Document Type

Thesis

Abstract

Several lines of evidence indicate landscape disequilibrium in the James River basin in the central Virginia Piedmont. The river longitudinal profile possesses pronounced convexities, and the James flows in a narrow inner valley incised into a discontinuous, low-relief upland. We hypothesize that this disequilibrium is generated by pulses of increased downwasting, and we focus here on a major ~five km long knickzone within the western Virginia Piedmont to determine if it is a translational feature. We combine soil analyses of grain size, pH, electrical conductivity, and Munsell color, cobble provenance examinations, and previously established cosmogenic dates of terraces along the James River to constrain the role of lithology, reconstruct past river profiles, and estimate rates of river erosion and knickzone propagation. We have identified numerous, disconnected terrace deposits, and reconstruction of past longitudinal profiles from these deposits suggests at least six separate terrace levels and hints at terrace generation by localized harder lithology. Longitudinal profiles of minor tributaries entering the mainstem within and downstream of the knickzone possess pronounced convexities whose upper elevations generally match elevations of the lowest terrace, implying increased incision since the abandonment of the lowest terrace level. Previous studies have dated high terraces near but outside the knickzone reach with in-situ Be profiles at ~1 m.yr., suggesting rapid river incision rates of ~55 m/my. This dating suggests disequilibrium erosion was initiated and has persisted here during the late Quaternary. We hypothesize this recent incision was induced by the shift to more rapid climate fluctuations in the early Pleistocene, leading to base level lowering and knickzone generation.

Date Awarded

2008

Department

Geology

Advisor 1

Gregory S. Hancock

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