Bare Bedrock Erosion Rates in the Central Appalachians, Virginia

Abstract

Long term average erosion rates were determined by measuring in situ concentrations of the cosmogenic radionuclide 10Be in the Blue Ridge of central Virginia. The time scale of the erosion rate measurements is on the order of 104-105 years. Several landscape variables were also investigated to determine which processes control erosion. Curvature and slope measurements taken at sample locations using ArcGIS showed slight correlations with erosion rate. Higher erosion rates had low slope values of <5% and curvature values that converged near 1.2. Location of the sample site was taken into consideration because many sites only had bare bedrock outcrops just off of their summits. Whether the sample was collected from a ridgeline or from a side slope had no effect on erosion rates. A slight negative correlation was observed between erosion rate and elevation from sea level throughout Shenandoah National Park, such that higher erosion rates were found at lower elevations. For sampling sites taken down the side of an individual peak no correlation was found between elevation and erosion rate. The dominant erosional processes present at the summits do not seem to have a significant effect on the erosion rates. Areas dominated by episodic events had lower erosion rates than sites dominated by grain-by-grain erosion. In contrast to previous studies, erosion rates found here were most heavily influenced by rock type. The metasandstone sampled had the lowest average erosion rate while the shale had the highest erosion rate average. The mean summit erosion rate for 20 bare bedrock samples is 9.72 m/My, but individual measurements varied from 2.46 to 41.00 m//My. Nearby long term fluvial incision rates are at least an order of magnitude larger than the summits erosion rates, indicating that the landscape is in disequilibrium, more specifically that the relief in the central Appalachians is increasing. The increase in relief is likely due to either high frequency climate changes during the late Cenozoic era or the local change in the Blue Ridge drainage network during the late Miocene epoch