Prior to the formation of the Thar Desert, the land in the West of the Aravalli mountain and upto the Indus plain in north-west, was fertile. A large number of rivers were flowing here. Discovery of Saraswati river is an evidence to it. According to the geologists, due to the rising up of this area, the drainage here merged with either the river Ganga or the Indus. Later due to scarcity of rainfall aridity increased and it turned into a desert.