Exploring the Potomac: Red Rock and Balls Bluff

Observations at Red Rock Park

I have been exploring the Potomac River from Washington to Harpers Ferry in stages in recent posts. Most of the rocks we’ve seen were Precambrian schists, sedimentary rocks deformed during the closing of the precursor of the modern Atlantic Ocean (The Iapetus). The Potomac River has eroded into the roots of the ancient mountain range that was created by this event, superimposing floodplain processes on these metamorphic rocks. Today I travelled a little further upriver and found different basement rocks, which are the reason I’m excited about today’s post.

Figure 1. Photo of outcrop of layered rocks along the VA bank of the Potomac River at Red Rock regional park (see Fig. 2 for location). This photo shows debris piled against this obstacle during recent high water.

Accessing the river at Red Rock park required a short walk along a narrow ridge, left by erosion of gullies into the rocks.

Figure 2. Google map of the study area. Red Rock park is located at the bottom center of the image (indicated by green color) and Balls Bluff Battlefield park is located at the upper left (NW corner) of image. Harrison Island formed in this large bend of the river, probably caused by changes in underlying basement rocks (e.g., faults and lithology).

Figure 1 was taken at Red Rock park, after following a steep trail about 100 feet to the river bed (Fig. 3).

Figure 3. Looking upstream reveals ledges (gray horizontal objects) that create a series of steps as the water flows towards the Potomac.

A close-up of the outcrop seen in Fig. 1 reveals medium-to-thin bedded, fine-grained sedimentary rock with a reddish hue (thus the name of the park), as seen in Fig. 4.

Figure 4. Photo of intercalated blocky and fissile siltstones of the Balls Bluff member of the Bull Run Formation. These rocks were deposited in streams about 200 million years ago (Ma), and have not been metamorphosed. They are tilted, however, at an angle of about 20 degrees to the ESE (about 30 degrees south of east).
Figure 5. Photo showing irregular bedding of the fissile (contains more mud than the blocky layers above and below), typical of mud drape over silty sediments on a river floodplain. We saw modern examples of this in recent posts. We’ll see some examples in this post. This was an environment not that different than what exists today in this area.
Figure 6. Hand sample of siltstone from Red Rock park, showing the characteristic red hue of terrestrial sediments. Siltstone is formed from the tiny slivers of quartz broken off large boulders as they roll along river beds. Thus, the grains are not round and there is often a lot of mud available, from the weathering of mica, feldspars, and other minerals contained in their parent rocks. In this case, the parent rock is unknown, having weathered 200 Ma to form these rocks. It was probably granitoid rocks formed at shallow depths during the Grenville orogeny (when the schists we’ve been seeing were deformed at greater depths).

The sedimentary environment implied by the siltstones and mudstone we see in Figs. 4-6 has (coincidentally) been reproduced today, with weathering of these same rocks and others found further west.

Figure 7. Photo of narrow floodplain at Red Rock park, showing a Pleistocene river bar on the left and a swale to the right. These sediments overlie the Triassic Balls Bluff rocks and they are finer, being recycled sediments from a previous epoch.
Figure 8. Photo of stream eroding a channel around the relict bar seen in Fig. 7, following more easily eroded sediments before entering the Potomac on its muddy shore (Fig. 9).
Figure 9. Terminus of stream seen in Figs. 3 and 8, revealing a muddy bank (seen along stream crossing image) cut into sediments deposited by the Potomac on this narrow flood plain, restricted by bell-shaped bluffs less than 100 feet in height.

Summary of Red Rock Park

Fine-grained sediments were transported along rivers similar to the modern Potomac about 200 Ma and deposited on a flood plain like we see today. This was when the modern Atlantic Ocean was just beginning to form as the supercontinent Pangea was being torn apart. There would have been mountains much higher than the modern Blue Ridge to the west, and a narrow but widening (~1 inch/year) ocean to the east. Erosion of the ancestral Appalachian Mountains continued for the next 200 Ma, creating thick piles of sediment on the continental shelf of North America, depressing the earth’s crust and burying even terrestrial sediments deep enough to create the Balls Bluff siltstone from mud and silt. As erosion wore these mountains down, the crust rebounded to expose these ancient rocks to the ravages of water and ice. Now, these sedimentary rocks are being eroded as the process continues.

Requiem: Balls Bluff National Battlefield

A little further upstream (see Fig. 2 for location) is the probable type-locale of the Balls Bluff siltstone, but good exposures weren’t accessible because the bluff is higher and the bank narrower, there being no floodplain as we saw elsewhere (Fig. 7). In fact, the only rocks we saw were at the top of Balls Bluff (Fig. 10) and along a trail that led to the river, where I was able to estimate strike and dip.

Figure 10. Photo of outcrop at the top of Balls Bluff. I was prevented from getting closer by a fence, a safety feature because the bluff is more than 100 feet high and quite steep. The reddish color is due to the presence of oxygen in the pore waters when the original sediment was buried (it had no where to go), which led to oxidation (rusting) of Fe-bearing minerals (e.g. the ubiquitous clays that would have replaced original, plutonic and metamorphic minerals) in the Triassic river sediments.

These rocks contained original sedimentary layering and I was able to estimate that they were dipping to the WNW at about 20 degrees, with a strike similar to the rocks at Red Rock park (i.e. about 30 degrees east of north). This is an important finding, because this is the opposite to what we saw only a few miles downriver. I’ll try and summarize this interesting observation briefly.

Tilting of layered rocks like these siltstones can occur by either folding or faulting. Check out the links to understand these processes. Folding creates great arcs of rock, like sine waves, or ocean waves, as lithified sediments (hard rock) are compressed from both ends. This is what led to the steep folds we saw in the Precambrian rocks at Great Falls and in Lynchburg; our limited observations can’t allow us to decide what happened to these rocks on our own, but we can turn to reliable resources. The sediments that formed the Balls Bluff siltstone have never been compressed; we know this because the Atlantic Ocean is still spreading at a slow and steady rate of about 1 inch/year.

Rocks also become tilted when the earth’s crust is stretched. Even though buried deeply, if they are not in a metamorphic pressure-temperature regime, rocks break and slide around to form faults. This is a well-understood phenomenon that is occurring today in the Basin and Range province of western North America. It doesn’t take much imagination to picture new ocean crust appearing while a continent (Pangea) is being torn apart, snapping rocks buried several miles beneath the surface, like a slab of concrete being removed by a bulldozer.

Crustal stretching produces a series of opposing normal faults that create grabens. These collapsing structures occur at every scale, from outcrops to the birthplace of Humans.

Closing Thoughts

It was good to see some younger rocks, especially Triassic river sediments that are direct evidence for the splitting apart of Pangea. It was a bonus to discover evidence of block-faulting of these same sedimentary rocks after they had been buried several miles beneath the surface. Geological processes occur on time scales of millions of years, with annual displacements of inches or less. Because of the juxtaposition of fast, river-based erosion and deposition and the slow pace of plate-tectonic movements, these rocks record their entire life cycle.

And it continues to this day, as slivers of quartz and oxidized clay minerals are transported yet again towards the same ocean into which they were originally flowing, before being trapped on a primordial floodplain.

Maybe they’ll make it this time…

Figure 11. Photo of stream cut ~15 feet into fine sediments at Balls Bluff. The bank revealed overall fine sediment with occasional boulders (~2 inches in diameter). This is a very dynamic and restricted environment, where rapid downcutting is excising young sediments and reintroducing them to the Potomac River…

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