Part of a huge hiking trail along the Potomac River. Spreads across both Loudoun and Fairfax counties. Many bridges that require you to carry your bike because there is a 2 foot step up onto.
Hidden trail along Goose Creek and Potomac River. There is an entrance along Riverside Drive in Lansdowne, VA.
http://www.singletracks.com/item.php?c=1&i=6010