When the land for the District of Columbia was surveyed it was a diamond shaped parcel crossing the Potomac River into Maryland and Virginia. In 1791 Andrew Ellicott, the surveyor, placed a boundary stone locating the West point of the diamond in what is now Falls Church Virginia. In 1847 the land in Virginia was given back to the State and the boundary stone became a historical marker. It survives today in a tiny park off a neighborhood street surrounded by the backyards of private residences.
38 of the original 40 boundary stones remain in or near the original locations although some have been replaced. More information can be found here
In addition to the West Boundary Stone, I also took a series of panoramas of the District of Columbia World War I Memorial. It is the only local memorial on the National Mall and is not familiar to most visitors or residents of the Capital. You can view these as well as a panorama of Andrew Ellicott Park here