Geological Bottleneck

Last week’s post showed some of the effects of erosion along the banks of the Potomac River, which flows lazily along a broad floodplain while not becoming so sluggish that it meanders. We know this condition doesn’t last, however, because the broad and anastomosed channels of the Potomac are forced into a narrow throat bordered by immovable Precambrian schist, as we discovered in a previous field trip. Today I am going to approach this series of cataracts from upstream and document the changing river morphology (Fig. 1).

Figure 1. Overview topographic map of the Upper Potomac study area.

Previous posts have described the floodplain morphology from Algonkian Regional Park (circled area to the upper left of Fig. 1) to the west (upstream), a terrain defined by relict riverbed topography incised by modern stream erosion from the surrounding terraces. At the other extreme, we visited Great Falls in a previous post, where we discovered Precambrian schists that resisted the river’s erosional power.

The field trip began at the Nature Center (black circle at top of Riverbend Park inset map). We followed a trail over poorly sorted gravelly sand (Fig. 2) cut by numerous channels, through what appeared to be a mature and healthy forest. I don’t know what kinds of trees they were but they were at least 80 feet in height.

Figure 2. Poorly sorted sediment including rounded boulders up to three inches in diameter.

The assortment of gravel and boulders seen in Fig. 2 is classified as a conglomerate. The rounded boulders and poor sorting suggests that these are fluvial. The boulders became round by rolling along the bottom of the river. This conglomerate (sediment is an unconsolidated rock to a geologist) is matrix supported, which suggests that high-flow events were common, but the majority of the sediment was the product of chemical weathering of rocks like the diabase we saw in a previous post. (Minerals with complex compositions react to water easily, compared to quartz.)

I will return to this later in the post.

However, there were angular pieces of a fissile, dark rock showing up as regolith along the trail (Fig. 3), suggesting that the conglomeratic sediments were a thin veneer over resistant bedrock.

Figure 3. Photo of a loose fragment of bedrock along the trail. Sometimes, smaller pieces of this rock (probably meta sediments but not quite schists) appeared to have been intentionally laid along the trail in places. (They weren’t.)

The trail dropped to the river where outcrops of resistant bedrock appeared within the shallow river channel (Fig. 4). This was a substantial change from a few miles upstream

Figure 4. The shallow Potomac channel seen from VA, showing outcrops and the bifurcation of the river into two channels, as suggested by numerous islands upstream.

Our path follows the red line along the river bank in the Riverbend inset map of Fig. 2. We are approaching Great Falls. The trail is no longer constructed on conglomerate, but now is traversing sandy silt sediments deposited during the Holocene epoch (Fig. 5).

Figure 5. The trail is following deeply eroded fine sediments. Note the young trees leaning precariously towards the river, a sure sign of rapid erosion.

Schistose rocks with nearly vertical layering appear along the riverbank, and begin to obstruct the trail (Figs. 6 and 7).

Figure 6. Fissile rocks (probably Precambrian schists deformed during the closing of the Iapetus Ocean) are now nearly vertically oriented and, despite significant mechanical weathering, have resisted the Potomac’s onslaught.
Figure 7. Massive exposure of Precambrian schist along the trail, less than 100 yards from the river.
Figure 8. Closeup of an exposure of schist. Note the rounded block of rock bounded by flow lines, at the top of the image. More malleable rock flowed around this large clast during medium grade metamorphism. Although not seen in the photo, the rock had a distinctive sheen caused by the alignment of platy muscovite crystals. The image is about 12 inches across.

Our short hike led to the Aqueduct damn, which supplies water to Washington DC (Fig. 9), where the river transforms into a raging torrent that is challenged only by experienced kayakers.

Figure 9. The Aqueduct dam maintains a minimum water depth to supply sufficient flow for the DC area’s municipal water supply.
Figure 10. View looking upstream from the Washington Aqueduct dam. This image shows a placid river flowing around islands, eroding its banks (note the young tree falling into the channel), and collecting behind the dam (Fig. 9). It doesn’t get any better than this.
Figure 11. Downstream view from the Aqueduct dam. Outcrops of Precambrian schist, metamorphosed sediments from a previous ocean deformed by the collision of continents to form Pangea, are more numerous. The channel is now defined by the rocks and not the erosion of the river. The rocks are in charge.

The transformation of the Potomac, from the placid stream in Fig. 4, to the convoluted and dangerous channel that follows no commonsense rules of river flow seen in Fig. 11, took place in less than two miles (see Fig. 1). I know because I walked the river bank, passing from one era to another before being confronted by a past that will not die…

There is one last point I’d like to make in this post. The imposition of Holocene erosion–streams fed by the Pleistocene highlands bounding the Potomac floodplain–applies here as well as in the gentler topography we saw upstream. The transition from the conglomerate we saw at a major stream draining into the Potomac (Fig. 2) to the more typical fluvial sediment (sand/silt/mud) we found further downstream (Fig. 5) reflects the input of erosion of bedrock only a few miles from the Potomac. This was documented in a previous post, which showed the breakdown of regolith into cobbles, which were transported inexorably to the Potomac.

It is my opinion that this is what has been recorded in the rocks along the banks of the Potomac River, creating the juxtaposition of sediment types along the path of a river that is draining the roots of an ancient mountain range.

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