SEDIMENTARY ANALYSIS OF A PERIGLACIAL SOLIFLUCTION FEATURE FORMED IN SOUTH-CENTRAL, PENNSYLVANIA DURING AND SUBSEQUENT TO THE LAST GLACIAL MAXIMUM (LGM)

During the LGM, the Laurentide ice sheet covered NE & NW Pennsylvania. LiDAR investigations of site PA-SHPO 36AD0569, located on South Mountain in south central PA has led to identification of solifluction benches and a lobate mass movement feature. Geologic evidence (landforms, sediments, soils, etc.) demonstrates PA’s Valley and Ridge experienced periglacial conditions. Since LGM, climate warming resulted in changes in surface/subsurface processes. Dominated by seasonal temperature variations, periglacial environments experience freeze-thaw cycles. These contribute to bedrock weathering, movement of surficial sediments as well as ground and surface-water flows downslope. Over hundreds of years, these contribute to distinct landforms such as solifluction benches (slope-perpendicular) and lobes (elongate slope-parallel). Lobes appear as portions of the larger benches that have surged downslope due to their inability to dewater the active layer above the immobile permafrost layer. The resulting slope failure can be a rapidly moving liquidized flow or an area of slow but fairly-cohesive creep that moves at a rate of cms/year.

This study uses ground-penetrating radar and sediment analysis to identify the origin of the lobate feature found among the three LiDAR-observed solifluction benches. Five 1-m square pits were dug to refusal, and sediments were collected in 10-cm intervals. Wet and dry sediment analyses were completed to quantify grain size trends. Native American rhyolite quarrying allows us to differentiate pre-and post- anthropomorphic disturbance assemblages within the soils on and adjacent to the lobate feature. Just off the feature (LP3), 1-m of debitage-rich soil shows evidence of reverse grading, illustrating infiltration of fines to the base of the profile. In contrast, a pit dug at the base of the lobe (LP2) illustrates evidence of normal grading below 50-cm in the sterile pre-disturbance zone, whereas the soil above displays a pattern like LP3. Wet sieve analysis of an enigmatic 7+ m soil profile at the head of the lobe (UP1) is normally graded. GPR analysis (reported elsewhere) reveals complex subsurface structures. This study is significant for identifying relict periglacial features 10’s to 100’s of miles south of the ice-front on the basis of sediment analysis.