The Qasim massif is a granite inselberg situated in a clay plain of the Balos area, southeast of Khartoum, Sudan. Conspicuous angular junctions are evident between the hill, the sandy pediment, and the plain. Subsurface erosion, including both chemical and mechanical eluviation, is active in removing decomposing feldspar from the weathered granite debris at the scarp foot. After removal of the feldspar, the debris contracts and becomes compacted, occupying less than 40 percent by volume of the original granite. The upper fringing pediment of the inselberg is mantled by moist debris with local pockets of water and the resulting intense weathering and subsurface erosion may explain the maintenance of the piedmont angle. The process, measurement, and results of mechanical eluviation are especially considered in this study.