Marker is located in Tucker County, on US 219 .25 miles south of WV Route 9 on the right driving north.
The inscription reads:
This monument, at the headspring of the Potomac River, marks one of the historic spots of America. Its name is derived from Thomas Lord Fairfax who owned all the land lying between the Potomac and Rappahannock rivers. The first Fairfax Stone, marked by "FX", was set in 1746 by Thomas Lewis, a surveyor employed by Lord Fairfax. This is the base point for the western dividing line between Maryland and West Virginia.
Headspring of the Potomac River.
From Wikipedia: http://en.wikipedia.org/wiki/Fairfax_Stone#References
The Fairfax Stone is located in Fairfax Stone State Park, just north of the marker.