Basins and bedrock: Spatial variation in 10Be erosion rates and increasing relief in the southern Rocky Mountains, USA

We used measurements of cosmogenic 10Be in alluvium to estimate rates on a 103–104 yr time scale for small (0.01–47 km2), basins in northern Colorado, southern Wyoming, and western Nebraska (western United States). Basins formed Proterozoic cores of Laramide ranges are eroding more slowly (23 ± 7 mm k.y.–1, n = 19) than adjacent basins draining weakly lithifi ed sedimentary rocks (75 ± 36 mm k.y. –1, n = 20). Erosion show a relationship to rock resistance and, for granitic rocks, basin slope, but not to mean annual precipitation. We estimated -term (>105 yr time scale) erosion rates for the granitic core of Front Range by measuring the concentration of 10Be and 26Al produced by muon interactions at depths 1.7–10 m below the surface. imply erosion rates of 9–31 mm k.y. –1, similar to -term erosion rates inferred from alluvial sediment. The spatial of erosion rates and stratigraphic evidence imply that in the southern Rocky Mountains increased in the late Cenozoic; relief probably dates from post-middle Miocene time.