SURFICIAL GEOLOGY MAPPING ALONG THE GLACIAL MARGINS OF NORTHWESTERN PENNSYLVANIA

A surficial geology mapping project of the Polk 7.5 minute quadrangle in Venango County has recently been conducted to update a portion of the currently mapped glacial geology of Northwestern Pennsylvania. A combination of digital terrain analysis and soil survey information was used in conjunction with detailed field-based geomorphic mapping and soil profile description as part of a faculty-student research project. Geographic Information System (GIS) technology was used to create the final map compilation based upon field observations and laboratory analyses.

During field work it was found that the intensity of sandstone and quartzite weathering and soil profile development were useful in differentiating glacial deposits of Wisconsin age from those of suspected pre-Wisconsin age. Laboratory soil analyses support field-based deposit age classification. Clast weathering and soil data could not consistently differentiate glacial deposits currently considered Illinoian from suspected pre-Illinoian deposits in the study area. However, a higher degree of topographic dissection and higher proportion of residual and colluvial deposits within the “pre-Illinoian” area aided in field mapping. Field observations and laboratory data generally agree with the previous mapped glacial limits that cross the Polk quadrangle with only minor exceptions. Based on these observations, the currently mapped borders are upheld within this mapping project. Colluvial, alluvial, and residual deposits were differentiated from those of glacial origin based on topographic setting, clast fabric and orientation, evidence of soil cryoturbation, gradational contacts with in-situ sedimentary bedrock, and soil morphological properties. At several localities, cryogenic features were observed within glacial units implying colluvial reworking. A high resolution understanding of the spatial distribution of both glacial and non-glacial surficial deposits in this portion of Northwestern Pennsylvania was made possible with this updated mapping project.