Desert of Rajasthan
Rajasthan is the only state in India which has within its territory a true hot desert. There are arid and semi-arid regions covering eleven western districts of the state characterised by preponderance of evaporation over precipitation, sparsed and restricted vegetational growth, extremes of daily and seasonal temperatures, prevalence of meagre inland drainage leading to salt basins and playas and dominance of brackish/saline conditions in the ground water regime and soils.
Physiography is marked by vast stretches of sand dunes of various kinds – longitudinal dunes, barchans, transverse dunes. In fact, the Great Indian Desert covering Barmer, Jaisalmer, Jalor, Jodhpur, Bikaner, Ganganagar, Churu, Jhunjhunun, Sikar, Nagaur and Pali districts, is a part of the Thar Desert extending into Pakistan. The total area of the arid tract is 233,100 square kilometres or roughly three-fifth of the State of Rajasthan. The greater part of this tract is covered by vast stretches of sand, both shifting and stable, the monotony being broken by occasional protrusion of bard rocks. In the Thar area the rock exposures are mainly of Aravali gneiss and schist, Malani granites and Vindhyans which confirm that the Thar is part of the western extension of the peninsular block. The rock exposures in Barmer-Jaisalmer-Bikaner tract to the extreme west and north belong to Jurassic and Eocene marine formations, whereas those in Jalor-Siwana-Jodhpur-Khinwasar tract in the central region are of igneous rocks, at times overlain by Vindhyan sedimentaries.
There is also evidence of great civilization which had flourished there in the past. The deep seated formations, such as the Lathis of Jaisalmer, are restricted zones of potable waters with local recharge facilities. Vast stretches of alluvium contributed by ancient rivers in their buried channels are superimposed by blown sand. Water table is relatively higher in those valleys, otherwise it goes as deep as 125 m and is often brackish. Recurring aridity at various stages of geological times, has endowed this tract with products like gypsum, potash, common salt, bentonitic and ceramic clays, and phosphor;tes, etc.