Rajasthan Plains are occupied by Thar or the Great Indian Desert.
This plain is an undulating plain rwave like whose average elevation is about 325 m above mean sea level.
The desert region is called Marusthali and forms a greater part of the Marwar plain.
It has a few outcrops of gneisses, schists and granites which proves that geologically it is a part of the Peninsular Plateau. It is only at the surface that it looks like an aggradational plain.
In general, the eastern part of the Marusthali is rocky while its western part is covered by shifting sand dunes.
The eastern part of the Thar Desert up to the Aravali Range is a semi-arid plain known as Rajasthan Bagar.
It is drained by a number of short seasonal streams originating from the Aravali and supports agriculture in some patches of fertile tracts.
Luni is an important seasonal stream which flows into Rann of Kuchchh. The tract north of the Luni is known as thali or sandy plain.
This region extends over 62,000 sq. km, north of the Luni river.
It is believed that this region during the Mesozoic era, was under the sea and this belief is made strong by the pieces of evidence found at the wood fossils park at Aakal, marine deposits around Brahmsar, near Jaisalmer.
These wood fossils were as old as 180 million years.
The south half of Marusthali forms a never-ending bowl, framed by flat-topped hills of sand, rocks, and limestone.
The north part has ridden closely packed with coarse sand.