GROUND PENETRATING RADAR (GPR) DETECTION AND SUBSURFACE CHARACTERISITCS OF AN ENIGMATIC (SOLIFLUCTION?) LOBE FEATURE IN ADAMS COUNTY, PENNSYLVANIA

Ground-penetrating radar is a shallow geophysical imaging technique used to objectively as possible identify characteristics of subsurface features (i.e. anomalies) found in radargrams (RGs) before more invasive techniques are used. If possible, RG-detected anomalies are ground-truthed and identified more precisely through excavation. This report continues previously presented work which investigated a slope failure in South Mountain, PA (PA-SHPO 36AD0569) within a post-LGM(?) periglacially influenced solifluction bench complex using 4 of 15 GPR transects. This study will present analysis of all 15 transects and their key characteristics in an attempt to identify the origin of this feature. The site is underlain by a combination of (unmapped) Precambrian metabasalt/diabase and metarhyolite from the Catoctin Formation capped by surficial colluvium within which quarries were excavated by Native Americans. GPR RGs helped identify enigmatic features present in the subsurface and carefully placed pits were excavated and soil samples were collected to explore anamolies.

The complex nature of the site presents challenges for analyzing RGs due to the abundance of natural float materials, clay from bedrock weathering, bedrock fracturing, variable bedrock depths, and anthropomorphic disturbance due to Native American quarrying and debitage disposal. The previous study identified “index layers” that were correlated across 4 GPR transects analyzed at this study site. The pits also helped identify key B horizon layers that were the main index layers that were traced in RGs. These layers and other observations were made using 5 excavated pits dug on site. Hand auguring near the top of the bedrock ridge in the “send” area of the lobe revealed an unexpected 7-m soil profile. Additional data are reported here as part of the larger project, to further analyze this landform by identifying sediment grain size distributions from the pits. The results of this study will continue to: 1) guide the planning of future research at this site, 2) help refine techniques and models for subsurface investigations in areas with similar geomorphology and anthropomorphic disturbance, 3) help constrain the timing of Native American quarrying, 4) and help identify characteristics of periglacial landforms found in south-central, PA.