The hilly region lies in the central part of Nepal and is less developed than the Terai but more developed than Himalaya. South of the Himalayan Mountain Region lies the complex zone of hills, valleys, basins and low tablelands of varying altitude and magnitude.
The central geographical region situated north to the Terai is the Hilly Region covering about 68% of the total land area of our country. Its altitude ranges between 600 m to 3299 m. This hilly zone rises from 600m. to 3600m. and varies in breadth from 80 km. to 128 Km. It mainly consists of a sloppy land structure that is made up of the Mahabharat range, valleys, foothills, Tars and river basins. Despite being the largest physical region, it is not so much prospered. It is the most extensive topographic unit representing about 50% of the total land surface of Nepal.
Nevertheless, the valleys and river basins are fertile, developed and densely populated. Some extensive valleys such as Pokhara and Kathmandu are its examples. The Churai Range, though dissected in some parts, runs parallel east to west into the southern part and Mahabharat ranges into the northern part. The climate is mild and soil is semi-fertile. This region is less developed in comparison to the Terai but more developed than the Himalayan. It accommodates the 43 % of the total population and density of people is 186 people per sq km. The deciduous forest grows into the lower part and coniferous into the upper part of the region.
On the basis of general configuration, this hilly region can be divided into three sub-units:
a. Midland of Hilly Region
Lying between the Himalayan Ranges on the north and the Mahabharat Lek on the south, this midland section comprises of main hills, basins, valleys and low tablelands of varied dimension and elevation. Since its width runs from 64 km. to 80 km., this topographic sub-unit is the most extensive in size and extent. Its hilly ranges attain an altitude running from 900 m. to 1800 m. and extend in diverse directions acting as watersheds for various river basins. Since the hilly ranges are relatively lower in elevation and less rugged in form, they are comparatively more populated than other sub-units of the hilly region.
Most of the large basins and valleys of the midland section are of tectonic origin. Smaller basins are, however, the results of river erosion. These river basins and valleys, being formed of fertile alluvial soils, are very significant for agricultural activities. Low tablelands (tars in Nepali) are, on the other hand, entirely dependent on rain and are, therefore, relatively less productive. Some of the notable tars are such as Palungtar and Salyantar on west and Rumjatar and Tumlingtar in the eastern sector of Nepal.
Major enclosed valleys (Upatyakas in Nepali) such as the Kathmandu and Pokhara Valleys are located in the midland section. The Kathmandu Valley is said to have originated from a tectonic lake formed in a down warp. It disappeared later on due partly to infilling with sediments derived from surrounding ranges and due partly to down cutting of its outlet at Chovar by an outflowing river. It contains lacustrine deposits whose depth after boring at Lagan, Kathmandu runs up to 4000 m. The Pokhara Valley too in Central Nepal is supposed to be a vast lake in the geological past. The present lakes existing in this valley are conceived to be the remnants of the past lake. The valley is now filled mostly with rock debris of glaciers from Annapurna Himalaya. As such, most deposits in this valley consist of rough conglomerates like those brought by snow and ice-melt water. The parts occupied by limestone beds and consolidated conglomerates have now been cut deep by local rivers and streams.
b. Mahabharat Lek
South of the midland section occurs the Mahabharat Mountain Range that attains a height ranging from 1500 m. to 3000 m. from sea level. Being parallel to the Himalayas, it also runs the entire length of Nepal; and as such it is also termed as Middle Himalaya. In structure, it is composed of main limestone, sandstone, shale, marble, granite, slate and other metamorphic rocks of varied geological ages. This mountain is rugged in form and has steep face towards the south. Since its elevation is much lower than that of the Himalayas, it is free from permanent snow-cover.
Like the Himalayan Mountain, it is also not a single continuous range. Rather it is broken at several points among which mention may be made of three major breaks— Chisapani (Karnali on the west), Deoghat (Gandaki in the middle) and Chatra (Kosi on the east). It has several spurs extending towards north and south of its east-west trend. The existence of deep and narrow valleys in between massive spurs is one of the major features of this mountain range. It is because very many rivers and streams originating from the springs existing in its various sections have carved out such valleys.
c. Churia Hills
These are the youngest and lowest ranges running more or less parallel to the Mahabharat Lek on the north. It distinctly runs up to the Kosi River only beyond which it exists in the form of isolated hillocks occurring quite close to the Mahabharat Range in the far eastern sector of the country. These Churia Ranges known in North India as Siwaliks attain an average height of 1200m. above sea level. In fact, its elevation runs from 600 m. on the east near the Kosi River to over 1800m. on the west just north of Kailali District. It reflects that the Churia Ranges grow higher in elevation from east toward the west. It must be due mainly to intense erosion induced by the higher amount of rainfall in the eastern parts of the country.
These Churia Ranges known also as the outer Himalayas were uplifted much later during late Pliocene period, and have no relation, therefore, with the northern ranges in point of the geological structure. These are composed of loose materials like sand, shale, sandstone, pebbles and conglomerate which are less compact than those forming the northern mountain ranges. In form, it has mostly hogback landscape with the steeper slope towards the south. The range is mostly forested and is highly asymmetrical in form as well as in direction. Moreover, it is broken into several detached sections by the rivers flowing down from the northern parts of the country. The detached ranges sweep far north and south in various parts of the country. Those which sweep down far to the southern border bear certain local names such as Dang, Deokhuri, Danduwa and Sumesar and occur in western and central parts of the country.
The hilly region in its totality is no less significant than the Himalayan Mountain Region. Here also, the Mahabharat Lek acts to a great extent as a climatic barrier inasmuch as it also prevents rain bearing monsoons. Besides giving rise to numerous rivers and streams useful for irrigating land and generating water power, it bears mineral deposits of great economic importance. Likewise, the forest-clad Churia Hills prove to be potential reserves of commercial timbers. Finally the broad river basins, valleys and tablelands of the midland section are of great agricultural significance to hill economy.