This week we didn’t have to go far to find a quiet place on the Potomac River, its confluence with Goose Creek, where low bluffs of reddish sedimentary rocks watched over the calm water of this sometimes violent tributary (Fig. 1).
However, things aren’t as geologically simple as the sedate image in Fig. 1 would suggest. This photo was taken within a few hundred feet of the contact with thermally metamorphosed sedimentary rocks that were probably part of the original Balls Bluff Siltstone (Site C in Fig. 2).
We saw diabase in an earlier field trip. Those dikes and sills were part of the same intrusive episode, when North America split away from Europe. Diabase is a fine-grained, intrusive rock with a chemical composition like basalt.
I was unable to measure the orientation of the sedimentary rocks seen in Fig. 1, but my “field estimate” is that they were dipping ~30 degrees away from the camera, with a strike of about (you guessed it) 30 degrees east of north. Such an orientation is consistent with what we saw in a previous post.
The presence of diabase crossing Goose Creek (see Site C in Fig. 2) is indicated by shallow water and a rocky bottom just west of Site C (Fig. 3).
Because of the shallow water over the diabase intrusion in an otherwise navigable stream, a canal was constructed to run from this point downstream to deeper water, parallel to the stream seen in Fig. 2. This was the Goose Creek Canal, finished in 1859 to reach grain mills as far as twelve miles upstream.
Moving back to the Potomac River, there are a few more surprises for us on this field trip (Fig. 8).
This post worked forward in geologic time, in reverse to the counterclockwise path we followed (thin black line seen in Fig. 8). I was surprised to find so much geologic history on a short walk along the shore, surrounded by golf courses and million-dollar houses.
To summarize, as Pangaea began to be torn apart, the mixed sediments of the Balls Bluff Siltstone were deposited in grabens for millions of years, until oceanic crust intruded as diabase dikes and sills, heating these now-deeply buried sediments and altering them into what is called a hornfels metamorphic rock. The rocks we saw today were deposited and thermally metamorphosed between about 237 and 145 Ma (equivalent to millions of years), which is a very long time. But that 88 million-year span was brief compared to the 145 million years that have elapsed since these diabase intrusions forced their way into the picture.
As we’ve seen everywhere along the Potomac, modern fluvial processes are struggling to overcome tectonic events from a bygone era…