The Ganga Geology - The Ganga: Water Use in the Indian Subcontinent - page 19

were deposited as flood plains of former Ganga-Brahmaputra river system, which
are almost the same as those of the recent flood-plain. Recent sediments are dark,
loosely compacted and have high water content and a variable but appreciable quan-
tity of organic materials. Pleistocene sediments, on the contrary, are well-oxidised
and typically reddish, brown or tan and mottled; water-content is low, resulting in
firmer and more compacted material. Organic material in these sediments is com-
monly confirmed to the surface soil profile. Much of the Pleistocene deposits have
either been eroded, or sank below recent alluvial deposits.
Himalayan rivers flow through poorly consolidated sedimentary rocks with a
number of folds, faults and thrusts. Owing to high gradient and velocity, these rivers
are highly erosive of softer materials, like pebbles, cobbles, boulders, coarse sands
etc. which cut and carve out deep gorges and remove the materials as fine particles
to act as sediment. Very often, landslides occur particularly during heavy rains and
increase sediment loads.
The sediments in Bengal rivers are made primarily of fine sands and silts with lit-
tle clay matrix. High sediment concentration makes the rivers constantly adjust the
configurations of their beds in diverse flow regions. Thus, high volume of sediments,
deposited in the basin, also erode because of change in flows. The deltaic region
is always subsiding, owing mainly to consolidation of recent sediments. Broadly
speaking, as Bose and Chakrabarti say, the whole of undivided Bengal is formed
by the deltaic processes of the Ganga and the Brahmaputra. To be precise, the
area bounded by the Bhagirathi-Hooghly on the west, by the Padma-Brahmaputra-
Meghna on the east and north-east and the Bay of Bengal on the south is a proper
delta, spread over about 54,750 km 2 within 21.5 and 25 ◦ N latitudes and 88.2 and
91 ◦ E longitudes (Fig. 3.3).
Fig. 3.3 Bengal delta (not to scale)