The third geographical division of India is connected with three of the most important rivers of India, namely Indus, Ganga, and Brahmaputra. This region also includes the river plains formed by these rivers. It forms almost one-half of India.
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. Average depth of alluvial deposits in these plains ranges from 1,0002,000 m.
The rivers which were previously flowing into Tethys sea deposited huge amount of sediments in the Tethys Geosyncline. Himalayas are formed out of these sediments which were uplifted, folded and compressed due to northern movement of Indian Plate.
During the initial stages of upliftment of sediments, the already existing rivers changed their course several times. Rivers also rejuvenated each time for that reason.
Head-ward erosion and vertical erosion of the river valley contributed huge amount of sediments. These sediments were deposited in the depression between peninsular India and the convergent boundary.
