LITHOLOGIC CONTROL ON TOPOGRAPHY IN EASTCENTRAL PENNSYLVANIA – IMPLICATIONS FOR APPALACHIAN LANDSCAPE EVOLUTION

Previous work in the eastern Pennsylvania portion of the Appalachians has shown that there is a good relationship between lithology and topography (Germanoski, 1999, 2001). Early workers (most notably Davis 1889, 1899) explained these landforms in terms of peneplains and partial peneplains. Nonetheless, the overall landscape configuration suggests that the regional topography is the result of differential erosion of rocks having systematic variability in resistance to erosion. Rocks that are more resistant to chemical and physical erosion such as the Shawangunk quartzite have elevations that average approximately 400 meters (amsl). Less resistant rocks underlay lower elevation landscapes. The most prominent surface in the region is underlain by the Martinsburg Formation (shale, slate, and sandstone) with an approximate average elevation of 170 meters (amsl). The next continuous surface is underlain by carbonate rocks with an approximate average elevation of 120 meters (amsl). The carbonate lowlands are interrupted by round-crested hills underlain by fault-bounded blocks of igneous and metamorphic rocks (Reading Prong rocks). Elevations of Reading Prong rocks are highly variable and range from approximately 100 meters (amsl) to over 300 meters (amsl). Variation in elevation of landforms underlain by similar rocks results from subtle differences in lithology and also from differences in the plan-view outcrop area. For Reading Prong rocks, there is a positive relationship between elevation and outcrop area.

Where less resistant rocks are small in outcrop area, but surrounded by more resistant rocks, they hold higher elevations than they do when their outcrop area is large. We refer to this condition as "protected isolation.” Conversely, isolated crystalline rocks of limited surface area are often eroded to lower elevations than similar rocks that are more extensive in outcrop area. We refer to these rocks as being in "exposed isolation" insofar as they are surrounded by less resistant rocks which afford them no protection from erosion. In this study, we improve upon the previous topographic analyses by using digital elevation models (DEM's) to evaluate relationships between rock type and topography. We also consider the significance of the main rivers being at elevations of approximately 60 meters (amsl).