Goose Creek

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).

Figure 1. Exposure of Balls Bluff Siltstone (237-201 Ma) on Goose Creek (photo looking north). These mixed sedimentary rocks have a reddish color (indicating deposition in rivers and lakes). They form a bluff ~20 feet high here. See Fig. 2 for location.

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).

Figure 2. Geologic map of the confluence of the Potomac River and Goose Creek. Figure 1 was taken at Site C, at the contact between the reddish and the purple areas. The metasedimentary rocks have been dated between 237 and 174 Ma, suggesting that they are part of the Balls Bluff Siltstone. They were altered by intense heating when several diabase intrusions (201-145 Ma) forced their way into the sedimentary rocks.

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).

Figure 3. Photo looking upstream along Goose Creek (west of Site C in Fig. 2), showing riffles over a rocky bottom.

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.

Figure 4. The stone lock constructed at Elizabeth Mills to allow boats to circumvent the shallow water seen in Fig. 3.
Figure 5. Gravel bed of the Goose Creek canal, west of the lock seen in Fig. 4.
Figure 6. Abutments to a bridge that spanned Goose Creek when there was an active mill at the site. The bridge was burned by the Confederate army in 1862.
Figure 7. Regolith soil profile near Site C, showing fine-grained sediment deposited by modern Potomac during flood, over boulders deposited by Goose Creek. Of course, there is also a contribution of material used to prevent erosion, including a few blocks of finished stone eroded from the Goose Creek canal’s sidewalls.

Moving back to the Potomac River, there are a few more surprises for us on this field trip (Fig. 8).

Figure 8. Summary of geology along the Potomac River downstream of its confluence with Goose Creek. The river’s flow along this stretch is determined by an outcrop of basement rock (probably Balls Bluff) at a narrow point (Plate 4). This resistant constriction (yellow area in the map) created a pattern of oscillations along the western shore for thousands of years(squiggly red lines in the map) that constructed minor headlands that collect debris during high river flow; Plate 1 is looking downstream whereas Plate 2 looks upstream. These two snags are separated by less than 300 feet. Plate 3 shows raindrop imprints in mud which will, if covered over before being disturbed (like in a sudden flood) become a feature recorded in sedimentary rocks.

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…

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