Research on the web revealed that the stones came from Washington, D.C. when the Columbian Harmony Cemetery was relocated to make room for development in 1960. The tombstones were left behind, and at least some of them were hauled away and ended up along the shore of the Potomac, some 75 miles away.
For more on this remarkable story, see this article.
Also, the Smithsonian Institution's archives has pictures of the Columbian Harmony Cemetery from 1960.
Today, a Metro station and shopping center occupy the land where the cemetery once stood.
These tombstones were all photographed exactly as they were found. They were not moved or cleaned in any way. Click on the pictures to enlarge