MASS WASTING ASSOCIATED WITH HEAVY RAINFALL AT JONES GAP STATE PARK, GREENVILLE COUNTY, SC

Approximately 20 cm of rain on 26-27 June 2006 triggered a mass-wasting event in Jones Gap State Park, which washed out the Jones Gap trail and introduced debris into the Middle Saluda River. Jones Gap State Park is located at the foot of the Blue Ridge Escarpment in the headwaters region of the Middle Saluda about 35 km northwest of Greenville, SC. The Middle Saluda flows from west to east in a deep, narrow gorge that is locally joint and fault controlled. At the slide site, the valley floor is ~488 m above sea level with adjacent ridge tops reaching 914 m. The valley width at that point, from ridge top to ridge top, is ~1,600 m. The Jones Gap mass-wasting event is best classified as a debris slide and is composed of colluvium made up of talus, sandy and silty sediment, and soil. The width of the slide varies from about 6 m at the top to as much as 25 m near the bottom. Talus is angular to rounded biotite gneiss with lesser amphibolite and ranges in size from cobbles to boulders. The maximum boulder size observed is 5x3x4 m. Rock clasts are commonly but not always supported by sand- and silt-sized quartz±feldspar and clay derived from mechanical and chemical weathering of the gneissic bedrock. Our interpretation is that high on the valley wall soil became saturated with water, which lubricated the impermeable gneiss below thus reducing friction and enabling sliding on the steep exfoliation surface (oriented N83°E 45°SE) of the biotite gneiss. The soil, vegetation, and loose rock added weight to the colluvium below, which was already water-saturated, thus triggering mass movement. Debris slid rapidly and violently downward as indicated by freshly broken corners and edges on boulders and bedrock. Thus failure occurred on bedrock and did not involve the movement of bedrock itself. Foliation in the biotite gneiss is variable, records at least two generations of folding, and bears no obvious relationship to the nature of failure on bedrock.