KNICKPOINT EROSION AND MIGRATION AT THE FALLS ON DINGMANS CREEK, PIKE COUNTY, NORTHEASTERN PENNSYLVANIA

Four falls, Dingmans (lower), 130 ft, Deer Leap, 34 ft, Fulmer, 78 ft, and Factory (upper), 34 ft, represent major interruptions, knickpoints, in the longitudinal profile of Dingmans Creek, a Delaware River tributary. The lips and faces of all falls are notched many feet down into former higher and wider lips and back (upstream) from former valley-wide, valley-head, vertical faces. Narrow valleys occur between the Delaware River valley and Dingmans Falls and between Dingmans and Deer Leap Falls. These valleys are incised into bedrock, have bedrock floors thinly veneered with gravel, have several low (<6 ft high) falls along their length, and are 1.5 and 1 mi. long, respectively. The valley below Dingmans Falls is narrower, deeper, and steeper sloped than the valley upstream because of differences in bedrock. The Deer Leap-Fulmer-Factory Falls complex is in a short (1,000 ft long), narrow, steep-sided valley with a bedrock floor. Dingmans Falls and the valley below are on bedding-parting deficient siltstone (Upper Devonian, Mahantango Formation) with widely spaced sub-vertical fractures. The other falls are on siltstone and sandstone (Upper Devonian, Millrift Member, Trimmers Rock Formation) that have closely- to moderately-spaced bedding partings, local fracture cleavage, and moderately-spaced, near-vertical fractures.

The following interpretations are made. Knickpoint erosion is by quarrying or plucking. The Mahantango Formation lacks closely spaced parting planes and is more difficult to erode than the Millrift Member. Erosion of falls and valleys by glacial ice was minimal because Dingmans Creek is normal to ice-flow direction in this glaciated area. The partly eroded, former valley-head falls faces represent pre-Pleistocene knickpoint positions. Notching and recession of falls lips and faces were multi-phase erosion events caused by meltwater flow during 3 or 4 deglaciations. Holocene erosion of the falls is minimal. Similar sequential events occur at Pinchot, Raymondskill, and Bushkill Falls, all falls in the same bedrock strike belt.