Nicolaus iron cross grave marker in section Circle-Left North at Prospect Hill Cemetery in Washington, D.C., in the United States in 2014. Prospect Hill Cemetery was added to the Register of Historic Places of the District of Columbia (a listing of historic sites maintained by the city) in 2005. The marker was erected in 1874. In the late 1800s, iron grave markers -- such as crosses or headstones -- were briefly in vogue. They were considered to be long-lasting and fashionable.