ON A BIMODAL GEOMORPHIC AGE OF STREAM MORPHOLOGY IN THE GLENSHAW QUADRANGLE, ALLEGHENY COUNTY, PENNSYLVANIA

In completing a revised bedrock map of the Glenshaw Quadrangle through the STATE MAP program, it was noted that streams appeared to exhibit bimodal characteristics of either youthful or mature geomorphic development. Outcrops in this urbanized portion of the Allegheny Plateau are constrained to road cuts, construction sites, or incised stream channels. The resulting traverses of many stream kilometers revealed that the geomorphic differences among stream reaches may reflect either Wisconsinan or Holocene drainage age. Wisconsinan morphology exhibits mature channel characteristics, including moderate floodplain development, greater sinuosity, floodplains with levees and yazoo streams, terraces, and talus cones at the base of steep gradient, first order tributaries. Elfinwild Creek best exhibited these characteristics, while certain reaches of Pine Creek, the major stream in the quadrangle, and its major tributaries, also exhibited substantial sinuosity. Holocene stream morphology displays straighter rill channels, exhibiting little floodplain development and steep valley walls. An end-member example of youthful drainage exists on a logging road, suspected to be about 100 years old, where a rill channel is incised into bedrock about a half meter deep and half meter wide, and consistently straight for half a kilometer. These bimodal features are best identifiable along lower order streams, where most appear youthful, while Elfinwild Creek is distinctively mature. Identification of these contrasting geomorphic characteristics should complement the descriptions of Pleistocene terraces present throughout the Pittsburgh vicinity, associated with the Allegheny, Monongahela, and Ohio Rivers.