SEISMIC REFRACTION SURVEY OF LANDSLIDE COLLUVIUM AND LACUSTRINE SEDIMENTS AT MOUNTAIN LAKE, GILES COUNTY, VA

Mountain Lake, located in Giles County, SW Virginia, is one of only two natural lakes in Virginia and has experienced extreme water-level fluctuations over the past 4200 years. The lake is situated within a broad breach of the northwest limb of a northeast plunging anticline in the Valley and Ridge Province. The lake itself is underlain by four distinct rock units of Ordovician and Silurian age. They are the limestone and calcareous shale of the Martinsburg Fm. along with sandstones of the Juniata Fm., the Tuscarora Sandstone, and the Rose Hill Fm. The lake is hypothesized by some to have formed when colluvium dammed the stream known today as Pond Drain and into which water flows over landslide deposits during full pond stage. Others hypothesize the lake to be primarily a karst feature. Seismic refraction was used in an attempt to estimate depth to bedrock at several locations in order to ascertain whether natural piping holes, through which water leaks from the lake bottom, drain through colluvial material or whether the underlying bedrock of the Martinsburg Fm. has dissolved to form “blind karst” features that provide the pathways for water loss. Findings to date will be presented and the implications will be discussed.