Rivers and valleys of Pennsylvania, revisited

Abstract

The 1889 paper by William Morris Davis on the “Rivers and Valleys of Pennsylvania” is a landmark in the history of geomorphology. It was in this manuscript that he set forth what came to be known as the Davisian system of landscape. It is important to understand that Davis' interpretation of landforms was restricted by the geologic paradigms of his day. Uniformitarianism was strongly entrenched and Darwin's theory of evolution had become popularly accepted. The concept of the landmass Appalachia and then current theories on mountain building affected the approach that Davis took in hypothesizing the origin and development of the Folded Appalachian drainage. All of these geologic precepts influenced the formulation and explanation of his theories. In his exposition he adapted, synthesized and embellished on ideas he derived from fellow geologists such as Gilbert, Dutton, Powell, and McGee. A number of the concepts he proposed in the 1889 paper quickly became the bases for geomorphic studies by others: the cycles of river erosion and landscape evolution and the peneplain (here called base level erosion). The cycle of erosion became the model for subsequent geomorphic analyses, and peneplain hunting became a popular sport for geomorphologists. Davis' hypothesis of the origin and development of Pennsylvanian drainage stimulated subsequent discussion and further hypotheses by others. In fact, many of the later theories were refinements and/or elaborations of ideas mentioned in this paper of Davis. He proposed the origin of the drainage as consequent streams, then antecedence, superposition, headward extension of divides by piracy, erosion along lines of weaknesses (faults, easily erodible beds) through resistant ridges and normal fluvial erosion. Thus, the hypotheses of regional superposition (Johnson), extended consequents (Ruedemann), consequents and local superposition (Meyerhoff and Olmstead), the utilization of structural weaknesses in development of transverse drainage (Thompson; Meyerhoff; Oberlander, among others), and migration of divides (Thompson), all had been suggested by Davis in 1889. Although the concepts of erosion cycles and peneplaination have waned in popularity in recent geomorphic research, the principles of formation of water and wind gaps, headward migration of divides, stream piracy and adjustment to streams to structure, so clearly and minutely explained in his 1889 publication, are still viable today.