LITHOLOGIC CONTROL ON THE FORM OF SOIL MANTLED HILLSLOPES

The smooth, convex form of soil-mantled hills is thought to reflect the increase in transport rates that necessarily must occur to maintain equilibrium further from the topographic divide when soil transport depends only on surface topographic gradients and empirical constants. Therefore the presence of a soil mantle is expected to yield smooth hillslopes regardless of the underlying geologic structure, an idea put forth by G.K. Gilbert over a century ago. However, emerging research and longstanding observations from soil science show that soil transport is often accomplished throughout the soil column, suggesting that in some situations soil thickness may also control soil transport rates. In turn, soil thickness is expected to adjust according to erosion rates and some properties of the underlying lithologies. Taken together, this suggests that changes in soil thickness that may accompany lithologic contacts could be expressed in some hillslopes, as slopes adjust to balance the flux of soil supplied from upslope with changes in soil thickness. Here we present observations from the Gabilan Mesa, CA, where soil-mantled hillslopes show parallel undulations in slope that correlate with changes in the underlying stratigraphy. Using a compilation of published data to document the variation in activity with depth of various soil transport processes, we propose a thickness dependent geomorphic transport law (GTL). Our analysis of high-resolution topography and measurements of soil thickness demonstrate that the proposed GTL provides a less biased prediction of soil transport than either a purely slope dependent prediction or one based on a simple product of soil thickness and slope. While soil thickness is a factor in this GTL, we demonstrate that in most cases transport may be relatively insensitive to variations in thickness, which affirms traditional formulations of hillslope transport.