Evaluation of lithologic controls on ridge erosion rates in the Teton Range, Wyoming

Rockfalls frequently occur in alpine environments and contribute to landscape evolution through ridge denudation and sediment accumulation. In the Teton Range, talus has accumulated beneath steep ridges and peaks and has shown evidence of coupling with fluvial and glacial processes. Quantifying hillslope processes and the effects on landscape evolution is challenging due to the timing and spatial distribution of individual rockfalls. This study investigates the lithologic properties that may control erosion processes on ridges and the contribution of hillslope failures to landscape evolution. Rock mass strength is tested as an alternative dating technique to indicate relative surface ages and is compared to other quantitative methods. Hillslope erosion is quantified with estimates of talus accumulation at the base of slopes beneath ridges extending from the center to the south of the range. These talus-based erosion rates are compared to rock strength estimated from a Schmidt type hammer and the Selby rock mass strength classification. The Selby classification uses surface weathering and spacing, width, and orientation of joints in the rock to estimate the potential for failure along a surface. Ridge and talus surface ages are also quantified with cosmogenic nuclides. Preliminary results indicate that erosion rates along ridges are similar across the range with occasionally faster rates occurring along the southern ridges. Similar erosion rates (0.1 mm/yr) were found on ridges composed of sedimentary and metamorphic rocks with similar Selby classification values. Selby classification results suggest that faster erosion rates occur adjacent to weaker rocks. The surfaces with stronger Selby classification values were located along ridges approaching the peak of Grand Teton, the highest peak in the range, however the strongest ridges do not correlate with the slowest erosion rate. The combination of rock mass strength and erosion rates may be useful to understand the spatial distribution of high peaks in the Teton Range, however rock strength alone is a poor indicator of variations in erosion rates on ridge surfaces.