of the side of this boulder, showing another trace fossil, Diplocraterion, as well as one of the Skolithos tubes.
Annotated version of the same photograph:
Finally, another piece of the Antietam Formation, this one only cobble-sized, showing another example of Diplocraterion:
GSW field trips are free and open to the general public. If you’re in the D.C. area, watch the D.C. Geology Events website for opportunities like this, and then come on along and join the fun!
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