ABSTRACT
DESERT SEDIMENTS Alluvial fans: banks of flood sediment and debris flows from mountain wadis. May coalesce into bajada, forming the mountain footslope with a sediment apron over a sloping bedrock pediment. Sediments are deposited rapidly, so are unsorted and poorly consolidated. Alluvial plains: extensive lower reaches of fans; mainly sand and gravel in shallow, braided wadi channels; coarse cobble beds remain from wet Pleistocene climates. Playas: flat floors of inland basins with fine, soft, weak silts and clays, often thixotropic. Salt and gypsum form in temporary evaporating lakes. Sabkha: similar, but with limestones, in coastal zones. Salt: may form thick beds in playas and sabkhas; also left by evaporation in clastic sediments. Capillary rise in fine soils may lift salt 3 m above water table, into roads and built structures. Salt crystal growth is major form of desert weathering of rocks and concrete. Impermeable dense concrete suffers less from salt breakdown. Duricrusts: surface layers of cemented sediment, mostly sand or gravel; mineral cement deposited by evaporating groundwater. Most common duricrust is calcrete, or caliche, cemented by calcite, about 1 m thick, over unconsolidated sediment; should not be confused with rockhead as bearing capacity is low. Loess: structureless, yellowish, calcareous silt, of grain size 0⋅02-0.06 mm, common in interiors of the northern continents; much was derived by wind deflation from Pleistocene glacial outwash plains. Dry or moist loess will stand in a vertical face; but it is easily gullied and piped by running water, and it disaggregates and collapses on saturation – hydrocompaction (section 27).
