The Last Few Miles

Figure 1. View looking uphill, along a small ravine in Rock Creek Park. This is a small tributary that shows what this post is about: the stream bed is interrupted by layers of rock every few yards.

This is going to be a brief post, mostly because it is very difficult to convey what I want to communicate in photographs; the camera lens (on my iPhone) simply doesn’t capture image depth well. For example, Fig. 1 was actually pretty steep, but it looks as unintimidating as my driveway.

Figure 2. Topographic map of Rock Creek Park. Note the steep gullies leading to Rock Creek from the west (indicated by dark shading). Figure 1 was taken in the deeply incised terrain east of the Nature Center (top-left of image).

I’ve been talking about the bedrock exposed along the bed of the Potomac in several posts (e.g., Geological Bottleneck and Great Falls), but those are specific locations. Those significant drops in river elevation are part of a larger pattern, one that is displayed even at the scale of Fig. 1. It doesn’t take much of a drop to generate enough potential energy to spin a waterwheel (Fig. 3), which can do a wide variety of work–from grinding corn, to operating a machine shop.

Figure 3. Waterwheel at Peirce Mill used to grind grains like wheat and corn into meal, constructed in the early 1800s, at the lower part of Rock Creek Park (bottom-center of Fig. 2). The stream’s flow was subdivided by channels like that seen in the left of the image to supply water to several mills in the area.

The staircase structure of streams along the transition from crystalline rocks to coastal plains (aka the Fall Line) is so important to the ecosystem that artificial barriers were constructed within the park to ameliorate the impacts of road and bridge construction (Fig. 4).

Figure 4. View looking downstream from a bridge near the top of the map in Fig. 2, showing blocks arranged to replicate the natural steps as seen in Fig. 1. This construction was completed to reintroduce the herring migration. They spawn in the upper reaches of Rock Creek.

Rock Creek National Park deserves its name, not just because of its rock bed. Cambrian sedimentary rocks exposure along the steep tributaries leading to the creek (river?) suggest that bedrock lies not very far beneath our feet (Fig. 5).

Figure 5. Large exposure of Cambrian sedimentary rock formation (image height is about 20 feet), consisting of interlayered sand, siltstone, and shale. Where sand is the predominant component, blocky outcrops like this occur. Siltstone and shale produce more fissile outcrops.

Water has been struggling with rocks for the last 200 million years, always trying to reach the sea. It exploits every nook and cranny in the bedrock until it forms a stream, then a river, and it cannot be stopped. Thanks to the perseverance of water, driven by the steady pull of gravity, the first European immigrants to North America were able to establish a toe hold on what was (to them) a new land…

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