Geology of the Shenandoah National Park Region - Csmres Jmu ...

Cross and plane beds are present and the sandstone likely represents a shoaling sequence in the marine.
Bedding dips 10˚ to 20˚ to the northwest and is cut by two sets of subvertical fracture sets.
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 at an elevation
of 933 meters (3,060’), is a shattered tor consisting of gently dipping (12°), broken quartzite blocks and minor,
less disturbed bedrock of the Harpers Formation (Fig. 16). 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 (Fig. 17). 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 is an undulating surface of
weakly developed benches and escarpments (Fig. 18).
Detailed measurements of clast volume and orientation show four sequences where boulder deposits exhibit
an increase in mean clast volume downslope, consistent with discrete events and landforms. The thickest
accumulations of debris are associated with large individual blocks that exceed 4 m in length (Fig. 18). The
fabric of the deposits is largely open framework, some sites exhibit distinct layers of finer materials underlying
coarser materials. The majority of the sites show a preferential downslope clast orientation of the long axis
(Fig. 16). 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.
These landforms and deposits are similar to 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.
Lowering of Blackrock Summit likely occurs through removal of blocks bound by weathered sub- horizontal
bedding planes and vertical joints, as well as toppling of isolated tors and direct removal by rain and wind
of granular material produced from weathering. To estimate the erosion rate at this site, rock samples were
collected the surface of bedrock outcrops, and the abundance of the cosmogenic radionuclide (CRN) 10 Be in
quartz (Fig. 9). 10 Be abundance in surface samples is interpreted as a steady-state erosion rate (Bierman,
1994) and calculations of surface production rate are corrected for elevation, latitude and horizon blockage.
The average erosion rate from three surfaces sampled at Blackrock is 17.9 ± 5.1 m/My, which is nearly 10m/
My greater than the average for eight other summit sites across the Park.
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