Geoscience Reference
In-Depth Information
bedrock. To achieve such a degree of intense leaching and new mineral formation
requires considerable rainfall, a relatively dense vegetation cover and very low rates
of physical denudation - conditions more reminiscent of the wet tropics than of the
arid tropics.
Perhaps the most striking evidence of previously wetter conditions are the remains
of the once integrated river systems that used to flow through every major modern
desert, providing the sandy alluvium that was later fashioned by wind into the impos-
ing sand seas and associated dunes popularly considered synonymous with deserts.
Indeed, one of the most characteristic features of all deserts is their current lack of a
perennial and integrated system of drainage (Cooke et al.,
1993
). Desert streams are
ephemeral. They flow episodically, for variable distances, depending on the intens-
ity and duration of sporadic rainstorms in their upper catchments. Even great rivers
that flow through deserts, like the Nile, the Tigris and the Euphrates, originate in
well-watered uplands far beyond those deserts. All rivers that flow through deserts
constantly lose water by evaporation and by seepage to the local aquifers. Most desert
rivers never reach the coast and instead flow into closed depressions, like the Tarim
Basin in China or the Lake Eyre Basin in Australia. Such rivers are termed
endoreic
,
in contrast to
exoreic
rivers like the Nile, which flow to the sea (de Martonne and
Aufrere,
1928
).
The fossil river valleys of the Sahara, the Gobi and western Australia have long
provoked the curiosity of geologists (
Chapter 10
). Today they are broad, linear depres-
sions filled with Cenozoic alluvium that is often cemented with iron, silica or calcium
carbonate. Some of these former valleys now show relief inversion and form low
erosional remnants or even extensive sheets of resistant ferricrete, silcrete or calcrete
(see
Chapter 15
). In Mauritania, Namibia and western Australia, these valley-fill cal-
cretes may also contain variable amounts of secondary uranium minerals precipitated
out of slowly moving groundwater originating from the Precambrian host rocks that
form the valley interfluves.
2.4 Evidence of previously greater aridity and desert expansion
Just as now arid areas retain evidence of once wetter climates, so is the converse
equally true. Along the now vegetated and stable margins of all the great deserts,
there is abundant evidence of former aridity in the shape of presently vegetated and
stable desert dunes (
Chapter 8
), salt lake and evaporite deposits (Chapters
11
and
12
),
and vegetated mantles of desert dust (
Chapter 9
). Some care is needed in using such
evidence to reconstruct past aridity, particularly in the case of desert dunes.
Desert dunes presently occupy about one-fifth of the Sahara and nearly two-fifths of
the Australian arid zone. In North Africa, the 150 mm isohyet is a good indicator of the
boundary between active and vegetated dunes, although precipitation is not the only
factor responsible for dune mobility. Sand supply, wind velocity, surface roughness
