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Desert Landforms

The landforms of deserts, like those of high mountains and the polar areas, are much more visible than those of more vegetated landscapes. Bareness also allows much more active surface processes in all these areas, but in different combinations. Deserts suffer much more wind erosion than any other environment. Additionally, if slopes are steep and when the rain does fall, they also experience very fast water erosion. Desert landscapes come in two categories: (1) "shield" deserts and (2) "mountain-and-basin" deserts (Cooke and others 1992 , Mabbutt 1977).

Shield deserts have developed on very ancient crystalline basements; that is, rocks that have been folded and faulted and hardened by heat and pressure over many millions of years. Granites, injected originally deep within the earth, have been unearthed by erosion and form steep-sided hills in many places (as at Uluru in Australia). The Sahara, the Arabian deserts, the southern African deserts, and the Australian deserts are in this group. Though very tough, the basement has been folded into gentlysloping swells and basins, and the basins have been filled over millions of years with sediments eroded from the swells, although these sediments have remained virtually unfolded themselves. They contain the best supplies of groundwater in the deserts, as in the northeastern and southern Sahara, and in Australia; and in some areas they are also rich in oil. Here and there recent volcanic rocks have overflowed at the surface, as in the Ahaggar and Tibesti mountains of the central Sahara.

In their long lives these landscapes have experienced many different climates (partly because they were moved round the earth by continental drift) and many features formed in the different climates survive. There are even ancient glacial features in parts of Arabia and the Sahara; there are many more ancient river gorges, and ancient soils like silcretes or even laterites - ancient soils that formed under wet tropical conditions. The deep rotting of the rock in wetter times penetrated further in softer than harder rocks, and when the loose rotted material was stripped off, the uneven surface of the sub-soil landscape was revealed: a process appropriately called etching.

Water is the main agent of erosion only on the few hills of the shield deserts, and cuts deep gullies on their edges. Elsewhere low gradients mean that water erosion is not very effective, and this leaves the field free to the wind: the great plumes of dust travel from the Sahara over towards Europe, southwest Asia and the Americas, removing much more sediment than do rivers from the same area, and have taken even more dust in recent geological periods. With the dust winnowed out, sand is left behind, and most of it collects in dunes, which cover 20-30 per cent of these landscapes. Some of the larger "sand seas" cover more than 300 000 square kilometres; their median size is 123 000 km .

Mountain-and-basin deserts are those in the much more recently folded and faulted rocks of the earth's active tectonic belts. Up-faulted mountains alternate irregularly with down-faulted basins. The American deserts, both North and South, are all of this kind, as are the deserts of Central Asia (where some of the basins, however, cover many hundreds of thousands of square kilometres). Water erosion in the mountains cuts deep, steepsided valleys and gorges, and takes the debris out into the basins, where the broadening and shallowing of the ephemeral washes (called arroyos in the Americas and wadis in Northern Africa and West Asia) first cause the coarser debris to be dropped in broad "alluvial fans." Occasional extreme storms may carry huge boulders onto these as well. Further down, the alluvial fans coalesce into a long slope of finer alluvium - the bajada.

Sand may be winnowed out of the alluvial deposits and form dunes (and in Central Asia, even a few sand seas). Only the finest debris (silt and clay) reaches the bottom of the basin, where it is deposited in ephemeral lakes or playas. The salts carried in the waters also accumulate there.

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