Geoscience Reference
In-Depth Information
to onshore winds, the incoming moist maritime air will be forced upwards. As noted
earlier, moist air is cooled adiabatically as it rises, attains vapour saturation and sheds
its condensed water vapour as rain or snow. The air then passes over the coastal ranges
and flows downhill, becoming warmer and drier. The country inland of the coastal
ranges is described as being in the rain shadow of the ranges. The inland-facing slopes
of high mountains are almost invariably drier than the coastal foothills, hence, for
example, the great aridity of the Tibetan Plateau in contrast to the extreme wetness of
the Assam foothills of the Himalayas. The wind-swept upland plains of the Bolivian
Altiplano and of Patagonia lie in the rain shadow of the Andes. The rain-shadow
deserts of New Mexico and Arizona are situated downwind of the Rockies. Other
examples are the very gently undulating, semi-arid western plains of Queensland and
New South Wales inland of the Eastern Highlands of Australia and the exceptionally
hot, dry and rugged Afar Desert bounded by the Ethiopian Highlands to the south
and west. Both the Afar Depression and the Dead Sea Rift are flanked by very high,
mountainous escarpments and occupy low-lying fault-troughs or rifts that in places
descend more than 150 m below sea level. If the region inland of the coastal uplands
contains high mountains, these will be a focus for some additional
orographic
,or
relief, rain, but if the area is lacking in relief, there will be no opportunity for any
such precipitation. The gravel plains (
serir
) of the southern Libyan Desert and the
sandstone plateaux (
hamada
) of the central Sahara, as well as the
gobi
plains of
northern China and Mongolia and the
gibber
plains of central Australia, are all good
examples of hyper-arid environments with very little surface relief. On these stony
desert surfaces, rainfall and run-off are at a minimum and plants and animals are
exceedingly rare, even by desert standards.
2.3 Evidence of formerly wetter climates in now arid areas
After considering why deserts are arid, it is pertinent to ask whether they have always
been so. The answer is unequivocally no. Scattered across the Sahara Desert are the
silicified trunks of tall trees that once grew in abundance in the forests that covered
this region more than 100 million years ago. The prehistoric hunters who roamed the
Sahara at intervals during the last million years made good use of this fossil wood
to fashion the stone tools they used to hunt the great herds of savanna animals that
also inhabited this now empty region. Later still, Neolithic pastoralists grazed their
brindled herds of cattle, sheep and goats at numerous localities throughout the Sahara.
They left behind them an enduring legacy of rock paintings and engravings on the
smooth rock faces of the Tassili sandstone plateau in Algeria, the granite slopes of the
Aır Mountains in Niger and the sandstone plateaux and granite massifs of the Libyan
and Egyptian deserts (Muzzolini,
1995
; Coulson and Campbell,
2001
).
Another indication of formerly wet climates in the drier parts of Africa, Asia
and Australia is the ubiquitous presence of deeply weathered and chemically altered
