What is the long-term history of landscape evolution in the Valley and Ridge Province of the Appalachian Mountains and how does this bear on modern patterns of erosion?
These mountains have been variously considered the type-example of an incised peneplain, with its concordant ridgelines and thus in a transient state of evolution, or the type-example of a mountain range in dynamic equilibrium, in which its topography is adjusted to lithology and erosion rate. The actual case bears upon our need to know and understand the background rates and patterns of erosion in this populated, farmed, mined, and logged region. I have approached this problem with stream profile analysis. Bedrock streams I have analyzed in central Pennsylvania have significant knickpoints that cannot be associated with lithologic contrasts. Above these knickpoints, a low-relief fluvial landscape covering >100 km2 is preserved. Projecting these streams beyond their knickpoints to the modern Susquehanna River, less than 10 km to the north, indicates 200-300 m of incision below this landscape has occurred. Results suggest the upper landscape’s erosion rate is not in equilibrium with modern climate or base-level fall rates. This work is consistent with analyses of cosmogenic 10Be in area stream sands that show watershed erosion rates are slower on ridge- and plateau-tops and presents geomorphic evidence of a transient landscape. Future work will expand the regional extent of such analyses and incorporate more surface exposure ages to understand better the pattern of landscape change and ultimately its cause.
Perspective view of orthophotos draped over a DEM showing the Rosecrans plateau in the Valley and Ridge province of central Pennsylvania.
Photograph of low relief plateau surface, ~350 m above surrounding valley bottoms.
Stream profiles and slope-area plots of two streams in the study area, showing low gradient streams on the plateau surface and high gradient streams incising its margins.