The third geological division of India comprises the plains formed by the river Indus, the Ganga, and the Brahmaputra. Originally, it was a geo-synclinal depression which attained its maximum development during the third phase of the Himalayan mountain formation approximately about 64 million years ago. Since then, it has been gradually filled by the sediments brought by the Himalayan and Peninsular rivers. An average depth of alluvial deposits in these plains ranges from 1,000-2.000 m. It is evident from the above discussion that there are significant variations among the different regions of India in terms of their geological structure, which has a far-reaching impact upon other related aspects. Variations in the physiographic and relief are important among these. The relief and physiographic of India has been greatly influenced by the geological and geomorphologic processes active in the Indian subcontinent.
Great Plains of India is also called the Indus-Ganga-Brahmaputra plains. It extends for a distance of about 3200 km and width of this plain varies from 150 km to 300 km. The plain was formed as a result of filling of marine depressions by detrital materials brought by water. Plains were formed after the upliftment of Himalayas. Majority of thus plain consists of Alluvial soils.