PEAVINE ISLAND GIANT POTHOLE, HOLTWOOD GORGE, SUSQUEHANNA RIVER, PENNSYLVANIA

Erosion has exposed the cross section of a giant pothole on Peavine Island in the Holtwood Gorge of the Susquehanna River (SR) in SW Lancaster County, PA. The pothole has a depth of 8.5 m (28 ft) from its top to water level. There is an unknown depth of water and sediment and the size and shape of the pothole suggests that it extends much deeper. The pothole is 1-2 m (4-7 ft) in diameter from just below its top to the water level and widens downward. It dips 8° upstream. The dip is normal to the dip of schistosity of the Wissahickon Schist into which the pothole was eroded and that may be the primary reason for the upstream dip.

The pothole is assumed to be the result of abrasive erosion by silt and sand rotated by a hydraulic vortex. Cavitation may have also contributed to the pothole erosion. The upper part of Peavine Island that approximates the pothole top has been dated at about 91 ka by ¹⁰Be dating (Rensser, et al, 2006). Thus, the river currents that created the hydraulic vortex were presumably related to meltwater from the Illinoian and probably also the pre-Illinoian glaciations. In addition, catastrophic failure of the glacial dam for Glacial Lake Lesley on the West Branch SR would have also created intense floods. Creating a pothole of this size seems to require long-lived and forceful water flow, probably with considerable water depth and velocity.

The giant pothole is an example of the type of activity that presumably contributed extensively to gorge erosion as evidenced by the many potholes along the several island margins. In addition, along the east side of the SR above, adjacent to, and below the Peavine Island locale are 5 narrow, elongate, bathtub shaped troughs, the Susquehanna Deeps, that extend from the general river bedrock surface of 110-120 ft asl to -10 feet below sea level. That potholing was a major erosion factor in these deeps is known from the work of Matthews (1918) who viewed the tops of potholes in a partly drained deep.