Physiography of North East India

Physiography 
N.E. India is dominated by three major surface features:

a. The young fold mountain Systems,

b. The alluvial Plains,

c. The Meghalaya-Karbi Anglong Plateau.

1. The Young Fold Mountain:

The mountain and hilly terrain in the region covers an area of about 150,000 sq.km or nearly 60% of the total area of the region. It is a predominant feature of the states of Arunachal Pradesh, Nagaland, greater part of Manipur, Mizoram and north eastern part of Tripura. It is along the Arunachal Pradesh and northern Nagaland that the Himalayan range takes its syntaxial bend. This changes its trend to south and south-east. This zone can be divided into two distinct units such as:

a. The Arunachal Himalaya extending from the eastern frontiers of Bhutan to Siang-Dihang rivers in the east and

b. The eastern hills comprising of the Dibang-Lohit-Patkai-Naga-Manipur-Mizoram-Tripura hills.

The Arunachal Himalaya, in turn can be divided into i). the lesser zone and ii). The greater zone. The syntaxial bend of the Himalayan chain is evident in the south-east in Lohit district of Arunachal Pradesh. The average elevation in the greater Himalayan zone is about 6400 meters. The altitude decreases north eastward in Upper Subansiri and west Siang to about 5200 meters. The prominent peaks in this zone are the kangto (7900m) and Gorichen (6538m).

The eastern hills appear from the Dibang-Lohit knot between Siang and Burhi-Dihang rivers. From here the ranges radiate westwards. The Namkin Mountain trends eastwards, while in the south the hills take the name of Patkai range. These ranges are of lower elevations particularly along the Brahmaputra front and increase as they reach the international frontiers in the east and north-east. The Patkai range forms a continuous wall along the Ind0-Myanmar border.

The Barail range enters the state of Nagaland at the southwest corner and has a northeastern trend upto Kohima where it merges with the main mountain range. It then extends to Manipur. The most important peaks are the Saramati (3826 m) and Matungse Kein (3420 m) in the extreme east. The Barail Range divides the state of Nagaland into northeastern and southwestern halves.

The Naga Hills and the Barail range continue westwards to divide the North Cachar into northern half aligned to the Brahmaputra valley and the southern half aligned to the Cachar plains. With an average elevation of about 1600 meters, it reaches its maximum elevation of 1953 meters in Haflong. To the west are the tertiary hills of north Cachar that gradually sink in the Kopili gorge - a structural transition between it and the Meghalaya Plateau.

The Patkai range with a northeast to southwest trend extends into Manipur and encloses the intermontane valley of Imphal. These ranges have an elevation of between 750 – 3000 meters. The highest points are found in the northeastern parts of Manipur. The area is dominated by deep gorges and steep slopes and covers more than 90% of the area of the state of Manipur.

The Patkai range loses its elevation as it enters Myanmar. Its western offshoot enters Mizoram in a general north-south trend. The highest elevations are found in the central part of the state. These ranges are in the form of parallel alternated with deep valleys. The ranges in the east are of higher elevation. In the western parts, these ranges have an average altitude of 500 meters; the highest point in the state is Blue Mountain (2157 m). These ranges continue south westwards to northern Tripura. Tripura ranges are low and form six distinct parallel ranges. The general elevation decreases from 700 meters in the east to 400 meters in the west.

2. The Alluvial Plains:

These are the Brahmaputra plains, the Barak plain, the Imphal valley and the Tripura piedmont plain. The Brahmaputra plain is a narrow elongated valley with an east-west distance of about 660 km and an average north-south distance of about 70 km. At its widest the valley is about 90 km and narrowest point is about 50 km near the north Cachar hills. The valley widens in the west with the retreat of the Meghalaya plateau.  It is narrow near Guwahati. The Plain is dotted with numerous inselbergs of gneissic hills as in Darrang, Barpetta, Kamrup and Goalpara. The slope of the valley in the southern banks is gentle while in the northern banks, it indicates a sudden drop of elevation of the Himalaya to 500 – 200 meters in the plains.

An important noticeable feature of the Brahmaputra plains is the presence of alluvial fan coinciding with the Barbar zone of Terai. The streams are braided and often disappear to reappear at a short distance. This created series of shallow depressions that developed into swamps and bils. South of this was the flood plains. Similar features are also evident in the south bank of the Brahmaputra.

The Barak plain is the headwater part of the Barak-Surma-Kushiyara plains lying largely in the Bangladesh. This plain dips westwards but has its lower parts in the middle course through which the river Barak flows. It has a sluggish meandering course with numerous ox-bow lakes and swamps. It also indicated elevated terraces and flood plains.

The Manipur plain or the Imphal valley is a lacustrine product like the Kashmir valley. An intermontane plain, it has steep gradients of about 75 cm per kilometer. It has numerous depressions, marshes and lakes. The most important lake is located at Loktak. The Tripura piedmont plain is an extension of the Bangladesh plains. It is largely made of erosional and depositional features.

3. The Meghalaya-Karbi Anglong Plateau:

It covers an area of about 32,281 sq.km or 12% of the area of the region. The plateau rises abruptly for about 1000 meters from the Sylhet plains in Bangladesh. It maintains a more or less uniform uplift throughout the southern front facing Bangladesh. The plateau continues to rise till it reaches the central part, where the Tura-Kailash range reaches its highest elevation at Nokrek (1412 m). In the Khasi hills, the highest point is the Shillong peak (1961 m). The plateau gradually loses its elevation in successive ranges that spread out to all directions. However, the eastern and the western parts of the plateau present a deeply dissected profile. They merge into the Brahmaputra plains through series of isolated hills.

The Karbi hills which cut into the southern banks of the Brahmaputra constricted the width of the valley. It is detached from the Meghalaya plateau by the Kopili gorge and by the headwaters of Diyung, Lumding, Lankajar and other rivers. On the south eastern side, it is separated from the Naga hills by the Dhansiri and its tributaries. The plateau is circular in shape and was the result of heavy denudation. The core of the Karbi plateau rises to an elevation of 1361 meters near Dambuko.