POSTGLACIAL BEDROCK GORGES IN NORTHEASTERN PENNSYLVANIA: PRODUCTS OF LOCALIZED STREAM DERANGEMENT RESULTING FROM BLOCKAGE BY GLACIAL DEPOSITS

Late Wisconsinan glacial retreat across the Appalachian Plateau of northeastern Pennsylvania left a series of "till knobs" in valleys sub-parallel to ice flow and "till shadows" in valleys transverse to ice flow. These deposits are often 100-150 feet thick in valleys typically 300 to 500 feet deep. They partly block the pre-glacial valleys and usually divert post-glacial drainage along one side of a valley to form "one-sided" bedrock gorges or across a divide to an adjacent valley to form "two-sided" bedrock gorges that are characterized by a series of waterfalls.

Over a thousand valley-blocking till knobs have been mapped in Pike, Susquehanna, and Wayne Counties. Several hundred of these sites have one-sided bedrock gorges 100+ feet deep, where post-glacial stream incision has migrated down the contact between the buried bedrock valley side and the till valley-fill. Hundreds of thick till shadows on the lee side of valleys transverse to ice flow have also been mapped and at nearly all these sites there are one-sided bedrock gorges. Prime examples of such gorges occur in tributaries of Starrucca and Tunkhannock Creeks. At some sites the stream incises vertically and starts to cut a two-sided bedrock gorge, in places leaving behind abandoned waterfall plunge-pools on the south side of the deepening gorge.

At many sites in the region, especially thick till deposits have diverted streams across saddles or noses in ridges to adjacent valleys. This forms knick-point gorges, 100+ feet deep, with a series of waterfall cascades. Excellent examples occur in tributaries to Snake and Wallenpaupack Creeks.