South African pediments and interfluves

Abstract

Abstract From fifty 1:50,000 scale topographic maps of South Africa, the following attributes were recorded at every intersection of one minute longitude and latitude: basic land facet description, annual rainfall, rainfall concentration, rainfall seasonality, monthly rainfall, local relief, contour interval, rock type, vegetation, pediment length or interfluve width, stream order at valley bottom, local drainage pattern, altitude, terrain morphology, physiographic region and postulated age of planation surface as defined by King ( The Morphology of the Earth: a Study and Synthesis of World Scenery , Oliver & Boyd, 1962) and Partridge & Maud ( South African Journal of Geology , 90 , 179–208, 1987). A strong relationship was found between the propensity for concave slopes (or escarpments) where there is low annual rainfall and high local relief. The relationship with rock type was not so strong, but concave slopes are shown to be more likely on fine-textured rather than coarser-textured rock types. The relationship between drainage density and annual rainfall decreases in value from less than 200 mm annual rainfall up to 400–600 mm annual rainfall, and then increases above this value. Concave slopes are particularly prone in lightly vegetated areas, require some local relief, and are more common on easily eroded rocks and older land surfaces protected from recent drainage dissection. Sheetwash seems the most likely agent of erosion.