The Ganga Geology - The Ganga: Water Use in the Indian Subcontinent - page 17

Fig. 3.2 Bengal basin
fault, or to a subsiding trough along the axis of the Ganga-Brahmaputra-Meghna
(GBM) river system. This is likely to have caused changes in the courses of major
rivers like these three.
Formation of a Delta
In the Tertiary period (16-15 million years ago), the Bengal Basin was filled by
sediments, brought by the rivers from the east and the west, as there was no gap on
the north. This was possible, because the three sides, especially the Himalayan side
on the north, were highlands with steep slopes. The land-building process must have
been accelerated by the Ganga and the Brahmaputra.
In the Pleistocene period (about 1.5 million years ago), the formation of the
Garo-Rajmahal gap on the north changed the pattern of sedimentation, because
of a complete change in the drainage of the Ganga and the Brahmaputra, which
flowed southward and fell into the Bay of Bengal. Subsequent world-wide glacia-
tions caused a significant fall in sea-level, which considerably increased the erosive