Pothole Alley

Potholes above Potomac River in relation to shoes
Potholes above Potomac River next to shoes

NPS / Nanette Nyce

SAFETY - Pothole Alley
Have you ever had a rock in your shoe? If you have, you’re aware of how deeply a small, sharp object can dig. When debris such as sand, silt, and gravel settle into cracks and depressions in the stream bed, they can swirl in tornado patterns, or “eddies” which will erode holes down into the rock like a drill.

Ancient potholes can be seen along Billy Goat A Trail, on a section called Pothole Alley, located between trail marker 1 and the Traverse. As you clamber through this portion of the trail, you will see dozens of potholes. Watch your step! They were formed when the Potomac River bed was much higher. Potholes come in a range of sizes. Across the Potomac River from towpath mile marker 13 on the Virginia side is a pothole that is greater than 6 feet across. Potholes can take thousands of year to form.

This same process continues and potholes can also be found in the current bed of the Potomac, leading to unexpected drops in the river bottom. For this reason (amongst others), swimming in this section of the Potomac is very hazardous and is illegal from the Great Falls region to the southern Maryland/DC line. Remember, your choices can affect yours and other's personal safety.

See https://www.nps.gov/choh/planyourvisit/safety.htm for various safety topics to improve the quality of your visit to C&O Canal National Historical Park.
Pothole with water, sand, and stones inside
Pothole with water, sand, and stones inside

NPS / Nanette Nyce

Discussion Questions: What is the purpose of safety warnings? Is the danger real if we can’t see the reason for the warning?

Additional C&O Geology pages: Locks 15-20 | Widewater | Devils Eyebrow | Purplehorse Beach | Paw Paw Tunnel | Flooding | Lamprophyre Dikes | Mather Gorge | Olmsted Island | Cumberland

Return to main Rock Talk page.

Large river pothole
View from across river channel; 16-ft-long canoe for scale; arrow point to area shown in next photo; Close-up view of pothole from first photo.

USGS - David F. Usher, USGS Professional Paper 1691

Last updated: March 2, 2024

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