Page 314 - Fundamentals of Geomorphology

AEOLIAN LANDSCAPES 297

AEOLIAN ENVIRONMENTS Deserts are regions with very low annual rainfall (less
than 300 mm), meagre vegetation, extensive areas of
Wind is a geomorphic agent in all terrestrial environ- bare and rocky mountains and plateaux, and alluvial
ments. It is a potent agent only in dry areas with plains that cover about a third of the Earth’s land surface
fine-grained soils and sediments and little or no vege- (Figure 12.1). Many deserts are hot or tropical, but some
tation. The extensive sand seas and grooved bedrock in polar regions, including Antarctica, are deserts because
the world’s arid regions attest to the potency of aeolian they are dry. Aridity forms the basis of classifications of
processes. More local wind action is seen along sandy deserts. Most classifications use some combination of the
coasts and over bare fields, and in alluvial plains contain- number of rainy days, the total annual rainfall, tem-
ing migrating channels, especially in areas marginal to perature, humidity, and other factors. In 1953, Peveril
glaciers and ice sheets. In all other environments, wind Meigs divided desert regions on Earth into three cat-
activity is limited by a protective cover of vegetation and egories according to the amount of precipitation they
moist soil, which helps to bind soil particles together and receive:
prevent their being winnowed out and carried by the
wind, and only in spaces between bushes and on such 1 extremely arid lands have at least 12 consecutive
fast-drying surfaces as beaches can the wind free large months without rainfall;
quantities of sand. 2 arid lands have less than 250 mm of annual rainfall;

Hyper-arid

Arid
Semi-arid

Figure 12.1 The world’s deserts.
Source: Adapted from Thomas (1989)