STRUCTURAL "PRE-DISPOSITION" FOR A LARGE BEDROCK LANDSLIDE ON JACK MOUNTAIN, WESTERN VALLEY AND RIDGE, PENDLETON COUNTY, WV

The effects of a large bedrock landslide can be seen along the western dip slope of Jack Mountain, Pendleton County, WV in the Appalachian Valley and Ridge Province of eastern North America. The event is Quaternary in age, suggested by the presence of colluvial materials such as soil, weathered rocks, and boulders in the debris flow generated by the landslide. Unique in magnitude and structure, this landslide is substantially larger (two times or more) in volume than and independent from other slope failures along the Jack Mountain front adjacent to the South Branch of the Potomac River.

Jack Mountain is an anticlinal ridge with a thrust fault on the eastern slope. Flexural folding generated minor structures within the Lower Silurian sedimentary rocks. Layer-parallel stretching on the exposed outer layer of Rose Hill sandstone was accommodated by tension fractures. Resistant to buckling, the inner arc of Tuscarora sandstone underwent parallel-layer shortening. Such strain within these rock units produced prominent joint sets and a network of irregularly spaced and oriented fractures. These structural discontinuities allowed a translational rockslide; the scarp face is controlled by joints, and the dip slope is controlled by bedding planes. A low-level seismic event may have initiated the extremely rapid failure. Possible candidates include an historical quake (1853) centered on the border of Highland Co., VA and Pendleton Co., WV with a calculated Richter magnitude of 4.4 and a recent (1986) quake centered approximately 10 miles to the east in Pendleton Co., WV with an approximate Richter magnitude of 2.5. The calculated epicenter of the historical earthquake lies within 1 to 2 miles of the large landslide making this seismic event the more likely initiator of the slide.