Locks 15-20

Lock 18 near Great Falls Tavern
Lock 18 near Great Falls Tavern.  Note the concrete and brick patches.

NPS / Nanette Nyce

LOCATION - Building stone of the C&O Canal: Seneca Red Sandstone

Adjacent to the Great Falls Tavern, you can see lock 20. The others (locks 15-19) are found further down the towpath. Look closely at each of them and you'll see a common building material. Seneca Red Sandstone is a local, attractive, and useful construction material. Its red color is due to the presence of iron oxide. An interesting fact is that Seneca Red Sandstone was actually much lighter in color when it was quarried. The stones became more red over time as the iron in the sandstone oxidized. This stone was quarried near Seneca Creek about 10 miles upriver of Great Falls. In addition to many structures in the C&O Canal National Historical Park, you can find it in buildings throughout the Washington, DC area, including the exterior stone at the Smithsonian Castle on the National Mall.

Observe closely the materials found in the various lock walls and structures. While Seneca Red Sandstone was used extensively in the original structures, flood damage repairs were often made with limestone, granite, concrete, and brick. When rebuilding, the easiest useful materials at hand were used. This resulted in a variety of visible lock surfaces.

Discussion Questions: Where do the building materials in your home & school come from? How does a material’s origin affect its use?

Canal Wall in Georgetown, Seneca Red Sandstone circled
Canal Wall in Georgetown, Seneca Red Sandstone highlighted

NPS / Nanette Nyce

Additional C&O Geology pages: Pothole Alley | Widewater | Devils Eyebrow | Purplehorse Beach | Paw Paw Tunnel | Flooding | Lamprophyre Dikes | Mather Gorge | Olmsted Island | Cumberland Coal

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Last updated: September 6, 2023

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