The formation of the Himalayas due to upliftment of sediments out of the Tethys Sea and subsidence of the northern flank of the Peninsular Plateau resulted in the formation of a large basin.
Over millions of years this depression gradually got filled up with alluvium deposited by the three major river systems – the Indus, the Ganga and the Brahmaputra flowing from the Himalayas in the north. Sediments were also deposited by the tributaries of these rivers rising from the mountains in the north as well as the Peninsular Plateau to its south. As a result, the fertile Indo-Gangetic or Northern Plains, and the Brahmaputra Plain in the northeast, were formed.
As the Himalayas gained in height, the rivers, glaciers and other agents of denudation became increasingly active in erosion. As a result, large amount of silt got deposited in the shrinking Tethys.
