Great Falls, Virginia: Deformation of Precambrian sediments when North America collided with Europe

This is my first post from Virginia. We went to Great Falls National Park on the Potomac River, only a few miles from our home in Northern Virginia. This is a fascinating area that reveals evidence of several cycles of collision between North America and Europe.

Figure 1.

The inset map shows where we went, situated between the Blue Ridge and Coastal Piedmont provinces. Notice the linear topography of the Blue Ridge, which is the result of crustal deformation when North America collided with Europe about 500 million years ago (Ma). The inset photograph (taken from the park’s web site) shows rocks that have resisted erosion and created the narrow gorge south of the circled area on Fig. 1. This narrow border between Virginia and Maryland is called Mather Gorge. The 76 foot drop in elevation is a local manifestation of what is called the “fall line,” where rivers drop out of the Appalachian Mountains to the coastal plain.

A photograph of the metamorphic rocks at this location reveals a polished appearance, with quartz veins (white areas) standing out against the softer matrix.

Figure 2. The yellow highlights indicate areas to concentrate on in the left two panels. In the right, they are intended to focus your attention on the white lines of quartz within the darker matrix.

Sediments deposited during the Grenville Orogeny included every imaginable lithology, from conglomerates to muds, over millions of years. These sediments were buried many miles beneath the earth’s surface and heated by the collision of tectonic plates about 500 Ma, when the proposed Iapetus Ocean was closing to form a supercontinent called Pangea. It is always difficult to infer original orientation of the precursors to metamorphic rocks like these schists, but they have a pronounced orientation of layers. For example, note the near-vertical lineations seen in Fig. 2. This overall structure is also visible at larger scales.

Figure 3.

Figure 3 is looking towards the northeast. Note the dark lines between layers of schist on the left (western) side of the photo. These lineations are approximately aligned with the quartz veins and blobs from Figure 2. Compare this image with the inset map from Figure 1 which shows the topography of the Blue Ridge mountains. The NE-SW orientation at the regional scale (inset map of Fig. 1) is seen at the outcrop scale (Figs. 2 and 3) at Great Falls.

Looking southward along the Potomac, into Mather Gorge (Fig. 4), everything falls into place.

Figure 4. Photo of Mather Gorge looking south from the falls.

I admit to some geological speculation, but I won’t suggest anything unrealistic. Compare Figs. 3 and 4, which look upstream and downstream (respectively), and then glance at Fig. 1, which shows how the Potomac River transitions from a broad riverbed flowing around resistant islands to become restricted to a narrow channel. The range of intermediate scale (what we could see during our field trip) structures in Fig. 2 suggest that Great Falls was a transition zone, where the stress regime changed suddenly when these rocks were deeply buried, possibly because of a change in the lithology of the original Grenville rocks, maybe because of a sudden change in the rate of plate-tectonic movement.

To conclude this post, I’d like to add that the rocks seen at the observation point (Fig. 2) were subjected to the same forces that the rocks at the bottom of the current Potomac River (Fig. 3) are experiencing. That’s why the outcrops are so rounded and the quartz veins and blobs stand out in relief. Quartz is a hard mineral (7 on the Mohs scale). The river excavated Mather Gorge with the help of boulders rolled along its bed for millions of years, polishing the very hard rocks that resisted erosion.

See you next time.

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