LARGE-SCALE BLOCKSLIDES AND ROCKSLIDES OF THE APPALACHIAN VALLEY AND RIDGE: REFLECTION AND DISCOVERY IN THE AGE OF 1-METER LIDAR DATA (Invited Presentation)

Newly available 1-meter lidar data for the central and southern Appalachian Valley and Ridge has highlighted the significant extent of large-scale blocksliding and rocksliding on dip slopes of sandstone- or quartzite-supported ridges. Numerous individual blockslides extend for several kilometers along strike and are likely 10’s of meters to possibly over 100 meters thick. Slide ages and kinematic details are largely unknown, and the slide scarps, bodies, and deposits are entirely reforested and very difficult to identify in the field without lidar support. 1-meter resolution lidar-derived imagery facilitates slide delineation by revealing tensile cracks or fissures, rotated bedding, toe folds, and grabens whose physical extent and subtle surface expression are largely obscured by vegetative cover. The slides reflect detachment of valley-dipping, mechanically strong sandstone sequences along weaker shale horizons, which are unstable in the combination of dip and slope geometry imposed by the overlying sandstone caprock. As the slides occur through a wide range of latitudes and drainage basins and in highly variable proximities to documented centers of seismicity, attributing slide development to specific external forcings is difficult. In the absence of a specific forcing event (seismicity) or period (e.g., climatic conditions), these large slides may represent an internally-forced phase of topographic evolution of some hogback ridges with interbedded sandstone-shale stratigraphy, in which the ridge continues to develop prominence until the collective strength of the rock mass (limited by internal weak horizons) is exceeded. The resulting large-scale mass wasting leads to net downslope rock transport and an ultimate reduction of ridge prominence. As hogback ridges in exhumed fold-thrust belt settings tend to be drained by numerous low-order channels which may be unable to effectively detach and transport large caprock fragments, the onset of extensive blocksliding could provide a means by which resistant rock is further disaggregated to facilitate channel transport and consumption of topography. Targeted, lidar-guided field investigation of these numerous slope failures may offer more insight into their context within the overall evolution of a strongly rock strength-controlled landscape.