EXTINCT RIVERS OF THE BLUE RIDGE UPLAND: A NEW PERSPECTIVE ON EROSIONAL PROCESSES IN THE SOUTHERN APPALACHIANS

The Blue Ridge Upland, a 700-1000 m plateau extending from northeast Georgia to Roanoke, Virginia, forms the southeastern margin of the Appalachian Highlands. Bounded by escarpments to the southeast and northeast, the Upland is framed by the Eastern Continental Divide, which separates low-gradient, westward-flowing Upland streams from the oversteepened headwaters of drainages flowing east to the Atlantic Ocean. The topographic disequilibrium suggested by the strongly asymmetric divde and Upland margin is rare in the Appalachians, and its persistence in the weak rocks of the Upland has been difficult to explain. Numerous stranded fluvial terraces preserved at the Upland rim as well as in its interior suggest that divide asymmetry is maintained by stream capture-induced escarpment retreat outpacing slow lowering of the Upland surface. Upland stability is evident in the good preservation of the terraces, and variation in clast lithology suggests the deposits are the remnants of several distinct drainages that may have also deposited a number of terraces previously documented in the eastern Valley and Ridge province. Roundness of vein quartz and quartzite clasts indicates many of the captured basins were large, extending 10’s of kilometers past the present margins of the Upland surface. The nearly complete dissection of these captured basins prior to erosional scouring of the terraces implies that capture-related retreat of the bounding escarpments exceeds the rate of Upland lowering by at least three orders of magnitude. Minimally weathered clasts atop the northeastern escarpment suggest its retreat is especially rapid and may ultimately cause the catastrophic capture of the New River near Radford, Virginia. While the extensive preservation of an ancestral drainage network atop the moderate relief of the Upland is itself significant, the implication for active, and rather rapid, retreat of the Upland margin suggests that some southern Appalachian landforms may be more dynamic than previously thought. Cosmogenic dating of sediments from captured basins and associated Upland terraces may enhance understanding of the role of stream capture in the continued evolution of the Appalachian landscape.