JOINT AND BEDDING CONTROLS ON WATERFALL MORPHOLOGY AND PERIGLACIAL MASS WASTING, RICKETTS GLEN, NE PA

At Ricketts Glen on the glaciated Allegheny Front in NE Pennsylvania, Kitchen Creek has carved out 22 waterfalls in gently N-dipping Upper Devonian and Mississippian strata. The sedimentary sequence consists, in descending order, of the Mississippian Pocono Fm., gray, conglomeritic sandstone; the Mississippian-Devonian Huntley Mountain Fm. (HMF), mostly gray, crossbedded, fine-grained sandstone; and the Upper Devonian Duncannon Mbr. of the Catskill Fm., fining-upward cycles of gray sandstone to red shale. All waterfalls are in the HMF and Duncannon. Mass wasting and ledge retreat are evident in the uppermost HMF and basal Pocono, the latter forming the caprock of the Front escarpment.

Bedrock joints, bedding, and crossbedding dictate the configuration of the individual falls. Two subvertical joint sets are dominant in all bedrock units: ENE-E and N-S. Small variations in joint orientation are associated with the occurrence of high joint density zones and splays. 20 falls are dominantly controlled by the E-W joints where the N-S joints are conduits for water flow, and two falls, Mohawk and Shawnee, are controlled by the N-S joints. In addition, the prevalent crossbedding of the HMF and Duncannon redirects the water and shapes the falls either through smooth ramps or alternating bedding dips.

Bedding and jointing control the periglacial mass wasting of the Pocono and HMF, where large blocks are displaced and moved down the slope of the Front. Three styles controlled by the E-W joints occur: listric, where blocks have tilted upslope; toppling, where blocks are tilted downslope; and creep, where blocks have moved out from the ledges with little or no tilt. The first two are particularly characteristic of the uppermost ledge of the HMF, while the third occurs in the main E-W ledge of the basal Pocono. Dip of some listric and toppled blocks is up to 40^o. Creep in the Pocono is evident as far as 40 m N of the face of the escarpment, the E-W joints showing through the soil cover as shallow, elongate depressions. Really spectacular is the creep of Pocono blocks at Midway Crevasse, a mass of jumbled blocks, which has moved out 30 m S of the main ledge. Some “shove” by the late Wisconsinan glacier, which reached its NW-trending border 10 mi SW of the Glens, may also have been involved in movement of these blocks.