Bihar is traditionally divided into
1) The North Ganga plain
2) The South Ganga plain
The North Ganga Plain
It extends from the base of the Terai in the north to
the Ganga in the south, covering an area of about 56,980 Sq Km. It spreads
over the whole of Tirhut, Saran, Darbhanga and Kosi divisions and has
a gentle slope towards the south.
The Ganga flows from west to east near
the southern margin of the plain. Towards the north and north-west in
the east and west Champaran districts, the country begins to undulate
and the alluvial plain gives place to broken hilly region known as the
Dun or Ramnagar Dun. This consists of a range of low hills. Below these
hills, large grassy prairies watered by numerous hill streams extend southwards
and eastwards. The soil even at the foot of the hills has no rocky formation
and whenever water can be impounded, rich growth of crop is possible.
The South Ganga Plain
The alluvial filling south of the Ganga is shallow, a
mere veneer and the Peninsular edge is very rugged. Many groups of small
craggy hills rise up to 488 meters from islands of bare rock or scrub.
In the west, where the stream Sone makes a great deltaic reentrance into
the older rocks, this alluvial strip is some 137 Km wide. But in the east
where the Rajmahal hills lies on the extreme north-east point of the Peninsula,
it goes almost directly on to the Ganga. The river bank itself lies high,
except in Bohjpur district and at high water the tributaries are flooded
and pushed back. The Punpun valley, parallel to the stream Sone on the
east, is thus annually flooded.
Both in the north and the south of Ganga, the construction
of railways across the drainage causes local but sometimes disastrous
water logging and flooding. Some of these temporary inundations are agriculturally
useful, either rabi crops are grown on them when they dry out or they
are bunted for producing dry weather rice.