Fertile lowland region of north-central India. It stretches westward, centred on the Ganges River, from the Brahmaputra River valley and the Ganges delta to the Indus River valley. It contains the subcontinent's richest and most densely populated areas. The greater part of the plain is made up of alluvial soil, deposited by the extensive network of rivers in the region. The eastern part of the plain has summer rainfall so heavy that vast areas become swamps or shallow lakes. It becomes progressively drier toward the west, where it incorporates the Thar Desert.