LANDSLIDE ACTIVITY ALONG MEADOW RUN, SHENANDOAH VALLEY, VIRGINIA

Recent landslide activity was examined along Meadow Run, a stream that emerges from the western flank of the Blue Ridge Mountains of Virginia near Crimora and flows into the South Fork of the Shenandoah River. The process of landslide reactivation that caused recent movement was found to be similar to previous episodes of landslide movement. Meadow Run meanders over a Pleistocene-Quaternary alluvial fan comprised of fresh and weathered sands and cobbles derived from quartzites and sandstones of the Antietam and Harpers Formations. The alluvium is underlain by the Shady Dolomite, which has limited exposure as saprolite in a few stream channels. Stream migration and undercutting of topographically higher fan surfaces creates bluffs of up to about 20 m in height that show signs of instability. At one site tilting of trees and slope surfaces indicates that rotation is the dominant type of slope movement. Dendrochronologic evidence of tilting of trees relates to the timing of previous landslide movements at this site correlating with previous flood episodes, such as Hurricane Fran in September, 1996. On September 18-19, 2003, Hurricane Isabel struck the mid-Atlantic Coast, bringing 174 mm of rain within 24 h to this locale and caused flooding in Meadow Run. When the Meadow Run landslide was subsequently examined on September 20, a portion of the landslide had been reactivated. The toe of the slide had been removed by stream erosion and a new scarp had formed midway up the slope. Currently, the reactivated landslide contains several scarps that define separate blocks. The sliding appears to be largely confined to weak saprolitized alluvial materials, in as much as some stronger saprolite (or bedrock) was observed along the flanks of the slide. Water was observed filling cracks and depressions in the lower third of the slide. At higher elevations on the slide no water was observed and, in shallow auger holes, no water was found. Subsequent observations and measurements following wet weather through October indicated that the landslide continued to develop fresh cracks and that new small scarps uphill were showing retrogressive landslide movement. By late October, the total displacement along the center profile of the landslide was about 4.5 m. The landslide continued to show signs of movement through at least November 25, 2003.