A SUBSURFACE INVESTIGATION OF A SUSPECTED SLOPE FAILURE, GELIFLUCTION LOBES, AND NATIVE AMERICAN METARHYOLITE QUARRIES USING GROUND PENETRATING RADAR (GPR): SOUTH MOUNTAIN, PENNSYLVANIA

Archeological studies often use a shallow non-invasive technique, i.e. ground-penetrating radar (GPR), to determine characteristics of the subsurface before excavation. Although GPR is used in geoarchaeology, sites have variable subsurface characteristics; therefore, analysis of radargrams (RGs) can be difficult and subjective without ground-truthing using excavated test pits. This project builds on work reported by Cornell et al. (2019), who analyzed preliminary GPR transects of this site. This study investigates what has been hypothesized to be periglacial slope-failure features on top of the Catoctin Metarhyolites of PA as well as some of the Native American metarhyolite quarries utilizing a combination of non-invasive GPR methods like hypothetical subsurface soil feature models to aid in RG interpretation, as well as carefully selected test pits all designed to characterize the features, and to constrain the chronology of feature emplacement and the timing of use of the site by Native Americans if possible.

The study site (PA-SHPO 36AD0569) has identifiable quarry pits on the SSE-facing slope which was first detected using LiDAR (light detection and ranging) imagery. Quarries were excavated by Native Americans who likely used the site for generations to obtain stone for tool making quite possibly as early as the Paleo-Indian period (~20,000-11,500 BP; Stanford et al. 2016). Pits vary in shape and 69 out of 73 are on the suspected landslide feature. This site presents unique challenges for analyzing RGs due to regional bedrock features that lie close to the surface (i.e. structural faults/fractures, depositional features within the Precambrian extrusives, and weathering/exfoliation fractures, etc.), overlying colluvium and soil horizons, tree roots, groundwater, generations of mining waste, etc. The primary objective here is to present inferred soil models and ground-truthing data from surface and sub-surface features to aid in objective RG interpretation and refinement. The results of this study will: 1) affect the planning of future excavations at this site, 2) help develop improved models for use at similar archeological sites on South Mountain, and 3) reveal important characteristics of periglacial landforms in this portion of south-central Pennsylvania and the timing of landform development.