AN INVESTIGATION INTO THE LITHOLOGIC CONTROL ON DIFFERENTIAL WEATHERING AND TOPOGRAPHY IN READING PRONG BASEMENT BLOCKS, EAST-CENTRAL PENNSYLVANIA USING HIGH RESOLUTION LIDAR DATA

Our previous work using 10-meter digital elevation models (DEMs) have quantified the long recognized relationships between bedrock geology and topography in the Great Valley section of Eastern Pennsylvania. Four distinct “surfaces” have been identified, each surface correlates with a specific lithotype. Early workers interpreted these surfaces as peneplains or partial peneplains (Davis, 1889, 1899). The four main terrains are: (1) the ridges and valleys including and north of Blue Mountain consisting of resistant sandstone forming ridges and intervening valleys underlain by shale and carbonates, (2) The Martinsburg surface underlain by relatively resistant slate and greywacke, (3) ridges underlain by Precambrian age, fault-bounded basement blocks of metamorphic and igneous rocks, (known as the Reading Prong), and (4) the lowest terrain, underlain by Cambo-Ordovician carbonates. Rocks that are more resistant to chemical and physical erosion such as the Shawangunk quartzite have elevations that average approximately 400 meters (amsl). The Martinsburg Formation (shale, slate, and sandstone) forms one of two prominent surfaces in the Great Valley and has an average elevation of 170 meters (amsl). The lower prominent surface in the Great Valley is underlain by carbonate rocks, with an approximate average elevation of 120 meters (amsl).

Whereas, the sandstone, slate, and carbonates form reasonably consistent “surfaces, there is more variability among the Fault-bounded Reading Prong, basement blocks. Elevations of Reading Prong ridges are highly variable and range from approximately 100 meters (amsl) to over 300 meters (amsl). In this study we used newly available high resolution (3-meter) LiDAR data to evaluate the effects of outcrop area, variation in the mineralogy within the Reading Prong rocks, and faults on topography. Because the landscape consists of prominent surfaces and the transverse drainage network is incised through these surfaces to elevations greater than 100 meters below the lowest continuous regional surface, we believe that topography is controlled primarily by differential weathering of bedrock rather than fluvial erosion.