Some reflections of the origin and land use of pediments on Ethiopians Highlands

Abstract

Pediments seem to be widespread on the Ethiopian highlands. They form smooth transition between the steeper
mountain or hill front and the flat surfaces downslope. The piedmont angle, characteristic of pediments elsewhere (arid
and semi arid zones) is, here, absent. This form is not due to the original structural surface formed by the aggradation of
the pyroclastic materials ar the foot of the volcanic hills or mountains, since pediments also occur at the base of
horizontally bedded remnant hills of Trap series lavas and sedimentary rocks. Where they are formed at the foot of
volcanic hills, they do not correspond to the much steeper original bedding of the pyroclastic deposits. They are cut in
rock (lava, pyroclastic, sedimentary) and are covered by a thin veneer of soil and colluvium. The relative absence of
linear erosion and the smooth, grading of the slope (both upward and down to the present day valley bottoms or flat
plains) suggest that the modelling of many of these pediments is either very recent or belongs to the present-day
processes (mainly sheet flow). These surfaces mainly act as transport slope for the finer particles (soil and pyroclastics)
brought from upslope. The sheet flow along the pediments is frequently changed into channelled flow because of the
presence of roads collecting water of the slope. Gullies are forming in the basal part of pediments, whose maximum
depth is close to the base of the bridge, where discharge is concentrated. Downslope, the depth decreases, until finally
the gullies disappear.