New observations on the geomorphology and origins of Mountain Lake, Virginia

Mountain Lake is the only natural lake of significance in the unglaciated southern Appalachian Highlands. It is located near the summit of Salt Pond Mountain, Giles County Virginia, at an elevation of 1177 m. It is underlain by Ordovician and Silurian non-calcareous shale and sandstone of the Martinsburg, Juniata and Clinch formations. Historical (250 years) and sediment (6000 years) records suggest that the size of the lake has varied periodically. In the 1930s lake origin was proposed as due to valley damming by a lateral landslide (Hutchinson and Pickford, 1932) or damming by scree (Sharp, 1933). A later theory modified the landslide hypothesis to the primarily vertical collapse of a canyon feature in the Clinch (Parker er al., 1975). Fracture trace analysis now reveals a regional lineation feature associated with the lake. This feature is present surficially both downgradient from the lake to the northwest, and upgradient to the southeast. Sonar bathymetry and diver reconnaissance show it expressed as a (relatively sediment-free) narrow open crevice in the deepest (33 m) portion of the lake, probably a fault. Hydrologic observation and resistivity suggest preferential water movement along this fracture, as well as leakage directly from the lake. The present study suggests conduit erosion within this feature and periodic vertical downsettling of overlying Clinch material as the primary mechanism of lake origin and water-level fluctuations through time. Copyright (C) 2001 John Wiley & Sons, Ltd.