DIFFERENTIAL WEATHERING AND RELATIONSHIPS BETWEEN TOPOGRAPHY, LITHOLOGY, AND REGOLITH THICKNESS IN EAST-CENTRAL PENNSYLVANIA

The rocks and topography at the interface between the Ridge and Valley and New England physiographic provinces in eastern Pennsylvania present a good example of the relationships between differential weathering and the control of lithology on topography. The landscape consists of four distinct terrains: (1) the highest terrain feature is Blue Mountain a continuous linear ridge with an average elevation of 385m amsl underlain by the Shawangunk quartzite, (2) a continuous surface with an average elevation of 198m amsl underlain by the Martinsburg shale/slate, (3) a continuous surface with an average elevation of 116m amsl underlain by early Paleozoic carbonates, and (4) highlands with an average elevation of 218m amsl underlain by Precambrian age fault-bounded basement blocks of metamorphic and igneous rocks. The Shawangunk is a quartz-cemented sandstone and conglomerate that weathers chemically by oxidation of disseminated pyrite. Nonetheless, it is the most resistant rock in the area and weathers macroscopically by freeze-thaw, especially during the Pleistocene. This rock has thin regolith on the ridge crest, and a thicker boulder colluvial mantle on its slopes. The rocks of the Precambrian highlands are more resistant to weathering and erosion than the shale/slate and carbonates, and weather chemically by oxidation of mafic minerals and hydrolysis of calc-magnesium silicates. Thick boulder colluvium on these rocks indicate that they were susceptible to periglacial freeze-thaw during the Pleistocene. Data from well drilling logs from the Pennsylvania Groundwater Information System (PaGWIS) suggest that the carbonate and slate terrains are mantled by similar thicknesses of regolith (14-20m on average), although these two terrains are topographically dissimilar. The carbonates are characterized by undulating karst topography with an average drainage density of 1.29 km/km² suggestive of significant internal drainage. The shale/slate terrain is an upland surface more deeply dissected by streams with an average drainage density of 2.10 km/km² suggestive of more effective fluvial erosion and evacuation of weathered debris. Whereas this landscape shows a clear relationship between lithology and topography, reconciling variations in geomorphic process and regolith thickness remains enigmatic.