PERIGLACIAL SLOPE PROCESSES AND DEPOSITS IN THE BLUE RIDGE MOUNTAINS OF CENTRAL VIRGINIA

Exposures of bedrock and block slopes near the summits of the Blue Ridge Mountains in central Virginia reveal a variety of deposits suggestive of periglacial slope processes. The summit of Blackrock (933 m asl), located in the Shenandoah National Park, is a shattered tor consisting of gently dipping (12°), broken quartzite blocks and minor, less disturbed bedrock of the Harpers Formation. The tor, consisting of isolated quartzite columns (3-8 m high), has undergone dislocation by cambering, leaving openings up to several hundred cm between prominent orthogonal joint sets. Below the ridge top where the slopes gradually increase, some remaining displaced bedrock columns show rotational movement which probably facilitated toppling collapse and production of large amounts of blocky debris. Prominent block slopes extend downward from both the east and west sides of the tor. On the west (dip slope) side the block slope extends over 500 m downslope on gradients that vary between 18-35°. The longitudinal profile of the block slope shows an undulating surface of weakly developed benches and escarpments. Detailed measurements of clast volume and orientation show four sequences where boulder deposits exhibit an increase in mean clast volume downslope, suggesting discrete events and landforms. The thickest accumulations of debris are associated with large individual blocks that exceed 4 m in length. The fabric of the deposits is largely open framework, with some sites showing distinct layers of finer materials underlying coarser materials. The majority of the sites show a preferential downslope clast orientation of the long axis. The lower half of this block slope narrows into a slightly sinuous path for ~150 m, and abruptly terminates with a distinct snout just above a small first order tributary of Paine Run. Additionally, circular shallow depressions measuring 1-3 m in diameter on the block slopes suggest an ice-rock mixture prior to melting and subsidence of these deposits, producing the depressions. The descriptions of these landforms and deposits are similar to accounts of solifluction and relict rock glacier deposits documented in the literature. Our continuing investigations will focus on the mechanisms and chronology of the slope processes, and their association with climatic events.