Revisiting William Morris Davis and Walther Penck to Propose a General Model of Slope ‘Evolution’ in Deserts

Abstract

Based on research from slopes on rhyolite domes of known age formed over a million-year continuum in eastern California, a classic geomorphic debate is reconsidered and a general model of desert slope development proposed. This study examines steep (∼25° to ∼35°) boulder-dominated slopes that include well, varnished, vertically oriented colluvial deposits. Such deposits are common throughout the arid southwestern United States. Basic field and isotopic dating methods are combined with two surface-dating techniques, cosmogenic chlorine-36 and rock varnish microlaminae, to produce a detailed slope development history with broad implications for geomorphic theory that includes the unresolved geomorphic debate between Walther Penck and William Morris Davis. Slopes in this study are dominated by the on-going desert slope processes of debris flows and in-situ grain disintegration as evidenced by active debris flow features, terminal Pleistocene ages of microlaminae, and chlorine-36 ages progressively younger than potassium-argon ages for slope genesis. Results also indicate that slopes retreat in a parallel fashion as postulated by Penck. Furthermore, the deposits do not exhibit significant changes in grain size, shape, or angularity from genesis to ∼0.6 Ma but change markedly after that time possibly indicating a geomorphic threshold between ∼0.6 and ∼1 Ma, or episodic erosional events throughout the mid to late Pleistocene.