Northeastern Section (45th Annual) and Southeastern Section (59th Annual) Joint Meeting (13-16 March 2010)

Paper No. 2
Presentation Time: 1:55 PM


MILLER, Scott R., Department of Earth and Environmental Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, MI 02139, SAK, Peter B., Department of Geology, Dickinson College, Carlisle, PA 17013, KIRBY, Eric, Department of Geosciences, Pennsylvania State University, University Park, PA 16802, BIERMAN, Paul, Geology Department, University of Vermont, 180 Colchester Ave, Delahanty Hall, Burlington, VT 05405 and REUTER, Joanna, Geology Department, University of Vermont, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405,

The central Appalachian Mountains have served as the proving ground for two, end-member conceptual models of relief evolution in mountain ranges: dynamic equilibrium and cyclical evolution. Here we present quantitative geomorphic and erosion rate data from the northern Valley and Ridge province of Pennsylvania that provide a test of these models. Analysis of streams draining a 106 km2 upland plateau reveals transient profiles, characterized by steep reaches incised into narrow, steep-sided valleys below knickpoints that are not associated with lithologic contacts or contrasts in rock strength. Streams atop the plateau are ~1/3 as steep and drain watersheds with a mean slope of 5° that cross-cuts bedding at a low angle (10°–15°). Normalized steepness indices in these channels correlate linearly with watershed-averaged erosion rates from cosmogenic 10Be in detrital quartz, indicating that steeper reaches are eroding at rates two to five times greater than those atop the plateau (23–80 m/Ma compared to 9–17 m/Ma, respectively). Reconstruction of relict profiles suggests that higher rates of incision are a response to ~200 m of relative base level fall on the West Branch Susquehanna River. Similar relationships between erosion rate, channel steepness, and estimated base-level drop occur on the Appalachian Plateau. Our results provide direct evidence that portions of the Appalachian landscape are still seeking equilibrium among lithology, topography, and erosion rate.