MEGA-BOULDERS, "TUMBLE TRACKS" AND FISSURE CAVES: A LIDAR-ENHANCED LOOK AT ROCK SLOPE FAILURE ALONG THE BLUE RIDGE ESCARPMENT, RUTHERFORD AND POLK COUNTIES, NORTH CAROLINA

Hillshade imagery derived from a new 0.5-m lidar dataset has led to the identification of numerous previously unknown rock slope failures along the Blue Ridge Escarpment in Rutherford and Polk Counties, North Carolina. Failures range from high-displacement, high-velocity events to larger rock slope movements that only slightly displace and disaggregate outcropping and near-surface bedrock. Movement of large boulders (>27 m3) is particularly widespread, although the emplacement mechanism of the boulders is not always apparent. In areas underlain by gneissic bedrock with widely spaced joints and foliation parting planes, boulders may be particularly large. The largest boulder identified has an estimated mass of 22,000,000 kg and appears to have traveled >200 m downslope while remaining largely intact.

Planform shapes of failure surfaces and displaced rock masses reflect the interplay of steeply dipping, intersecting brittle regime joints sets with well-developed metamorphic foliation and Blue Ridge Escarpment topography in facilitating rock mass detachment and gravitational transport. Timing and cause of these rock slope failures remains unknown in most cases, as do the kinematic aspects (i.e., sliding or tumbling) of large boulder movement. Lidar-derived imagery is, however, sufficiently detailed to reveal travel paths of rock debris as well as small surface depressions associated with subsurface voids developed in the early stages of rock mass displacement. Enhanced understanding of the distribution, prevalence, and mechanisms of these rock slope failures, supported by lidar-derived visuals, will positively impact public understanding and preparedness for potential slope movement events in this dynamic landscape.