PHYSICAL WEATHERING AT DEVIL'S DEN, GETTYSBURG NATIONAL MILITARY PARK, PA

A preliminary geomorphic analysis of physical weathering features at Devil’s Den in Gettysburg National Military Park, PA was conducted. Devil’s Den is composed of the York Haven Diabase in the Gettysburg basin that formed during Mesozoic extension. The location is approximately 150 km southwest of the Pleistocene glacial limit in northeast Pennsylvania and experienced periglacial activity throughout the Pleistocene. The most obvious sign of physical weathering includes numerous fractures of the diabase, which likely formed during enhanced freeze-thaw processes associated with periglacial activity. Approximately 240 fractures have been measured, and additional measurements will be collected. Fracture width varies from less than 0.001 m to 2.20 m, and fracture depth ranges from less than 0.001 m to 7.4 m. Preliminary analyses demonstrate that there are a large number of micro-fractures present (width less than 0.01 m), and they have a mean width of 0.004 m. Fractures that have a width larger than 0.01 m display a wide range of variability in depth and width. The size and orientation of long axes of boulders was measured, and a weak to moderate northwest-southeast fabric was observed. Exfoliation weathering occurs at Devil’s Den as well. Large blocks appear to have peeled away from the primary outcrop in at least three locations. Additional weathering features of unknown origin were also observed, and analysis of these features is ongoing. Aspect, slope angle, and percent lichen cover on boulders will also be measured.