THE PEDOGENIC RECORD OF HILLSLOPE EROSION AND DEPOSITION PROCESSES

Erosion and deposition processes have a significant impact on soil development, consequently soil profiles and soil landscapes contain information on the spatial and temporal intensity of these processes. The conceptual basis for using soils to analyze erosion and deposition lies in soil stratigraphic relations; the geomorphic surface concept developed by Ruhe and Butler's K cycle Both Ruhe and Butler's concepts define time stratigraphic units which allow estimates to be made of surface stability in soil landscapes. A soil develops in a landscape when the rate of pedogenesis exceeds the rate of erosion or deposition Thus development of a soil records relative surface stability and the formation of a geomorphic surface. The degree of soil development provides an estimate of the age of a soil and the duration of stability of part of the landscape. A geomorphic surface contains both erosional and depositional elements, and soils forming in the different elements will be morphologically and taxonomically different. A soil forming in an erosion scar developed in bedrock will be very different from the soil forming in the associated deposits. However, both are the same age and form part of the same geomorphic surface.

As a soil forms over time, soil horizons reflecting the leaching and accumulation of weathering products develop vertically within the soil profile. Intergrade elements of geomorphic surfaces are recorded by alteration of the location and properties of these soil horizons. When a soil is experiencing a low rate of erosion, then the upper (leached) soil horizons are often absent and the soil represents a degrading geomorphic surface. Similarly, a soil undergoing deposition, will have buried surface horizons indicating an aggrading geomorphic surface. Application of these concepts to understanding hillslope erosion processes will be described at different scales ranging from a drainage basin to a simple hillslope.