Deserts of the World - Design with the Desert: Conservation and Sustainable Development - page 8

FIGURE 1.7
Photograph of Kalahari Desert dunes. (Courtesy of Robert H. Webb.)
is especially noted for the extensive development of calcretes 37 and of silcretes, 38 while the
Kalahari Sands (which are either in situ or reworked aeolian materials for the most part)
stretch over large tracts of country (c. 1 million miles 2 ) between the Orange River in the
south and the Congo River in the north. The river systems of parts of Zambia and Zimbabwe
inherit their alignments from a previously more extensive aeolian cover, 39 and throughout the
Kalahani, there are extensive fossil drainage networks that are now either ephemeral or dry. 40
The internal basin of the Kalahari, in which these sediments accumulated, was created
by tectonic processes in mid-Jurassic times at the final division of Gondwanaland. There
are various subbasins and graben structures within the area, and the greatest depths of
Kalahari sediments occur in the Etosha Pan area of northern Namibia, locally exceeding
1200 ft in thickness).
There is abundant evidence for environmental changes in the Pleistocene within the
Kalahari, with ancient dunes and palaeolakes being the most striking manifestations. 41
The dune systems of the Mega-Kalahari occur in areas where mean annual rainfall cur-
rently exceeds 31 in., and they have been mapped in the Hwange and Victoria Falls areas
of Zimbabwe, Zambia, and Angola 42 and in Botswana. 43 They indicate formerly very exten-
sive late Pleistocene aridity (with rainfalls less than 5 in. per annum), and there is also
some suggestion that wind directions may have been different from those pertaining in
the region today. 44 The presence of sandy and relatively clayey lunettes in association with
small-closed depressions (pans) has also been used to indicate the hydrological fluctua-
tions of the late Pleistocene (Figure 1.8). 45
The greatest palaeolake in the area, however, is that of the Makgadikgadi Depression in
northern Botswana (Figure 1.6). Strandlines extend to an altitude of 3080-3110 ft (c. 160 ft
above current pan floor level), and the maximum extent of the lake was probably around
23,600 miles 2 , about the size of today's Lake Victoria and larger than palaeolake Bonneville
in the United States. 46 The palaeoclimatic significance of the Makgadikgadi lakes is,
however, uncertain given the increasing body of evidence for tectonic instability in the
Okavango delta region.
The third desert of southern Africa is sometimes called the Great Karoo semidesert. It
occurs as a plateau at an altitude of 1970-3280 ft, tends to be underlain by horizontally
bedded Palaeozoic sediments of the Beaufort Series, is bounded on the north and south