Reconnaissance of the Bengal provinces of East Pakistan and India indicates that structural activity, primarily faulting, has significantly influenced Quaternary geology. Two areas of Pleistocene terrace border the Bengal basin on the east and west and flank Tertiary and older hills of India. Two large inliers of Pleistocene sediments within the basin are surrounded by Recent flood-plain deposits of the Ganges and Brahmaputra rivers and their combined deltaic plain.
Block faulting and echelon faulting have so disturbed the topography of the Pleistocene terrace that the reconnaissance was insufficient to permit determination of whether multiple terraces are present within the basin. Faulting and structural uplift have continued into the Recent epoch, necessitating a physiographic subdivision into an early and a late phase.
Changes in the courses of the Ganges and Brahmaputra rivers through Bengal during the last few hundred years can be attributed to faulting and resultant tilting of fault blocks. These changes have caused the Ganges to abandon numerous western distributaries in favor of joining the Brahmaputra-Meghna system to the southeast. At present about 12,000 square miles of former Ganges deltaic plain in southwest Bengal has been abandoned.
A series of surface echelon faults plus evidence of structural control of stream courses suggest the presence of a subsiding structural trough or major fault at depth. This active structural zone apparently has controlled both the Brahmaputra and Ganges rivers in their lower reaches. Subsurface information is lacking, but this subsiding trough may possibly be related to the arcuate chain of mountains in adjacent Burma.