HACK VS. DAVIS: REVISITING LANDSCAPE EVOLUTION IN AN ANCIENT OROGEN

The central Appalachian Mountains have served as the type area for two competing end-member models of landscape evolution, cyclical evolution (Davis) and dynamic equilibrium (Hack), with one line of debate centered on the origin of plateaus and accordant ridge elevations. Here we present geomorphic data from a 106 km² plateau in the northern Valley and Ridge province of Pennsylvania that tests these models. Knickpoints characterized by a threefold steepening of fluvial channels lie near the perimeter the plateau but are not associated with lithologic contacts or changes in rock strength. The region of low steepness corresponds to a low relief, plateau-like surface with mean slope of 5° that cross-cuts bedding at a low angle (10°–15°). In contrast, steep reaches occur in narrow valleys at the plateau margin with valley-side slopes ~20°. We infer these knickpoints are the result of transient stream incision. Reconstructed relict profiles, based on extrapolations of slope-area relationships above knickpoints, are on average 196±45 m above the modern base level, the West Branch of the Susquehanna River ~3 km north of the plateau. Late Cenozoic climate change or acceleration in base-level fall has likely driven an increase in erosion rates manifested in these knickpoints. Evidence for relict surfaces and transient relief suggests the parts of the Appalachian landscape do not represent an equilibrium among lithology, topography, and uniform erosion rate.