Climate change in arid and tropical humid regions
695
clays. As a whole, the data indicate that nowadays there is a phase of incision and,
previously, two stages of alluviation occurred. This alternation seems to be related to
climate changes. In the river Solimoes toward the northwest, the same sequence
of accumulation and incision can be observed and there are some abandoned meanders on
the terrace. Here, the important regression that occurred during the glacial period triggered
the dry climate in which an intense incision developed. Therefore, the origin of these
terraces seems to be eustatic and not climatic (Tricart, 1985).
The classic works of Bigarella and Andrade (1965) and Bigarella et al. (1969) deduced
dry stages in the Pliocene on the basis of analyses on pediments. The study area is located
in the Serra do Mar and its continuation toward southern coastal areas of Parana and Santa
i--.-I
i
Guinea
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Gulf of
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GABON
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iJ" J ~"~~ZA!RE
10 ~
20 ~
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30 ~
[" "11 Semiarid zone
~ Area underlain by sanda
of the Kalahari System
Town
Figure 25.47. The extent of desert, and deposits of desert origin, in southern Africa. Note that the
northward extent of the Namib Desert is controlled by the extent of the Benguela Current, and that the
Kalahari Sands reach across the Zaire River although they are now covered by forest and savanna
woodlands in the northern area (after Selby et al. (1979)).