Geoscience Reference
In-Depth Information
Today
North Tropic
Equator
South Tropic
Active sand seas
At Last Glacial Maximum
North Tropic
Equator
South Tropic
Active sand seas
Figure 17.11
The distribution of active sand seas today and as suggested for the Last Glacial Maximum (after Sarnthein, 1978).
dune activity, rainfall and vegetation (e.g. Grove, 1958),
with the role of wind energy now recognised as critically
important too (e.g. Lancaster, 1987).
The identification of inactive or degraded sand dunes
and palaeo-sand seas has been facilitated by the ex-
amination initially of aerial photography (Grove, 1958,
1969) and subsequently satellite imagery. In linear dune-
fields distinct vegetation zonation aids identification (Fig-
ure 17.13). Compared to interdune areas, the dune crests
may be relatively sparsely vegetated, as in much of Aus-
tralia (Madigan, 1936), or densely covered by trees be-
cause the soft sands favour the development of deep root
systems, as in the northern Kalahari (Thomas, 1984). In
areas where smaller dune types occur and their morpho-
logical patterns are less obvious, identification is still fea-
sible using imagery because quartz-rich sand has a high
reflectivity (Muhs, 1985).
The extent of ancient dunefields is well established in
Africa India, Australia, North America and to a lesser ex-
tent in South America. Reviews of the fixed sand seas of
the Kalahari have been provided by Thomas and Shaw
(2002), for Australia by Wasson (1986) and for North
America by Wells (1983). Goudie's (2002) review of
desert environments includes summaries of sand sea his-
tories from around the globe.
A range of criteria has been used to designate fixed or
relict dune status (Table 17.5). In the Thar Desert (India),
Makgadikgadi basin (Botswana) and Chad basin (Nige-
ria), dunes have been flooded and overlain by lacustrine
deposits (Singh, 1971; Cooke, 1984; Grove, 1958). Other
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