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Denn, Alison

Geology

2017

MS

The change of topography with time and the consequent structure of Earth's surface is dependent on the production and transport of weathered bedrock. I use measurements of in-situ cosmogenic 10Be to investigate erosion rates and exposure ages of boulders, streams, and hillslope sediments in central Pennsylvania, a landscape shaped by glacial/interglacial climate cycles and changes in base level. I measure rates of landscape change at three separate sites, Hickory Run boulder field, Young Womans Creek watershed, and Garner Run, a small upland catchment bounded by two ridgelines. Hickory Run Boulder field is the largest of its kind in the eastern United States. This enigmatic, 1-km-long field of boulders has been attributed to frost-induced processes during the last glacial maximum, when the Laurentide ice sheet margin was nearby. My isotopic data demonstrate that Hickory Run, and likely many other similar boulder fields in eastern North America are ancient, multigenerational features that have persisted over many glacial-interglacial cycles. These findings add nuance to the conventional view of periglaciation as a force that 'wiped the slate clean' in the Appalachian Mountains in upland areas with resistant lithologies, I show that the landscape was reworked, but not reset by repeated periglaciation. Young Womans Creek is a 230 km2 drainage basin in the headwaters of the Appalachian Plateau physiographic province, where I investigate the role of non-equilibrium topography on the rates of erosion at a basin scale. Here, I use in-situ 10Be to understand the influence of fluvial incision into the landscape, contrasting erosion rates in undissected uplands with those in incised valleys. Erosion rates are positively correlated with slope, but correlate negatively with normalized channel steepness, ksn, and downstream distance. My results demonstrate the effects of lithology and base level on erosion rate are difficult to disentangle, and that when studied on a small scale, lithology exerts the strongest control over the spatial variability of erosion in a transient landscape. Resistant sandstone ridgelines such as those at Garner Run are common features in the Ridge and Valley province of central Pennsylvania. At this site, I strive to understand the influence of periglacial activity on the generation and movement of sediment downslope. My measurements demonstrate that sediment in small upland catchments such as Garner Run can have cosmogenic nuclide concentrations equal to and exceeding 100,000 years of surface exposure history. Despite perturbations by multiple glacial/interglacial cycles throughout the Pleistocene these sandstone-underlain environments are not young, and the exposure of surficial materials predates the last major advance of the Laurentide Ice Sheet at ~26,000 years ago.