In ancient times the Northern Plains of India were a part of Tethys Sea, dispersed in water. This low land slowly filled with sediments brought by the perennial rivers of the Himalayan region as also the mud, silt and pebbles shed by Gondwana and Angara land. During the core movements of the earth, they drifted toward north of the peninsular plateau and the Tethys sea started opening. The stock of deposits thus, began traveling and got elevated. The highest of these elevations formed the Himalayan Mountains and the plain, but higher than sea level landforms constituted the Northern Indian Plains, consisting of the Ganga-Yamuna-Doab, the Ganga-Brahmaputra Delta and the Indus plains. On the basis of the relief features, The Northern Plains of India are further divided into four regions; Bhabar, Khadar, Bhangar and Terai.