Page 327 - Fundamentals of Geomorphology

310 PROCESS AND FORM

Plate 12.9 Nebkha dunes formed from gypsum-rich sands in central Tunisia. Note that the palm trees in the background
are growing on an artesian spring mound.
(Photograph by Dave Thomas)

2
kum and peski in Central Asia, and nafud or nefud in an area of 192,000 km and average thickness of 26 m,
3
Arabia. They are regional accumulations of windblown houses 4,992 km of sand. The Namib Sand Sea is more
3
sand with complex ancestry that are typically dominated modest, storing 680 km of sand (Lancaster 1999). Sand
by very large dunes (at least 500 m long or wide or seas that have accumulated in subsiding basins may be
both) of compound or complex form with transverse or at least 1,000 m thick, but others, such as the ergs of
pyramidal shapes (Figure 12.8). They also include accu- linear dunes in the Simpson and Great Sandy Deserts of
mulations of playa and lake deposits between the dunes Australia, are as thick as the individual dunes that lie on
and areas of fluvial, lake, and marine sediments. Sand seas the alluvial plains.
are confined to areas where annual rainfall is less than Dunefields and sand seas occur largely in regions lying
150 mm within two latitudinal belts, one 20 –40 N downwind of plentiful sources of dry, loose sand, such
◦
◦
and the other 20 –40 S. The largest sand sea is the Rub’ as dry river beds and deltas, floodplains, glacial outwash
◦
◦
al Khali (the ‘Empty Quarter’) in Saudi Arabia, which is plains, dry lakes, and beaches. Almost all major ergs are
2
part of a 770,000-km area of continuous dunes. About located downwind from abandoned river courses in dry
fifty comparable, if somewhat less extensive, sand seas areas lacking vegetation that are prone to persistent wind
occur in North and southern Africa, Central and West- erosion. Most of the Sahara sand supply, for instance,
ern Asia, and central Australia. In South America, the probably comes from alluvial, fluvial, and lacustrine sys-
Andes constrain the size of sand seas, but they occur in tems fed by sediments originating from the Central
coastal Peru and north-western Argentina and contain African uplands, which are built of Neogene beds. The
very large dunes. In North America, the only active sand sediments come directly from deflation of alluvial sed-
sea is in the Gran Desierto of northern Sonora, north- iments and, in the cases of the Namib, Gran Desierto,
ern Mexico, which extends northwards into the Yuma Sinai, Atacama, and Arabian sand seas, indirectly from
Desert of Arizona and the Algodones Dunes of south- coastal sediments. Conventional wisdom holds that sand
eastern California. The Nebraska Sand Hills are a sand from these voluminous sources moves downwind and
sea that has been fixed by vegetation. A single sand sea piles up as very large dunes in places where its transport
may store vast quantities of sand. The Erg Oriental, with is curtailed by topographic barriers that disrupt airflow