GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 260-2
Presentation Time: 9:00 AM-6:30 PM


CORNELL, Sean R.1, MARR, Paul G1, STEFFY, Paige1 and WAH, John S.2, (1)Department of Geography and Earth Science, Shippensburg University, 1871 Old Main Drive, Shippensburg, PA 17257, (2)Matapeake Soil & Environmental Consultants, P.O. Box 186, Shippensburg, PA 17257

Recent high-resolution LiDAR was used to detect small (~1-10m) depressions from the South Mountain region of south-central PA (site number 36AD0569). Field investigation revealed these to be Native American quarry sites where PreCambrian Catoctin Metarhyolite was excavated for material to make stone tools. Further analysis focused on the pits, which co-occur with enigmatic landforms along a SE-facing slope. Several questions emerged from this preliminary study. What is the origin of the landforms and why were the quarry pits concentrated on one landform given the widespread availability of metarhyolite rocks in the neighboring landscape?

Our LiDAR-based analysis revealed 3 terrace-like benches separated by steep-sloped risers, and at least 2 lobate landforms that extend downslope perpendicular to the benches. Potter and Delano (2011) described similar landforms at other sites in PA and interpreted them as periglacial gelifluction lobes formed during the Late Pleistocene (14,000-16,000 bp; Potter et al. 2014). We collected GPR (using a Mala X3M, 250Mhz) transects across and along one of the lobes. Longitudinal profiles reveal evidence of fractured bedrock and a mantle of colluvium/soil of variable thickness (~<1-2m) underlying the benches. Transverse profiles of the lobe show evidence of a thicker mantle of debris (2-3m thick). This is interpreted to be a flow that moved downslope over the original land surface.

In addition to GPR, the location, weathering, and petrology of metarhyolite boulders (>0.25 meters) were mapped and entered into a geodatabase, as were the location of 73 confirmed quarry pits. These data reveal that 69 quarry pits (conical, bowl-shaped, and trough-shaped) were clustered on the main gelifluction lobe. Only 4 occurred off the lobe. Although we continue to investigate reasons for the cluster, Native Americans preferred to excavate blocks from the lobe and not from either side of the lobe. It is unclear why they would excavate volumes of overburden rather than mine blocks for processing tools from easy to access surface exposures. With regard to the timing of quarrying, archeological investigations in PA and MD have revealed increased occurrence of metarhyolite from South Mountain in Archaic Period artifacts (Raber et al. 1998). Although archeological investigations have yet to be planned for this site, it is likely that this site (and others in the region) was used over an extended period of time before the landscape evolved from periglacial to forested landscape conditions.