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Physical Divison of Nepal

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The physical build of Nepal is of extreme diversity. Its surface configuration consists of mainly mountains, hills, plateaus, basins and valleys of varying altitude and magnitude. It also ought to be noted that the variation in landform along latitudinal direction is far greater than that along longitudinal one. That the topographic elevation increases from below 100 metres in the terai region on the south to over 8,800 metres in the Himalaya Mountain on the north proves the validity of this statement. It is estimated that about 75% of its land surface is built of varied mountains and hills including river basins and tectonic valleys entrapped in between them. On the basis of landforms and their elevation, the Kingdom of Nepal can be divided into three broad physiographic divisions each having three sub-units as noted below:—

1. The Himalayan Region
The Himalayan Mountain situated on the north part of Nepal is the youngest and loftiest fold mountain of the world. Its tremendous height is supposed to be due to subsequent uplifts during later periods of Tertiary Era. This mountain zone encompasses about 25% of the total area of the country. The height of the mountain runs from about 3,600 m. to 8,848 m. above sea level. Here, snowline occurs at the altitude of about 5,000 m. and the portions above this line are perpetually capped with snow. It is alleged that 4 peaks of over 8,100 m., 13 peaks of over 7,600 m. and 240 peaks of over 6,000 m. are confined to this mountain region.
The Himalayan Mountain extends in an east-west direction for about 850 km. and commands a north-south breadth ranging from 24 Km. to 48 Km. The glaciers formed out of sliding snow have produced several U-shaped valleys and lake basins. Almost all the prominent rivers of Nepal are conceived to have originated out of the melting of glaciers. These rivers and glaciers have greatly moulded the original landscape of this mountain zone.
Though stretching for the entire length of north Nepal, the Himalayan Mountain is not a single continuous range; for it is carved at its various sections by various antecedent rivers like the Karnali, Gandaki, Kosi, etc. Near the river-breaks on or close to the border are located various high passes such as Kodari, Rasuwa, Mustang, Namja and so on. But most of the Himalayan passes, owing to thick deposit of snow, remain closed during winter months. Trade and communication between Nepal and Tibet are, however, conducted through those passes during other months of the year.


2. The Hilly Region
South of the Himalayan Mountain Region lies the complex zone of hills, valleys, basins and low tablelands of varying altitude and magnitude. This hilly zone rises from 600m. to 3600m. and varies in breadth from 80 km. to 128 Km. It is the most extensive topographic unit representing about 50% of the total land surface of Nepal. On the basis of general configuration, this hilly region can be divided into three sub-units:
(a) Midland — Lying between the Himalayan Ranges on the north and the Mahabharat Lek on the south, this midland section comprises of mainly 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 upto 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 Annapurana 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 for the entire length of Nepal; and as such it is also termed as Middle Himalaya. In structure, it is composed of mainly 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. 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 upto the Kosi River only beyond which it exists in the form of isolated hillocks occuring 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 west. It must be due mainly to intense erosion induced by higher amount of rainfall in the eastern parts of the country. These Churia Ranges known also as outer Himalayas were uplifted much later during late Pliocene period, and have no relation, therefore, with the northern ranges in point of 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 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 midland section are of great agricultural significance to hill economy.

3. The Terai Region
It constitutes a broad stretch of level land extending from beyond the Mahakali Rivers on the west to the Mechi River on the east of Nepal. Because of the occurence of hills and mountains on the north, the general gradient of this level region runs towards the south. On the basis of structure and surface relief, the terai region is relatively far more homogeneous than any other topographic units of the country. This terai region including inner terai basins encompasses approximately 25% of the total land surface of Nepal. It has been formed out of the age-long deposition of sediments carried by various rivers and streams debouching from the northern hilly and mountainous parts of the country. Like the northern hills and mountains, even the terai region on the basis of location is popularly divided into three sub-units as: (a) Eastern Terai (b) Central Terai and (c) Western Terai. But more significant from the geographic point of view are the three north-south sub-units introduced below:


(a) Terai (Outer or Open Terai) — It is the vital part of the terai region and represents some 13 % of the total area of Nepal. Its elevation from sea level rises from below 100 m. in the eastern section to the maximum of 300m. in the western one and attains the maximum width of 56 km. It is not quite continuous; for it disappears at places where the Churia Ranges extend upto the border. Its eastern section is relatively much broader than the western one. Though limited in size and extent, this terai section, being composed of fertile alluvium, is of substantial significance from the viewpoint of agricultural production.
(b) Inner Terai — There exist several synclinal valleys bounded by the Mahabharat Range on the north and the Churia Range on the south. These are of tectonic origin and are known as Inner Terai or Doons. In point of dimension, their east-west length varies from 32 km. to 64 km. Though their average width is reckoned to be 16 km, they are much broader in central and western parts where the Churia Ranges extend upto the border. Their elevation from sea level runs from about 200 m. on the east to 400 m. on the west. These inner terai basins altogether represent some 8 % of the total area of Nepal. These are basically composed of alluvial soil which grows coarser towards the marginal parts close to the Mahabharat and Churia Ranges. The most widely known inner terai basins from west to east are Dang, Deokhuri, Chitwan, Makawanpur, Sindhuli and Udaypur. In several physical aspects, they nearly resemble the outer terai plain and prove, therefore, to be potential agricultural areas of Nepal.


(c) Bhabar Tract — It constitutes a narrow stretch of level land which gradually merges with the outer terai plain on the south and happens to be on the southern foot of the Churia Ranges. It is roughly 200m. high from sea level and represents about 4 % of the total area of the country. This tract is formed of coarser materials like sand, pebbles, conglomerates and rock detritus carried and deposited by the rivers and streams debouching from the Churia Ranges. Since its surface is in this way capped with such loose and coarse materials, water rapidly disappears from the surface; and hence this part of the terai region is of little significance from the standpoint of agricultural production. Almost all parts of this tract are covered since long with sub-tropical evergreen forest.
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