The Geological Study of the area of Ganges and its Tributaries: Structurally the region is a segment of the great Indo-Ganga trough; however, it has some marginal portions of the other two major formations, that is, the Siwaliks in the northern part of Champaran District and the fringes and projections of the Peninsular Block in the south. The Himalayan foot-hills region of the Siwalik formations covering entire Northern parts.
It is evidenced that although the average alluvial filling may not exceed 1300-1400 meters, it varies widely from one part to the another; particularly the depth increases up to 8-10,000 meters as the Himalaya are approached while it thins out as a mere veneer on the peninsular margins. There is one large trough, which is called Raxaul-Motihari trough, over 8,000 meter deep in the north, and 1,500 meters in the south, except a few patches in the Ganga-Ghagra interfluve being deeper.
It is indicated that the entire region has suffered great down-wrapping due to the Himalayan upheaval. The basal rocks seem to have faulted transversly at places and dislocations in the earth's crust along such pre-existing faults or cracks cause earthquakes. Fault lines can be seen running N-S or NE-SW, one through west of Muzzafarpur and Patna and the other east of Purnea.
The nature of the alluvial sediments has been already noted; here kankar formation is relatively less because of more riverian character of the plain, and more particularly of the higher incidence of Khadar lands.
Hemmed between the foot-hills of the Siwaliks and the Bhadar in the north and the Peninsular uplands in the south, the region is almost a synonym of a homogenous level, seemingly featureless plain from one end to the other, and the monotony of releif never appears to lose until th hills are actually approached; in small topographic facets, however, the plain is not without interest. in general it is below 100 meters above the sea-level, except that it gradually rises up to 150 meters in the south incorporating the projections of the souhtern uplands; in the east the Kosi Plain ranges between 30 meters in the south to 75 meters in the extreme north. Heterogeneity, if any, in the physical lanscape is produced actually by local eminences such as river levees and bluffs, or sandy features like the oxbow lakes, Tals, Chaurs, Dead arms or remanents of the river channels, or occasionally available badlands and ravines, or not infrequently perceptible notches and slopes carved by the rivers at the outer edge of the bhangar tracts. A more pronounced relief is occasioned when the plain meets the hilly area; in the north, beyond the the Tarai, are the Bhbar plains, actually not falling within the region, but bearing the stamp of their looselt-set gravelly nature and talus slopes over the Tarai, particularly in the extreme north, where the surface appears to be broken by large rivers, like the the Gandak, the Baghmati, the Kosi, etc., which comb the region with their affluents in an intricate pattern.
Obviously, it is difficult to divide the region into physical sub-units on any prominent foundation of relief, except through the help of the river-systems which generally carve out somewhat inter-distinguishing relief and slope, differential nature of drainage based on the rainfall regimes and the proximity of the hills, and the all-resultant sub-soil water table of varying depths in different parts of the region. Based on these somewhat cogent foundations, the regions may be divided into the Ganga Plain North and Ganga Plain South.
The Ganga Plain North can be further divided into (i) Gandak-Kosi interfluve, and (ii) the Kosi-Mahananda interfluve. As Geddes has worked out, the broad relief features of the entire Saryupar and N. Bihar Plain are a series of alluvial cones formed by master streams alongwith the inter-cones or the interveneing slopes between them. Generally and quite naturally the inter-cones or the intervening slopes between them. Genrally and quite naturally the inter-cones have lower gradients than the cones. Excepting the small chunk of Siwalik formations, the vast N. Ganga Plain is absolutely featureless, without a single hill, the surface variations are all the result of the drainage lines as discussed above.
The Gandak-Kosi interfluve covers a large area in the North Bihar Plains where rivers like Bagmati, Burhi Gandak, Balan, Kamla and Masan with others drain southwards. There are numerous oxbow lakes, tals and dead or deserted channels of the rivers. The northern tarai has a moist belt in the extreme north followed by the sub-tarai belt of marshy lands with intervening uplands, which is again followed in the south by another, rather wider belt of marshy lowlands, devoid of uplands and having marshy lakes and depressions. Towards the south lies the upland zone till the Gandak levees or flood-plains are approached.
The Kosi-Mahananda interfluve has more of these features as the river is notorious for its shifting channels. However, both the Gandak and the Kosi have been confined in most places by the artificial embankments.
The Ganga Plain South can be divided into four parts - (i) Karmnasa - Sone interflute, (ii) the Lower Son Valley and (iii) Magadh-Anga Plain. As is obvious from the figure, the ganga Plain South is defined by the 150 meter contour rather irregularly because of the spurs and the scarps of the Peninsular uplands. It is very narrow west of the Jargo river (Chunar), and wide up to 100 to 140 km in the Son Valley, again shrinking to 40 km opposite Giddhaur Hills and widening to 65 km in S. Monghyr and bhagalpur but tapering hardly to a 3 Km-wide ribbon when the Rajmahal hills project themselves against the Ganga to separate the region from the Lower Ganga Plain. The alluvium deposited hare has been brought from the southern uplands and is relatively caorser; it also lacks the ox-bow lakes which are so commonly found in the northern plains.
The relief east of the Karmnasa is free from the pent-land topography. The souhtern edge is relatively more regular, particularly in the bhojpur Plain, while the general surface cast of the Son is more or les even and is dotted with residual conical hillocks, sometimes forming elongations up to the bank of the ganga. The lower Son valley is a physiograhic unit in its own right, although the flood-plain is itself narrow. East of the Son in the Magadh-Anga Plain, there are a number of hills like the Barabar, the Rajgir-Jethian (446 meter at Rajgir) and more significantly the Kharagpur hills, running in SW-NE elongated ridges; the Kharagpur hills throw themselves against the Ganga near Monghyr and their sub-alluvial projection causes the river to have a sharp bend. One rather peculiar feature is the extension of the low lying area called the tal in Patna district, south of the levee of the ganga, perhaps marking the old bed of the Son.
Chotanagpur is composed mainly of Archaean granite and gneiss rocks with patches of Dharwar rocks (phyllite, mica-schists etc.) in the northern and southern margins, the latter being very conspicious in the iron ore and series covering greater part of the Singbhum district. The Dalma range marks the belt of Archaevan lava flows. "The structural base of the region is provided by a series of batholithic intrusions of granite into Dharwar strata, which were intensely metamorphosed by orogenetic movements. The earliest floor, on which the Dharwars were deposited has not been recognised anywhere since it was subsequently metamorphosed". This part of Chotanagpur is characterised by complex geological structure. There is evidence of a peneplained "ancient fold mountains, extending east-west across north Singbhum and south Ranchi and into north Dhalbhum and south Manbhum..". Jamshedpur lies on the central axis of this pre-existing range; the hills streching east and west of the old mountan system. The greater shear or thrust zones run approximately ENE-WSW and E-W for nearly 100 miles turning to the SE near the eastern end.... There are two almost parallel thrust zones to the north of the great shera zone, one marking the northern limit of a broad belt of lavas (Dalma Traps) and the further north. They are almost parallel to the Satpura trend and converge near Goilkera, east of Chakradharpur."
In the structural trough of the Damodar valley occur Gondwana rocks consisting of sandstone pf great thickness with some slates and clay. The Pat land on the western margin is covered with Deccan trap which is converted into laterite and bauxite due to weathering. The Rajmahal hills in the north-eastern fringe of the plateau are also covered with lava flows which were probably linke with the initiation of of the himalayan orogeny. Rajmahal Traps consists of dolerites, basalt and andesites.
Of greater importance for understanding the existing gemorphology in the incessant erosion from Creataceous period accentuated by uplifts during the Tertiary era in the follwing succession :
(i) An early Tertiary peneplain was uplifted by 300m to the south with a tilt to the north-east;
(ii) A further uplift of perhaps 300m sometime between Middle and Late Tertiary;
(iii) After an interval sufficient to permit the formation of quite a well defined peneplain, a futher uplift of above 100m took place with at least a sharp unwrap in the Subarnarekha plain.
Towards the southern edge of Chotanagpur the upward movements were culminative whereas futher north, close to the edge of the Ganga alluvium and particularly around Rajmahal hills, there was no apparent differential movement- there may have been subsidence but not certainly not uplift.
The river Ganga flows right across Bihar from West to East dividing it into two unequal parts, the Southern portion being almost double the Northern Portion. Thus, the main line of physiographic division is between
