It is the original western boundary cornerstone for Washington, DC and is located in a small park in Falls Church, Arlington County, VA. You may, or may not remember that Maryland and Virginia ceded land for the Nation's Capital to be built. In 1846, the area west of the Potomac River was returned back to Virginia.
I went to this one and the southern stone. today. The south side was a disappointment because a lighthouse had been build on it. You can see the top of it through a small skylight (it's under a walkway).
Yes, they are placed within iron fences. The existing boundary stones are the oldest Federal monuments.