MATERIAL PROPERTIES OF RIDGE-FORMING LITHOLOGIES IN THE VALLEY AND RIDGE AND BLUE RIDGE OF THE SOUTHERN APPALACHIAN MOUNTAINS

Periglacial bedrock fracturing in the mid- to southern Appalachian Mountains is generally thought to have affected rock at higher elevations, where mean annual temperatures were colder and slopes are steep enough to result in extensive natural outcrops. As a result, the lithologies most likely to have been shaped by periglacial conditions are, coincidentally, the most resistant to erosion in the region (i.e., the ridge-formers). Although certain lithologies are well recognized as ridge-formers in the Blue Ridge and Valley and Ridge, little is known about their mechanical properties, how they weather, or what renders them resistant to erosion. Preliminary data from ridge-formers in the Virginia Valley and Ridge, including the prominent Silurian Tuscarora Formation, show shared traits, including high quartz content, low permeability and porosity, comparably high strength (Point Load Strength Index values of ~6-7 MPa), and high discontinuity spacing (>0.5-1 m). Total thickness of resistant intervals is also important, with the most prominent ridges occurring in resistant units that are at least several tens of meters thick. There are clear differences in these properties and those of lithologies that floor the valleys of the region. Ongoing work is expanding to obtain comparable data from both ridge-forming and valley-forming lithologies of the Blue Ridge, including a selection of the highest mountains in North Carolina and Virginia. Preliminary results suggest that measurable properties can be linked to topographic prominence in this province as well, but that the petrologic conditions that render formations resistant are more variable than in the Paleozoic strata of the Valley and Ridge. In addition to offering insight into the landscape evolution of the Appalachians, these data help provide a framework for understanding the susceptibility of individual lithologies to periglacial rock fracture.