Abstracting several metric tons of deeds has given me an appreciation of the science of surveying, so I was fascinated by this website about the Boundary Stones of the District of Columbia.
They created the boundary lines of the capital by clearing 20 feet of land on each side of the boundary and setting a uniquely marked stone at each mile interval. On each stone, the side facing the District of Columbia displayed the inscription "Jurisdiction of the United States" and a mile number. The opposite side said either "Virginia" or "Maryland," as appropriate. The third and fourth sides displayed the year in which the stone was placed (1791 for the 14 Virginia stones and 1792 for the 26 Maryland stones) and the magnetic compass variance at that place.
The boundary stones are the oldest federal monuments. Although several have been moved or replaced, 38 boundary stones remain in or near their original locations, including all 14 in the land that was returned to Virginia in July 1846. A 39th is in storage and the 40th is marked by a plaque.