Abstract

The common wisdom is that steady-state hilltops should display convex shape with both uniform soil thickness and uniform rates of lowering of the soil–bedrock interface. A 120 m hilltop transect on Bodmin Moor, southwest UK, displays a parabolic hilltop form, which implies it is in geomorphic steady state, yet the measured depths to bedrock range almost 6-fold from 14 to 80 cm, and the soil production rates derived from 10Be concentrations in rock at the soil interface range by 2-fold from 10 to 20 m/My. We address this conundrum by developing a theory to explain the scatter in soil depths that would result from block-by-block lowering of the soil–bedrock boundary, and support our theory with a simple model involving block release and diffusive transport. We show that over long timescales a hilltop can indeed be in geomorphic steady state while at any point in time exhibiting a broad range of depths to bedrock. Variable depth to bedrock results in variable 10Be concentrations in the rock sampled at the base of regolith, which yields calculated soil production rates that vary about the long-term average. Hence, scatter in plots of soil production as a function of soil depth are to be expected in many landscapes, and if the landscape is in geomorphic steady state, this scatter reflects the stochastic nature of the block release mechanism by which the bedrock interface is lowering. We find in our models that the commonly observed humped and exponentially declining soil production functions are only preserved during the transient approach toward geomorphic steady state.

Citation

Riggins, S.G., Anderson, R.S., Anderson, S.P., and Tye, A.M. (2011): Solving a conundrum of a steady-state hillslope with variable soil depths and production rates, Bodmin Moor, UK . Geomorphology, 128: 73-84.. DOI: 10.1016/j.geomorph.2010.12.023

This Paper/Book acknowledges NSF CZO grant support.