From
oha.ci.alexandria.va.us:
Congress authorized $5,000 in 1852 to build the Jones Point Lighthouse, one of America's oldest inland waterway lighthouses. The "brilliant" whale-oil lantern was first illuminated May 1, 1856, and also provided light for ice skaters on the river during winter nights. The Mount Vernon Chapter of the Daughters of the American Revolution took over responsibility of the lighthouse in 1926 and spearheaded the building's preservation.
http://www.boundarystones.org/view.php?stone=SOUTH
http://en.wikipedia.org/wiki/Boundary_Markers_of_the_Original_District_of_Columbia