Beneath this stone repose the bones of two thousand, one hundred and eleven unknown soldiers gathered after the war from the fields of Bull Run and the route to the Rappahannock. Their remains could not be identified, but their names and deaths are recorded in the archives of their country. And its grateful citizens honor them as of their noble army of martyrs. May they rest in peace. September A.D. 1866.
Officially, this is the Civil War Unknowns Monument. The Arlington site notes that it was the first such monument dedicated to unidentified war dead. Both Union and Confederate fallen are here.