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

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 subsurface 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.

Contrasted Perceptions of Uluru

“…This accession of water favors a relatively dense vegetation cover. It may also lead to the flushing of fines (Ruxton, 1958). In addition, water-related chemical alteration could cause volume decrease in the regolith, leading in turn to compaction and to surface subsidence (Trendall, 1962).…”

On the Origin and Significance of Basal Notches or Footcaves in Karst Terrains

“…The lowest areas occur along the tower bases. Thus runoff generally flows toward and concentrates at the bases of the towers where ill-defined scarpfoot depressions have formed, possibly as a result of the flushing of fines (Ruxton, 1958), and/or pronounced dissolution, volume loss, compaction, and subsidence (Trendall, 1962). In some places, short-lived ponds occur along the bases of towers.…”

Suspended sediment in rapid subsurface stormflow on a large field plot

“…This principle has a well-known application in the design of sand filters. However, the fact that fine particles in suspension can readily be transported by flow through cracks and macropores in the soil has been demonstrated by observations in the laboratory (Hallsworth, 1963), in the field (Ruxton, 1958) and in a groundwater recharge basin (Goss et al, 1973).…”

“…The process was also studied in the laboratory in Russia, where it is termed 'lessivage' (Mel'nikova and Kovenya, 1971; Kovenya et al, 1972). Field evidence has been reported (Ruxton, 1958) for subsurface transport of sediment based on geological and mineralogical considerations rather than direct measurements. The available evidence indicates that only very fine particles entrained within the soil profile are transported, that transport rates are much less than the velocity of the subsurface water, and that very large time scales of months to hundreds of years are required for appreciable movement of sediments.…”