Soil production on a retreating escarpment in southeastern Australia

The functional dependence of bedrock conversion to soil on the overlying so il depth (the soil production function) has been widely recognized as essen tial to understanding landscape evolution, but was quantified only recently . Here we report soil production rates for the first time at the base of a retreating escarpment, on the soil-mantled hilly slopes in the upper Bega V alley, southeastern Australia. Concentrations of Be-10 and Al-26 in bedrock from the base of the soil column show that soil production rates decline e xponentially with increasing soil depth. These data define a soil productio n function with a maximum soil production rate of 53 m/m.y. under no soil m antle and a minimum of 7 m/m.y. under 100 cm of soil, thus constraining lan dscape evolution rates subsequent to escarpment retreat. The form of this f unction is supported by an inverse linear relationship between topographic curvature and soil depth that also suggests that simple creep does not adeq uately characterize the hillslope processes. Spatial variation of soil prod uction shows a landscape out of dynamic equilibrium, possibly in response t o the propagation of the escarpment through the field area within the past few million years. In addition, we present a method that tests the assumpti on of locally constant soil depth and lowering rates using concentrations o f Be-10 and Al-26 On the surfaces of emergent tors.