There is one park in the system where you are allowed, and are encouraged to, walk atop some sandstone arches - Big South Fork National River and Recreation Area, on the Tennessee/Kentucky line. It is 7/10ths of a mile to Twin Arches in the western side of the park via the aptly named Twin Arches Loop Trail. These magnificent structures stand approx. 100 feet long and 70 feet tall. The trail passes beneath as well as on the arches, and continues on past Charit Creek Lodge, historic homesteads, massive blufflines, and other features for one of the South's best easy dayhikes. Big South Fork NRRA is home to one of the largest collections (if not the largest) of rock arches outside of Arches NP. Learn more at http://www.nps.gov/biso/naturescience/sandstonearches.htm and http://www.nps.gov/biso/naturescience/twinarches.htm