It's one (er...two) of the many quarries in the area:
http://maps.google.com/?ll=38.824798,-77.491636&spn=0.015948,0.020256&t=h&z=16
If you move northwest of that one you'll find another - in fact, they just rerouted a road so they could dig more.
Head a little more north to Route 50 and there's another.
That is to say, all of us to the east, southeast, and northeast (!!! That's downwind of them all!) of these guys are full of rock dust.
Pity the poor wealthier homeowners to the east of that recently-expanded site...