The tract lying between 25°8' & 27°31' North Latitude and 83°20' & 88°17' East Longitude is Bihar. It is bounded on the north by the Kingdom of Nepal, in the south by Jharkhand, in the east by West Bengal and on the west by Uttar Pradesh. Roughly rectangular in shape it comprises mainly the Gangetic Plains, the dissected highlands of Kaimur plateau and the Himalayan foothills occupying a very small tract of the state.
The North Gangetic plain extends from the base of the Tarai in the north to the Ganga in the south, comprising an area of about 22,000 square miles. Except for the Someshwar and the Dun hills in the extreme northwest, it presents an almost flat appearance and has an elevation of less than 250 feet above sea level.
The Someshwar and the Dun hills cover an area of about 364 square miles in Champaran. They are the lowest and the outermost of all the Himalayan ranges, immediately overlooking the plain, and form part of a long range which runs along the whole length of Nepal, at the southern base of which lies the swampy submontane tract called the Tarai. The Someshwar range runs along the northern frontier of the State for a distance of about 46 miles and varies in altitude from a few hundred feet to 2884 feet at Fort Someshwar, which commands a magnificent view of the Himalayas wrapped in the grandeur of eternal snow.
The Dun hills lie to the south of the Someshwar range from which the Dun Valley separates them. They are a range of low hills land extends from about 20 miles from the northwest to the southeast. The Tarai that lies towards their southern margin forms a narrow belt of submontane forest, followed by a prairie land of long reedy grasses. It is mostly marshy and unhealthy. This area of hills is but sparsely populated by an aboriginal tribe called the Tharus. But for this small area of hills in the northwest, one looks in vain in any direction for la hill or mound, which may interrupt the continuity of the level surface.
The North Gangetic plain is the playground of rivers- the Gogra, Gandak, Bagmati, Kosi, Mahananda and a host of minor streams, which descend from the Himalayas and make their way to the Ganga in frequently changing channels. The Kosi particularly has earned the bad name of being the most changeable stream in India, shifting its bed all the time. Because of the large quantities of silt they deposit the rivers at many points flow on ridges slightly elevated above the general level of the country and frequently inundate the low-lying lands on either side during the rains. The South Gangetic plain occupies the greater part of what is popularly known as South Bihar. It is wide towards the west and the middle, and extremely narrow towards the east. The outlying hills and undulations of the Chotanagpur plateau encroach upon the plain from the south, until near Munger they extend in the Kharagpur hills as far north as the Ganga itself, and after a brief recession run along the bank of the river for a considerable distance as the Rajmahal hills.
The South Gangetic differs in many respects from the North Gangetic plain. It is higher in the south and slopes towards the Ganga, but the slope is not so gradual as in the North Gangetic plain. While the North is liable to flood and change, the South Gangetic plain is stable and not subject to floods except in limited areas. Moreover, it is much more diversified than the North, and a great many hills spring as islands of high rock from the level alluvium.
Notable among the hills that lie scattered in the South Gangetic plain are the Barabar hills, the Rajgir-Jethian hills, and the Kharagpur hills. These hills lie in the districts of Jehanabad, Nalanda and Munger. Though not quite naked, they have been highly denuded, and most of the vegetation has either been cleared away with the axe or has disappeared with the erosion of the surface soil. They therefore present semi-bare rocky surfaces having a thin covering of grass and scrub. In the Kharagpur hills, areas of jungle still survive as but poor remnants of the dense forests which once clothed these hills. The Barabar and the Rajgir hills are generally less than 1,000 feet above sea level, but they exceed 1,000 feet at several places.
The Kharagpur hills have several peaks rising above 1,600 feet. High banks on either side contain the trough occupied by the channel of the Ganga. After the close of the monsoon, there appear within the trough large expanses of sand and silt deposits called diaras which vary greatly in extent and position every year. The Rajmahal hills form the northeast angle of the Chotanagpur plateau and are the only considerable mass of hills that approach the Ganga in its course in that region. Although nowhere do they rise higher than 2,000 feet they present an extremely varied and picturesque topography. On its eastern margin, the plateau has a height of less than 1,000 feet but the general configuration is still that of an undulating upland. The Kaimur hills, separated from the rest of the plateau by the valley of the Son, are structurally very different from Chotanagpur. They are the easternmost termination of the Vindhyan range and form an undulating tableland rising abruptly from the plain in bold and lofty precipices, to eminences of about 1,200 feet above sea level.
