As a Licensed Guide for Washington, I have been asked the question, "What is the oldest monument in D.C.?" There's not one, but forty of them, all identical. Dubbed the Boundary Stones, these sandstone markers were laid out by surveyors Andrew Ellicott and Benjamin Banneker between 1791 and 1792 to mark the boundaries of the newly formed city of Washington D.C. These stones run along the original diamond-shaped borders at one-mile intervals.
On one side, the inscription reads "Jurisdiction of the United States," while the other side reads either "Maryland" or "Virginia." The other two sides bear the year it was placed and the compass variance. Although some have been lost, destroyed, replaced, or repositioned, history buffs have taken to finding the rest throughout the Maryland and Virginia suburbs. One-third of the stones are in Arlington and nearby Falls Church and Alexandria.