By: magic-gigapans
Image: Robin Rohrback, Mid-Atlantic Geo-Image Collection
License: Creative Commons Non Commercial ⧉

Uploaded: 13 May 2020
Last Updated: 15 May 2020

922 megapixels
50,760 x 18,160 pixels
169.2 in X 60.5 in at 300dpi

The Rocky Islands are exposures of Proterozoic-age metagraywacke of the Mather Gorge Formation. Graywacke is a "dirty" sandstone deposited by turbidity currents in submarine fans (a.k.a. "abyssal fans"), which are essentially big piles of sediment in deep ocean basins. This is likely the original environment where the Billy Goat Trail's mud and graywacke deposits accumulated. Turbidity currents are dense, sediment-laden flows which flow along the bottom of the ocean, starting from a source area adjacent to a land mass. As they slow down, they deposit their largest grains first, and their finest grains last, creating the graded beds characteristic of graywacke. In addition graywacke metamorphosed and tilted in the Taconian orogeny, the Rocky Islands display evidence of ongoing geologic processes perpetrated by the Potomac River. "Bathtub rings" around the islands' lower reaches tell of the river's changing level in response to climatic factors such as drought and precipitation. When the river floods, it deposits logs, trash, and other debris higher up on the islands, and bends trees on the islands to point downstream. If the water level is right and you know where to look, you can watch potholes the river scouring out potholes. This was taken on the towpath, about 100 meters upstream of the Section A trailhead.