Page 325 - Fundamentals of Geomorphology

308 PROCESS AND FORM

Plate 12.7 A large star dune, over 200 m high, near Sossus
Vlei in the Namib Desert.
(Photograph by Dave Thomas)

and are very common in coastal dunes and in stabilized
(vegetated) dunes around desert margins.
Plate 12.6 Partially vegetated and sinuous linear dunes in
the south-west Kalahari. Dunefields and sand seas
(Photograph by Dave Thomas)
Dunefields are accumulations of sand, occupying areas
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of less than 30,000 km with at least ten individual
dunes spaced at distances exceeding the dune wave-
Plants may act as foci for dune formation, and three length (Cooke et al. 1993, 403). They contain relatively
types of dune are associated with vegetation. The com- small and simple dunes. They may occur anywhere that
monest type of plant-anchored dune is vegetated sand loose sand is blown by the wind, even at high latitudes,
mounds, also known as nabkha, nebkha, shrub dunes, and there are thousands of them. In North America,
or hummock dunes (Plate 12.9). These form around a dunefields occur in the south-western region, and in
bush or clump of grass, which acts as an obstacle for sand intermontane basins such as Kelso and Death Valley,
entrapment. Parabolic dunes,or ‘hairpin’ dunes,are California.
U-shaped or V-shaped in plan with their arms opening Sand seas differ from dunefields in covering areas
2
upwind. They are common in vegetated desert margins. exceeding 30,000 km and in bearing more complex and
In the Thar Desert, India, they may attain heights of bigger dunes. In both sand seas and dunefields, ridges or
many tens of metres.They are also found in cold climates, mounds of sand may be repeated in rows, giving the sur-
as in Canada and the central USA, and at coastal sites. As face a wavy appearance. About 60 per cent of sand seas are
to their formation, it is generally thought that parabolic dune-covered, while others may be dune-free and com-
dunes grow from blowouts. Blowouts are depressions prise low sand sheets, often with some vegetation cover.
created by the deflation of loose sand partly bound by Sand seas have several local names: ergs in the north-
plant roots.They are bare hollows within vegetated dunes ern Sahara, edeyen in Libya, qoz in the Sahara, koum or