Washington is a town filled with monuments. They're everywhere. This small one is near the base of the Washington Monument, and is a spot most people miss. I would have missed it if I hadn't read about it and stumbled across it on my last day in DC. This is the Jefferson Pier, and marks the intersections of a line drawn due South from the White House and due West from the Capitol Building. I believe that Jefferson originally wanted this to be a sort of Prime Meridian for the US, but it never worked out that way. If you zoom in on the face of the Pier, there's some well-weather text, one line of which seems to have been chiseled off. I can't find any reference to what it used to say, unfortunately.
You can see the White House in the background of this shot looking North from the Pier. It's hard to get the Capitol Building in a similar shot looking East, due to the hill on which the Washington Monument sits. Due South of the Pier is the Jefferson Memorial, but you can't see that for all the trees.
I did find this shot, taken from the Zero Milestone, that shows the Jefferson Memorial, way on the other side of the Tidal Basin, and you can see the Jefferson Pier, nearly centered below the statue of Jefferson. I took this two days before the other shots above, so I didn't know the Jefferson Pier was there. If I'd known, I'd have stepped to the left a little and gotten the two monuments perfectly aligned.