Page 135 - Sedimentology and Stratigraphy

122 Aeolian Environments

graphy to broad areas of the continent covering many
thousands of square kilometres.

8.5.1 Water table

The land surface in sandy deserts is mainly dry, but if
the substrate is porous sediment or rock there will be
groundwater below the surface. The level below the
surface of this groundwater, the water table, is deter-
mined by the amount of water that is charging the
water-bearing strata, the aquifer, and the relative
level of the nearest lake or sea (Fig. 8.14). Charge to
the aquifer is from areas around the desert that
Fig. 8.11 Aeolian dune cross-bedding in sands deposited in
a desert: the view is approximately 5 m high. receive rainfall, and direct precipitation on the desert
itself. The level of a lake in these settings will be
largely determined by the climate and, if the erg
The dominant factor in determining the distribu- borders the ocean, the sea level will be controlled by
tion and extent of sandy deserts is climate. Arid con- a number of local and global factors (23.8). A rise in
ditions are necessary to inhibit the development of the water table will affect aeolian processes in the erg
plants and a soil that would stabilise loose sediment, if it comes up to the level of the interdune areas: wet
and an absence of abundant surface water prevents interdune sediment will not be picked up by the wind,
sediment from being reworked and removed by fluvial so it becomes stable and not available for aeolian
processes. Sand accumulates to form an erg where reworking (Fig. 8.14). A rise in water table therefore
there are local or regional depressions: these may tends to promote the accumulation of sediment
range from small build-ups of sand adjacent to topo- within the erg. Conversely, a fall in water table from

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Fig. 8.12 Depositional environments in arid regions: coarse material is deposited on alluvial fans, sand accumulates to form
aeolian dunes and occasional rainfall feeds ephemeral lakes where mud and evaporite minerals are deposited.