The Dynamic Potomac

Figure 1. View of the south channel of the Potomac River looking toward Tenfoot Island (see Fig. 2).

This post returns to the Potomac River. We have previously discussed several features along this stretch of the famous waterway: Precambrian metamorphic rocks at Great Falls; sediment contributed by tributaries as well as erosion; and emplacement of intrusive rocks during rifting of Pangea to form the Atlantic Ocean. This time we’ll see evidence of recent erosion, as evidenced in Fig. 1, which shows a large block of stone that has collapsed along the steeply eroded south bank. The bank consists of silt and clay just like further upstream.

Figure 2. Study area in Northern Virginia (see inset), showing locations for other figures in this report.

Site A (see Fig. 2) is where Fig. 1 was taken. The meandering stream to the east in Fig. 2 is Sugarland Run, which we examined further south, near its headwaters, in a previous post. The Rock-D geologic map suggests that the river is underlain by a Triassic (237-203 Ma) fining-upward sedimentary sequence consisting of sands to shales. A close-up image of the boulder at Site A (Fig. 3), despite a covering of mud and some biological material, reveals no apparent bedding. This is contrary to the description of the Balls Bluff (sedimentary) Member of the Bull Run Formation, contemporaneous with the Newark Supergroup although no longer considered stratigraphically equivalent.

Figure 3. Close up of boulder shown in Fig. 1, revealing a reddish, fine-grained texture. No clean surfaces were available, but neither individual crystals nor bedding are visible . Image is about 2 inches across.

Figure 3 doesn’t look sedimentary to me, but more like the diabase we saw further upstream on Sugarland Run. The streak of white material in the upper-left corner looks like quartz. If these are Triassic sedimentary rocks, they wouldn’t have been metamorphosed, so this exposed block is anomalous. It is possible that this is an outlier of sedimentary rocks that were thermally altered when intrusive rocks were emplaced during Triassic. The area consists of faulted and folded diabase, sedimentary rocks, and metasediments–intruded, deposited or altered, respectively, during the breakup of Pangea during the Triassic period.

Figure 4. Looking upstream with Tenfoot Island to the right. Note the trees falling into the river along the recently eroded shoreline. The concrete and steel is the remains of an electrical power plant that was removed.

The bank is steeply eroded with trees collapsing into the river, indicating rapid lateral erosion during the lifetime of a typical tree (less than a century).

Figure 5. Image along the south bank (Site B in Fig. 2), showing logs deposited during a previous flood. The bank is low at this location, the swale winding landward around a relict river bar, deposited before the Potomac cut downward to its current elevation.
Figure 6. Downstream end of Tenfoot Island with Maryland on the other bank (see Fig. 2 for map view).
Figure 7. Natural levee along the VA side of the Potomac River at Site C (see Fig. 2 for location).

This section of the southern flank of the Potomac River is characterized by a wide floodplain covered with hummocks that represent bars on the original river bed. Superimposed on this older morphology is a natural levee (Fig. 7) that varies in height, steepness, and distance from the modern channel along the river. This landscape has been cut by numerous streams that drain the Pleistocene highlands overlooking the incised river.

Substantial islands (Figs. 5 and 6)divide the Potomac river into two anastomosed channels (see Fig. 2) that have become unstable during the last ten thousand years, as demonstrated by rapid downcutting and lateral instability (Figs. 1 and 4).

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