Deserts of the World - Design with the Desert: Conservation and Sustainable Development - page 19

they extend well beyond the present desert areas. They are relict features of Pleistocene
aridity, and some may be of late Pleistocene age. This applies to the stable dunes of the
northern part of the Great Sandy Desert that extends beneath the Holocene alluvium of
the Fitzroy Estuary 97 and that thus formed at a time of low sea levels. Lacustrine and
lunette sediments in the Willandra Lakes area of New South Wales also afford evidence
for a period of dune encroachment and lake desiccation in the late Pleistocene. 98 This
glacial aridity may have become prevalent after 25,000 year BP, before which lake lev-
els in the southern parts of Australia appear to have been relatively high. Northeastern
Australia, however, was significantly less humid than today well before 25,000 year BP,
for the pollen spectra from Lynch's Crater in Queensland indicate a drastically reduced
rainfall (100-20 in. year −1 ) in the interval between about 80,000 and 20,000 year BP. Overall,
the combined evidence from marine cores on the Timor continental shelf, pollen analysis,
and the study of lake and dune deposits indicates that at the time of the last glacial maxi-
mum (c. 25,000-18,000 year BP), much of Australia was drier and windier than today and
was surrounded by a much broader continental shelf. 99 Soon after 17,000 year BP, tem-
peratures, precipitation, and sea levels began to rise, and most of the desert dunes were
probably becoming stabilized by c. 13,000 year BP. In the early Holocene, rainfall levels
were higher, and forests became more widespread. There is some evidence for renewed
dune activity in western New South Wales beginning c. 2500 year BP, 100 and this may sig-
nify the passing of the mid-Holocene humid phase and a brief return to relative aridity.
1.8 South America
The main desert areas of South America are inextricably associated with the Andean cor-
dillera. The most extensive zone of aridity includes the coastal Peruvian and Atacama
deserts to the west of the mountains from c. 5°S to c. 30°S; to the east of the cordillera lie
the Monte and Patagonian deserts of Argentina (Figure 1.17).
The Monte and Patagonian deserts both lie essentially in the lee of the Andes. The Monte
Desert, which is more or less continuous with the deserts to the west, is composed of basin-
range topography, including mountain blocks, extensive piedmont surfaces, and largely
internal drainage. Volcanic features are also to be found. The evolution of the region is
not well understood, but Walter Penck was amongst those who have contributed toward
the description of slope evolution and piedmont development. The Patagonian Desert
stretches for over 313 miles between the Andes and the sea. It owes its aridity to the moun-
tains, which block the rain-bearing winds from the west, and to the cold Falkland Current
off the coast. The region is dominated by piedmont plains that slope eastward toward
the Atlantic, where they are terminated by marine surfaces, by ephemeral rivers that are
entrenched into them, and by enclosed drainage basins. Volcanic, glacial, and fluvial
deposits occur extensively in the region. Several glacial episodes during the Quaternary
in the Patagonian Andes certainly influenced the evolution of this arid area strongly, espe-
cially in feeding fluvioglacial gravels into the desert. 101
West of the Andes, the Peru-Chile desert has several distinctive features. Climatically,
the aridity is created by subtropical atmospheric subsidence reinforced by the upwelling
of cold coastal waters associated with the north-flowing Peru current. As a result, it is one
of the world's driest areas, although precipitation does increase eastward with elevation in
the Andes (Figure 1.18). The coastal zone is characterized by fogs ( camanchaca ) that roll in