RELATIONSHIPS BETWEEN TOPOGRAPHY, GEOLOGY, AND DIFFERENTIAL EROSION IN EASTERN PENNSYLVANIA

William Morris Davis’ famous landform evolution model, "The Geographical Cycle" was based on observations of landforms in the Lehigh Valley and the surrounding region and the topic of Appalachian landform evolution still attracts a substantial amount of consideration. The even-crested ridges and topographic surfaces of the area led Davis to the conclusion that each surface represented an episode of uplift followed by either nearly complete planation (peneplanation), or planation interrupted by an episode of uplift (partial planation). Although the ridges appear in topographic accordance, elevations vary by hundreds of meters. The only ridges that are in near accordance are those underlain by silica-cemented quartz sandstone or conglomerate and ridge-top elevation can vary significantly among adjacent ridges if the lithology differs. The correlation between topography and geology is so strong that crude geologic maps could be constructed on the basis of topography. Whereas the relationship between geology and topography is consistent, close examination shows significant variations in elevation within single lithotypes. Variation in elevation of landforms underlain by similar rocks results from differences in lithology and also from differences in the areal distribution of the outcrop area. The variation in areal distribution of outcrop causes less resistant rocks to stand at higher elevations than they otherwise might ("protected isolation") and also to allow more resistant rocks to be reduced to lower elevations than they would if they were exposed in larger outcrop area ("exposed isolation"). Less resistant rocks stand at higher elevation when surrounded by rocks of higher resistance because they are "protected" from downwasting by the more resistant rocks. Conversely, isolated crystalline rocks of limited areal exposure are often eroded to lower elevations than similar rocks that are more extensive in outcrop area. These rocks are in "exposed isolation" insofar as they are surrounded by less resistant rocks which afford them no protection from erosion. Overall, the correlation between lithotype and topography suggest that the topography was produced by differential erosion of rocks of variable resistance to erosion rather than through peneplanation or partial peneplanation.