Geoscience Reference
In-Depth Information
east and the Taklamakan Desert to the west was studied by the explorers Sven Hedin
in 1899-1902 and Aurel Stein in 1906-1908, and both found evidence of wetter
historic climates near the lake, which began to shrink soon after the third century
(see
Chapter 5
). Berkey and Morris (
1927
) also reported evidence of previously more
humid climates in presently arid Mongolia. In contrast to the evidence of once wetter
climates in the form of now shrunken lakes and defunct river systems, the vast loess
deposits of central China described by Baron Ferdinand von Richthofen (Richthofen,
1877
-
1885
; Richthofen,
1882
) seemed to argue for drier, windier conditions. The Silk
Road from western China into central Asia and across to Europe was dependent on
secure staging posts, of which the fabled oases of Bokhara, Tashkent and Samarkand
are the best known. As in the Sahara and Arabia, these oases are situated in natural
depressions watered from shallow groundwater, and so escape the tyranny of drought.
The drier parts of India display very different landscapes to those of central Asia and
western China. The Thar Desert in north-west India and its southern margin in Gujarat
contain a long record of past eolian, fluvial and pedogenic activity (Khadkikar et al.,
2000
; Chamyal et al.,
2003
; Juyal et al.,
2006
; Singhvi et al.,
2010
). The semi-arid
sandstone plateaux of theVindyanHills andKaimur Ranges that lie between theGanga
and Yamuna valleys to the north and the Son and Belan valleys to the south consist
of sparsely wooded escarpments dissected by narrow valleys. The Son and Belan
valleys contain an alluvial record extending back well-beyond the last interglacial
125 ka ago, as well as a record of human occupation from Lower Palaeolithic times
onwards. Further to the west, the sandstone plateaux are capped by thick Cenozoic
basalts, some of which are deeply weathered and capped by ferruginous duricrusts
or laterites (see
Chapter 15
). Where such laterites occur in now dry areas, the former
climate must have been much wetter.
19.4 Cenozoic tectonism, cooling and desiccation
Uplift of the Tibetan Plateau as a result of the collision of India and Asia around
45 Ma ago caused a major change in the distribution of land and sea and was followed
by severe desiccation of the region to the north and east of the plateau, notably the
Taklamakan, Badain Jaran and Gobi rain-shadow deserts (
Chapter 8
,
Figure 8.12
).
These deserts are still subject to earthmovements and the deformation of Cenozoic and
older sediments (
Figure 19.3
). The inception (or intensification) of the Asian winter
and summer monsoons has also been attributed to these tectonic events, although
other factors, such as the global cooling associated with the formation of permanent
ice caps in Antarctica 34-33 Ma, may also have played a role. As Dettman et al.
(
2001
) have pointed out, the Tibetan Plateau is a key driver of the Asian monsoon
today. The surface of the plateau becomes hot in summer, the warm surface air
rises and the ensuing low atmospheric pressure attracts moist air from the ocean,
causing the heavy summer rainfall that is the hallmark of the Indian summer monsoon.
