HICKORY RUN BOULDER FIELD 3: MORE OBSERVATIONS AND NEW INTERPRETATIONS

We reported earlier (Helmke, et al., NEGSA, 2007) about using an Unmanned Aerial Vehicle (UAV) to obtain low-altitude aerial photographs of Hickory Run Boulder Field (HRBF), Hickory Run State Park, Carbon County, PA. We further reported (Potter, et al., NEGSA, 2007) initial observations from those photographs of previously unreported surface features, particularly the presence of abundant sorted stone rings (SSRs) and the non-random occurrence of different boulder shapes on this very low gradient (1oW), periglacially-originated, boulder accumulation. Subsequent study of the UAV photographs, supplemented by field-surface observations and data mapping on a quality photo-mosaic of the field surface both clarifies some of the complexity and raises new questions.

We conclude that the initial HRBF (A) comprised mainly rounded, semi-equidimensional boulders of red sandstone and conglomeratic sandstone derived locally from Upper Devonian, Catskill Fm. rock. Surface boulders larger than 1 m long are common and boulders increase in roundness and decrease in size with increasing depth below the surface. The boulders were emplaced by an unclear process of gelifluction that transported, rounded, and vertically sorted the boulders during the first glacial advance into PA (>770,000 ya). The exposed, upper surfaces of undisturbed surface boulders are deeply weathered, pitted, and gray in color because of loss of iron.

In axial parts of the upper 2/3 of the field (A) is replaced to an unknown depth by red, tabular-shaped boulders that have mappable differences in edge-rounding, surface weathering, and SSR abundance. These differences are interpreted to indicate significant time separation between episodes of periglacial emplacement of tabular boulders. Multiple periglaciations in an area of multiple glaciations are real, but application of the concept to HRBF is complicated because at least two ice sheets have passed over the site.

We hypothesize that long, periglacial periods during which tabular boulders were eroded from outcrops and transported onto HRBF both preceded and succeeded three continental glaciations, two of which passed over HRBF with minimal disturbance and erosion of the boulders. We are searching for substantiating data, e.g., identifiable erratics, etc.