RIVER EROSION AND BLUE RIDGE ESCARPMENT EVOLUTION IN THE JOCASSEE GORGES AREA OF WESTERN NORTH CAROLINA AND NORTHWESTERN SOUTH CAROLINA, USA

Ongoing investigations of rivers in the Jocassee gorges area of western North Carolina and adjacent South Carolina have provided insight into how the Blue Ridge Escarpment in this region is undergoing modification and evolution. The Blue Ridge Escarpment is a passive-margin escarpment related to initial rifting at ~200 Ma, and thus is an ancient and long-lived prominent, topographic feature. Field work and digital elevation models along the Whitewater, Thompson, and Horsepasture Rivers and Bearcamp Creek reveal that each river is eroding into the escarpment differently, but that there are some common controlling factors. The Whitewater is distinctive in that it has two prominent knickpoints with a long stretch of intervening channel with no bedrock exposure and abundant large boulders derived from the steep gorge walls. By contrast the Thompson has more bedrock exposure and multiple slides (running directly down the dip of bedrock foliation) and pools with no prominent knickpoints at the scale of the Whitewater. The Horsepasture has multiple knickpoints with little intervening bedrock exposure and a boulder-choked channel. Bedrock in all three rivers is the 1.1 Ga striped Toxaway Gneiss composed of biotite (10-15%), quartz (20-30%), microcline (35-40%), and plagioclase (15-20%). With such consistent bedrock and bedrock foliation (NE strike/SE dip) across the rivers, lithology alone cannot account for the differences in channel erosion. All three rivers are eroding primarily by plucking along prominent NS & EW joint sets and secondarily by abrasion as indicated by polished bedrock and potholes. Bearcamp Creek, a much smaller stream, erodes both Toxaway Gneiss and garnet muscovite schist. Where flowing over Toxaway Gneiss the stream has low shoals and one notable knickpoint. By contrast in schist, Bearcamp generated a series of tight, steep-walled slucies, differences in lithology clearly playing a role. In the cases of the three larger rivers, the channels are headwardly eroding into the escarpment and are closely approaching the continental divide in the Blue Ridge uplands. The presence of knickpoints and disequilibrium in the longitudinal profiles suggest rejuvenation of Appalachian topography as the result of Cenozoic mantle dynamics.