SEDIMENTOLOGY AND GEOMORPHOLOGY OF BLACKROCK BLOCK STREAM, SHENANDOAH NATIONAL PARK, VIRGINIA

Blackrock Summit, located in Shenandoah National Park in the Blue Ridge Mountains of central Virginia, is a tor composed of thick units of quartzite strata of the Harpers formation. The tor is highly fractured, producing spires that show downslope rotational movement. Block deposits extend downslope from this ridge top primarily on the northeast and western slopes of the mountain. The size and fabric of these boulders were examined to determine the mode of transport, and to provide insight into past climatic conditions. The specific site studied is located on the anti-dip slope on the northeastern side of the tor and is 50 m in length. The size of the blocks decreases with distance from the tor. The clasts near the top of the profile show a random orientation, suggesting movement by rockfall. Farther downslope the long axes of the boulders begin to show a northeasterly trend that parallels the slope. This trend continues to the bottom. The profile displays an undulating topography, and the blocks have an open framework that lack interstitial material. The western block field, examined in a previous study, shares similar features to the northeastern site; however, this slope has an extent of over 500m due to its location on the dip slope. Both profiles show a long-axis orientation that is predominantly downslope. The features and trends observed at Blackrock are indicative of periglacial processes described in the literature where ice acts as the main agent of transport.