extensive north-central section of the Indian subcontinent, stretching westward from (and including) the combined delta of the Brahmaputra River valley and the Ganges (Ganga) River (Ganges River) to the Indus River valley. The region 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 three main rivers and their tributaries. The eastern part of the plain has light rains or drought in the winter, but in summer rainfall is so heavy that vast areas become swamps or shallow lakes. The plain becomes progressively drier toward the west where it incorporates the Thar (Great Indian) Desert (Thar Desert).