- © 2016 Association of Environmental & Engineering Geologists
Mountain Lake’s unusual location, near the summit of Salt Pond Mountain, VA, in the non-glaciated portion of the Appalachian Mountains, has prompted geologists to study its origin for decades. The northeastern end of the lake abuts an area of heterogeneous colluvial deposits that contain large rectangular blocks of hard Tuscarora Sandstone. This area is known as “Rock City” because of the resemblance of the gaps between the rock blocks to streets and alleys in a city. The purpose of this study was to investigate the origin of Rock City and whether the colluvial deposits within its boundaries are part of a landslide that is possibly responsible for the formation of Mountain Lake. Mapping of Rock City included taking global positioning satellite readings at the corners of rock blocks and along the boundaries of other outcrops, then using ArcMap software to generate maps. Using stereonet analysis, the mode of rock-block displacement was investigated by comparing the measured orientations of principal discontinuity sets forming the rock-block boundaries with discontinuity orientations of undisturbed outcrops. Discontinuity data analysis indicates that Rock City is most likely a landslide that dammed the valley of Pond Drain, forming the lake. The primary mode of slope movement involves lateral spreading associated with extension occurring along discontinuities. The Tuscarora Sandstone blocks comprising Rock City were detached from a scarp face along a northwest-southeast–trending joint set and were displaced laterally toward the west. A seismic event may have triggered slope movement; however, more extensive analysis must be done to rule out other interpretations.