The functional dependency of bedrock conversion to soil on the overlying soil depth (the soil production function) has been widely recognized as essential to understanding landscape evolution, but was quantified only recently. This paper presents soil production, erosion, and river incision rates calculated from ²⁶Al and ¹⁰Be concentrations in granitic bedrock and used to interpret the processes and rates of landform change in two field areas in southeastern Australia. First we show quantification of the soil production function in the context of a retreating escarpment, focusing on the soil-mantled hilly slopes in the upper Bega Valley, NSW. Radionuclide concentrations in bedrock collected at the base of the soil column show that soil production rates decline exponentially with increasing soil depth, defining a function with a maximum soil production rate of 53 m/My under no soil mantle and a minimum of 7 m/My under 100 cm of soil. The form of this function is supported by measurements of soil thickness that show an inverse linear relationship between topographic curvature and soil depth, but also suggest that simple creep does not adequately characterize the hillslope sediment transport processes. Spatial variation of soil depth and therefore soil production rates show a landscape out of dynamic equilibrium, possibly in response to the propagation of the escarpment through the field area within the last few million years. Additionally, a method is introduced to test the assumption of locally constant soil depth and lowering rates using concentrations of ¹⁰Be and ²⁶Al on the surfaces of emergent tors. We find strong support for this assumption by comparing our data to predicted nuclide concentrations.

The second field site is on the southeastern highlands, east of Bredbo, NSW, and is typical of a granitic landscape where chemical weathering has differentiated the bedrock into saprolite and emergent core stones. Soil production inferred for radionuclide analyses from samples of the weathered saprolite may define a much steeper inverse exponential function of soil depth with a maximum production rate of about 140 m/My under zero soil depth. There were no observed soil depths between 20 cm and zero, however, and all exposed bedrock samples were emergent core stones. The maximum soil production rate is therefore about 50 m/Ma and similar to the Bega Valley site. Curvature-depth data from the highland site show a small range of soil depth for a broad range of curvature, which suggests the possibility of nearly constant erosion rates across the site. The catchment average erosion rate is16 ± 1 m/Ma for this site, similar to the average hillslope erosion rate of 15 ± 1 m/Ma, with both rates determined from radionuclide analyses of granitic sediments. Bedrock incision rates of the Bredbo River, which sets the base level for the highland site, average 9 m/Ma and suggest that the higher rate of average hillslope erosion may be in response to the propagation of a local knickpoint that is observed in the long-profile of the Bredbo. Nuclide analyses of the outcropping tors yield bedrock erosion rates of about 4 m/Ma, with higher rates for the more weathered tor tops. A profile of nuclide concentrations above the ground on the tor surface is not consistent with the simple model of locally constant soil depth and lowering rates, suggesting that there is a legacy from the of processes dominant during the Pleistocene climates that still may be effecting the highlands. We explore this hypothesis briefly by showing predicted soil depths from a simple landscape evolution model and comparing them to our observed depths.