Weathering and Subsurface Erosion in Granite at The Piedmont Angle, Balos, Sudan

Abstract

Abstract Sub-surface erosion is intensely active in weathered granite debris at the scarp foot of Jebel Qasim. Powerful flushes of sub-surface water after heavy rain remove much of the decomposing feldspar partly by solution and partly by mechanical eluviation. The sedentary debris gradually contracts as the feldspar is removed and when removal is nearing completion the debris may become compacted. The compacted debris occupies less than 40 per cent by volume of the granite from which it was derived. The upper fringing pediment around this hill is mantled by moist incoherent debris holding small local pockets of water which persist throughout the dry season. Annual recharge of this sub-surface water ensures intense weathering in the debris. The combination of intense weathering and sub-surface erosion with ensuing contraction of the debris occurring just below the piedmont angle may be sufficient to explain the maintenance of this angle during slope retreat.