No, esteemed sage Dan Casey does not do geography issues. âThis is all yours,â the metro columnist emailed the answer desk in the wake of a pair of queries he fielded, the first asking him to clarify matters of geography. The key question involved Interstate 81 signage. Q: Just before one reaches the base of Christiansburg Mountain at mile marker 129.9 southbound on I-81 there is a sign indicating motorists are now entering the New River Valley. Yet near there another sign identifies the North Fork of the Roanoke River. How can this be? Is there some quirk of geography that allows the New River Valley to extend beyond the North Fork of the Roanoke River? Or is it that VDOT simply placed the New River Valley sign in the wrong place?