CHRONOLOGY AND IMPACT OF LARGE BOULDERS ON HILLSLOPE AND CHANNEL EROSION IN THE SOUTHERN APPALACHIANS

Large, chemically resistant boulders sourced from quartz-rich ridgetop lithologies are ubiquitous features in the southern Appalachians and exert a primary geomorphic control on the landscape. In Giles and Montgomery Counties, Virginia, areas with high concentrations of boulders cover ~9% of hillslope area and ~20-90% of headwater channel bed area. This boulder cover reduces erosion by armoring hillslopes and trapping sediment as it moves downslope. In headwater catchments, many of the larger boulders are fluvially immobile under current hydrologic conditions and essentially act as bedrock substrate that is more resistant to erosion than the underlying in-situ shale and limestone. Understanding the timing and mechanism by which these boulders are detached and deposited will shed light on our understanding of the processes that set the pace for erosion in boulder-dominated hillslopes and channels.

A prevailing hypothesis for such boulder production in the southern Appalachians is that the boulders were detached and emplaced by enhanced periglacial frost cracking beyond the limits of Pleistocene ice sheet maxima. We use ¹⁰Be surface exposure dating to determine the age of these boulders and help constrain their mechanism of emplacement. Our results include ¹⁰Be ages from Devil’s Marbleyard, a Blue Ridge quartzite boulder field in Rockbridge County, Virginia, range from 18.5 ± 1.7 to 28.5 ± 2.6 ka (n=6). These ages, coincident with the Last Glacial Maximum (~26-16 ka), support periglacial frost cracking as the dominant mechanism for the formation of the boulder field. We are also determining 17 ¹⁰Be ages of boulders from four Valley and Ridge sites in southwestern Virginia. These samples are at PRIME lab awaiting AMS analysis. By comparing the timing of boulder deposition with the timing of Pleistocene glaciations we will determine if climate cycles influenced boulder production and in what ways the pace of erosion continues to be influenced by past periglacial conditions. Pleistocene glacial-interglacial cycles provide an analog to modern high-latitude climate change and, thus, examining the geomorphic impact of past periglacial processes in the Southern Appalachian Mountains may allow us to better understand and predict the geomorphic evolution of similar modern high-latitude periglacial landscapes.