GEOLOGY AND BLUE RIDGE ESCARPMENT EVOLUTION ALONG THE THOMPSON RIVER IN NORTHWESTERN SOUTH CAROLINA AND ADJACENT NORTH CAROLINA

Understanding river erosional processes is necessary in conceptualizing escarpment evolution and migration. Previously, escarpment evolution was thought to be a process of slow, uniform retreat, but new evidence suggests that the Blue Ridge Escarpment (BRE) experienced episodic and irregular retreat. This research examined the Thompson River in upstate South Carolina and adjacent North Carolina as a case study in order to understand what erosional processes are influencing escarpment retreat along the BRE. Field studies along the Thompson River examined structural and petrologic features of the bedrock. The dominant bedrock is Toxaway Gneiss, and thin section analysis reveals the primary mineralogy to be quartz (20-35%), microcline (30-35%), plagioclase (15-25 %), and biotite (10-15%) with accessory muscovite, epidote, and hornblende. Whole rock chemistry shows a range of silica content from 64 to 78 wt. %, which corroborates well with quartz abundance seen in thin section. A total of 32 joints in bedrock were measured, and the major joint sets have orientations striking primarily NS and EW with nearly vertical dips. A total of 32 foliations in Toxaway Gneiss show that the dominant foliations were striking uniformly NE with a gentle dip to the SE in the direction of river flow, making the Thompson River predominantly a down-dip river. Digital Elevation Models (DEM) and longitudinal profiles reveal one major knickpoint with many intervening down-dip foliation slides and 1-2 meter waterfalls. The Atlantic-draining Thompson River has a watershed area of 29 km² and flows primarily on bedrock at the bottom of a deep gorge with an average gradient of about 120m/km (620ft/mi). Minor potholes and smooth, polished slide surfaces demonstrate abrasive forces at work. However, intersecting joints and rockfall at the major knickpoint suggest erosional plucking outpaces the abrasive forces as the primary mechanism for headward erosion.