UNCOVERING THE HIDDEN LANDSCAPE: MAPPING BEDROCK ELEVATION FOR THE GLACIATED REGION OF NORTHWESTERN PENNSYLVANIA

Northwestern Pennsylvania was covered by Quaternary ice sheets that shaped the landscape and influenced drainage patterns. A regional bedrock elevation map was created for two counties (4456 km²) using information from more than 10,000 water, oil, and gas well records. Bedrock elevation was contoured by hand and digitized. The map presents evidence for a revised interpretation of the controls on both modern and pre-glacial drainage systems. Extant literature argues that prior to glaciation regional drainage flowed north to the St. Lawrence River. Our results allow us to compare bedrock topography and modern surface drainage for two major valleys. In the first valley, a 21 km transect oriented parallel to flow, bedrock elevation reaches a high of 335m, flanked on either side by elevations of 240 m asl. The presence of this bedrock knob coincides with the modern surface drainage divide and suggests that a similar pre-glacial drainage divide also existed. In the second valley, bedrock elevations along a 17 km transect oriented parallel to flow increase in elevation to the southeast from 115 to 365 m asl. Paleodrainage appears to have been unidirectional to the northwest. A modern drainage divide occurs in the middle of this valley near a bedrock elevation plateau where a thin (8-20 m) end moraine was deposited. Blockage of northwest streamflow by the edge of the ice sheet, followed by rerouting of the drainage system to the southeast has created the modern drainage divide.