The Great plain of Northern India lies on the south of the Himalayas. It is, in fact, bounded by the Himalayas on the north and the Deccan plateau on the south. This plain stretches for about 2,400 km from east to west and 200 to 400 km from north to south. It covers an area of about 5,80,000 sq km.OriginThe Great plain of Northern India was formed by the sediments brought down by the Indus-Ganga-Brahmaputra and their tributaries and it is popularly known as the Indo-Ganga-Brahmaputra plain. Geologists suggest that there was a shallow trough or geosyncline in between the Himalayas and the Deccan plateau during the latter geological period of the formation of the Himalayas.
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. FORMATION… The northern plain of India is formed by the interplay of three major rivers – Ganga, Indus and Brahmaputra. In the Tertiary period there was a sea , Tethys , with extension to north and west . Due to the movement of tectonic plate the Indian lithospheric plate under thrusts the lighter Eurasian plate on the north .This shallow basin marine sediments began folding due to compressive forces acting from north and south .The evolution and upheaval of Himalaya began. The plain was the fore deep of the folded mountain. Thus numerous streams took birth and flowed down the Himalayan along with load of sediments . Sediments continued lying down on the uneven basement of the fore deep .Orogeny continued till its third phase along with making Himalaya higher and higher. And sediments filled the 3-6 km deep trough . However this deepness of trough is also due to subsidence because of increasing load of sediments. so the Himalayan orogeny and filling erodes from Himalaya in it made this fertile land for us…. And the fertile plain of India was formed.