Environmental Engineering Reference
In-Depth Information
FIGURE 1.10
Lag gravels in the Thar Desert, India. Mohangarh (Jaisalmer) playa (exposed at center) is located in Jaisalmer
district of Rajasthan and is the largest (about 501 ha) gypsum quarry in India. (Courtesy of Navin Juyal; from
previously unpublished data. With permission.)
wind-energy levels, the Thar is one of the least energetic of the world's deserts. Large
sayf dunes that occur in the west of the desert, especially near Umarkot in Pakistan, have
some seasonally inundated lakes in their swales ( dhands ) and may be derived as blown-out
parabolics. 55
Locally, within the Thar, there are closed basins and salt deposits (Figure 1.10). Some may
be the result of the blocking of drainage systems by dunes as at Sambhar or on some of the
tributaries of the Luni River, 56 while others, such as the Jaisalmer and Pokaran Ranns, may
be related to faulting. 57 Coastal deposits are also important areas of salt, and the Rann of
Kutch is a major sabkha area. 58
Possibly, the most contentious issue surrounding the Thar Desert is its age and origin.
Fossil evidence for pre-Pleistocene climates is scanty in Gujarat, Sind, and Rajasthan, but
the records of Dipterocarpoxylon malavii , Cocos , Mesua tertiara , and Garcinia borocahii from
the Tertiary beds of Kutch and Barmer and the Eocene lignite at Palna near Bikaner may
be suggestive of conditions rather similar to those currently pertaining in eastern Bengal,
upper Burma, and Assam. 59 However, it is far from clear when the desert became estab-
lished. Many authors have maintained that the desert is only of Holocene age and is the
result of postglacial progressive desiccation.*
The stratigraphy of the Rajasthan lakes at Sambhar, Didwana, and Lunkaransar has
shown conclusively that a major aeolian layer predates Holocene freshwater deposits, 61
and there are also hypersaline evaporite layers that date back to the last glacial maxi-
mum. 62 There are several phases of dune formation in the late Pleistocene, and the dune
fields were formerly more extensive and active than they are today. 63 Many of the dunes
are now stable, vegetated, gullied by fluvial action, and overlain by slopewash depos-
its, and, in the case of the coastal and near-coastal miliolites, they have been strongly
cemented into material used for building. The lake stratigraphy also shows evidence for
fluctuations of humidity in the Holocene, and this may have influenced the fortunes of the
Harrappan civilization in the Indus Valley and its margins. The late Pleistocene aridity of
the area may be confirmed by the presence of loess layers in river terrace deposits dated
to c. 20,000-10,000 year BP in the Allahabad region 64 and by high dust loadings in Indian
Ocean cores. 65
* See, for example, M.S. Krishnan, V.M. Meher-Homji, and B. Allchin et al. 60
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