Walking around the mid-Atlantic Piedmont (my home territory), we find a lot of these fellows lying around. They are cobbles of the Antietam Formation (a Cambrian quartzite from the Blue Ridge) which were weathered out and transported eastwards (~60 miles or so, as you can probably deduce from their rounding). They were then deposited as part of the Potomac Group (Cretaceous river gravels draped over the metamorphic rocks of the Piedmont; preserved today on Piedmont hilltops and as the basal layer of the Coastal Plain). The cobbles display the vertical trace fossil "Skolithos" (sometimes spelled "Skolithus"), usually interpreted as a worm burrow. Each burrow is 2-3 mm in diameter. Here I've got a few photos: a cross-sectional view, a "plan" view, and a shot of one of the boulders in a stream in Arlington, VA.