PLEISTOCENE PALEOCHANNEL ABANDONMENT OF PENNS CREEK, UNION COUNTY, PA

Penns Creek in southern Union County, Pennsylvania unexpectedly turns south and crosses a resistant ridge through a water gap near the town of Swengel. The change in course is unexpected because the valley that the creek runs through continues east of the water gap. High, wide terraces are also present along the creek throughout the valley, but do not parallel the creek near the gap. The purpose of this study was to determine if the creek once flowed in a more northerly course, and if so, what caused it to change its path. This study was accomplished by topographic analysis of the current creek’s terraces and possible terraces to the northeast. Soil pits were dug in four different terrace surfaces and compared to six pits within the suspected paleochannel. Seismic and resistivity studies were also done on the suspected paleochannel to determine its subsurface structure.

Topographic and soil evaluations indicate that Penns Creek did once occupy the paleochannel running between the towns of Millmont and Mifflinburg. A well-weathered moraine found at the western terminus of the channel indicates that the creek was most likely blocked by a glacier during the middle Pleistocene. Seismic studies have shown the subsurface of the paleochannel to be composed of sand and gravel deposits overlying bedrock, as would be expected in a fluvial environment. Ground resistivity has also provided good data on the type of sediment overlying the valley bedrock, showing that its electrical properties are consistent with sand and gravel.