LOCATION, SITUATION AND HISTORICAL BACKGROUND 

Location and Situation

North-East India is Located between 20^o N lat, and 29^o 30 N lat. and 89^o 46 E long. and 97^o30�E long. Its northern and eastern boundaries are natural barriers represented by the high Himalayan Mountains and Patkai Hill ranges respectively. However, its southern and western boundaries are more political than natural.

To the north of the region lie Bhutan and Tibetan part of China. The eastern boundary is covered by Myanmar. The Arakan Yoma of Myanmar and Chittagang and Tippera hill and Surma Plain of Bangladesh lie to the south of the region. To the west of the North- East India there lie West Bengal and Bangladesh.

North-East India is commonly called the land of the �Seven Sisters�. Of the present seven states, viz Arunachal Pradesh, Assam, Nagaland, Meghalaya, Mizoram, Manipur and Tripura, the first five were parts of undivided Assam, from the view point of administration, during the pre- Independence period, while they were subsequently constituted into individual states to fulfill the socio political aspiration of the people inhabiting them. Manipur and Tripura, on the other hand, were princely states which become Group C states after independence and later become full- fledged states.

North- East India is surrounded by hill and mountains on there sides. It is only towards the west that the region is bordered by plains. But at the time of attainment of independence in 1947, the country was so partitioned that the eastern part of the then Bengal was constituted into East Pakistan (now Bangladesh ) and this almost isolated the north eastern part of India from the rest of the country , except for a narrow corridor of about 40 km width along the Bhutan foothills. While this corridor lies between two foreign country (Bhutan and Bangladesh), the main part of North-East India is surrounded by four foreign countries, viz China, Myanmar, Bangladesh and Bhutan . The region, therefore , assumes a great strategic importance.

The total area of the region is 255036 km² , representing about 7% of the country. It supports 31,386911 souls (1991), accounting for 3.90% of India�s total population.

The Frontiers and Boundaries : Since the shape of North - East India is like that of a reverse triangle with its apex at the southernmost tip of Mizoram, there is no distinct southern boundary of the region .

To the north of the region lie the Himalayan kingdom of Bhutan and snow -clad Great Himalayan Mountain ranges, separating Tibet of China from India . The international boundary between Bhutan and India runs along the junction between the plains and hills and hill promontories jutting into the plains. Therefore, the Indo-Bhutanese boundary , although largely physical , has certain parts (like the Eastern Duars and Subankhata areas ) determined through political negotiations. There are several routes linking the two countries, which are traditionally known as �Duars� (doors).

The eastern part of the northern boundary runs from the north -eastern corner of Bhutan to the tri-junction of China Myanmar and India for a distance of 1140km. This boundary is know as the Mc. Mahon Line and was determined as per Simla Agreement of 1914 between India and Tibet (then an Independent country). It runs along the highest Himalayan Range, much above the snow line. The passes like Bumla, Thaga La , Yangiya La, Chaka, Lamdu , Tulung La, Lucha, Dom La, Andra La, Yanga La, Yangiya La, Chang Kang Kaya La, Diphu, etc. connect Arunachal of India with Tibet of China across the Himalayan Range.

The south -eastern boundary follows the general alignment of the hill ranges and runs partly over the Patkai Range and partly over the ranges lying in the same alignment as the Patkai in Manipur and Mizoram . The ranges enter into Myanmar in the name of the Arakan Yoma. although the south-eastern boundary separating Myanmar from India runs mostly along high ranges , it is cut across by many rivers that flow into the Chindwin and a number of passes. The important passes across this boundary are Kumjawang pass through which the Allied Forces built the famous Stilwel Road from Lekhapani in India to China in World War II and it is Tuzu Pass through which the Indian National Army, marched into India during the same period.

The south-western and western boundary is man-made and hence runs mostly along an arbitrary line over villages and crop-fields. It separates Mizoram, Tripura and Karimganj districts of Assam, Meghalaya and part of Dhubri district of Assam from Bangladesh. In the northernmost part of this boundary, the line separates West Bengal from Assam It is through this part that North-East India is connected with the rest of the country by roads and railways.

Present North -East India consists of ancient Assam, Manipur and Tripura and the surrounding hill tracts. The ancient Assam used to be know as Pragiyotisha in the earlier epic period and as Kamrupa in the later part of the first millennium A.D. i.e. in the age of some Puranas and �Tantric� books. The capital of the ancient Pragiyotisha and Kamrupa was most of the time located at Prajyotishpura, which is identified as present Guwahati. Like ancient Kamrupa, ancient Manipur and ancient Tripura also have their own history. In the absence of any written records the history of other hill states of North -East India is not clearly known. But all the peoples living in these states have their rich legends and traditions that run through since time immemorial.

The boundary of the ancient Kamarupa used to fluctuate in different periods of history. At one stage it is said to have extended upto Karatowa (a river, consisting in the past , of now abandoned channels of Koshi, Mahananda and Tista ) in the west and included a large part of the northern parts of West Bengal and Bangladesh. Both Yoginitantra and Kalika Purana mention the spread and sway of Kamrupa from Dikshu River , (which perhaps existed in the present day Sadiya region) in the east to the Karatowa in the west, and from Kanchan Giri      (Kanchenjunga in the north -west to the confluence of Lakshmi and Brahmaputra (now in Bangladesh in the south).

Between the fourth and twelfth centuries A. D. the plain part of North East India, then called Kamrupa, was ruled by the Barman , Salastambha and Pala dynasties. Kumar Bhaskar Barman,(594 AD-650 AD), a contemporary of Harsha Vardhana, was a famous king and it was during his reign that the Chinese traveler Huen-Sang visited Kamarupa and wrote in praise of beauty and prosperity of the country . At the beginning of the twelfth century Muslim invasions started (1206AD) from the west, and the Ahoms entered from the east under the Leadership of Sukapha (1228 AD). With the entry of these two forces into the political arena of North-East India, there occurred a series of political changes. The capital of the western part of Kamata kingdom was shifted to Kamatapur (a place located 32 km south of Cooch Behar ), the Bhuyan feudal lords come to hold their sway in the middle of the Brahmaputra Valley, the Kacharis and Chutiyas carved out their Kingdoms in the southern and north -eastern parts of the Valley respectively, while the Ahoms established their kingdom over the area between the Burhi-Dihing and Dikhow rivers. With the passage of time the smaller chiefs and kings were wiped out and towards the sixteenth century there remained only two powers in the Brahmaputra Valley-- the Ahoms in the east and the Koches in the west. In course of time the Koch kingdom also broke into two portions and subsequently its western part went under the rule of the Mughals, while the eastern part came under the control of the Ahoms.

The origin of the name Axom (corrupted to Assam ) is not clear. The scholars, however, believe that it perhaps came from �Ahom� the people who ruled a major part of the Brahmaputra Valley for long six hundred years from 1228 AD to 1826 AD. Assam was a powerful kingdom in North East India during the late seventeenth and early eighteenth centuries. It was from the first Moamaria rebellion i.e.1769, that the Ahom kingdom began to decline. The third Moamaria rebellion took place in 1786 and at the same time, the prince of the vassal Daramgi state also started a rebellion against the Ahom king Gaurinath Singha. The desperate Ahom king sought the help of the of the East India Company and the British Governor General, Lord Cornwallis, stationed at Calcutta, sent six companies of soldiers under Captain Wales to help Gaurinath Singha quell the rebellion. Although it was effectively done, there appeared feuds and quarrels among the ruling Ahom princes and generals and their kingdom was ultimately invaded by the soldiers of the northern Burmese kingdom in 1816, 1817 and 1821 and ravaged the whole Brahmaputra Valley. The Ahom forces were defeated in each of the their attempt to prevent the onslaught of the Burmese forces. The Burmese then trespassed into the onslaught of the British territory and the British declared offensive on March 5, 1824 and pushed back the Burmese invading army to their territory. On February 24, 1826, a treaty was signed at Yandaboo and the Brahmaputra Valley was annexed by the East India Company. Although the Valley was annexed in 1826, its north -easternmost part comprising present Sadiya sub- division and Dibrugarh district were under the Khamti and Matak chiefs respectively. Similarly , the present eastern Assam districts of Golaghat, Jorhat, Sibsagar and Lakhimpur were allowed to be ruled by the Ahom vassal prince. By 1839, all these areas were brought under direct British rule. The westernmost part of the Brahmaputra Valley and Garo Hills has been earlier annexed by the British from the Nawab of Bengal.

The Barak Valley or the undivided Cachar was taken over by the East India Co. from its local Kachaei king in 1832. The Jaintia and Khasi hills districts were ceded to the Company�s territory in 1835 and the Barak / Surma Valley was linked with the Brahmaputra Valley in by a cart-route. Between 1839 and 1854, the present North Cachar Hills District, and in 1866 the major part of Nagaland, in the name of Naga Hills District, were taken over. The Bhutan Foothill region of the percent Kokrajhar District was known as the �Eastern Duars�. For a year or two it was kept as a separate administrative district and later on merged with the then Goalpara district. Till 1874, all these newly annexed parts of North - East India were under the Bengal Presidency. In that year the Province of Assam was constituted under a Chief Commissioner with Golapara, Kamrup , Darrang , Nowgong, Sibsagar, Lakhimpur, Naga Hills, United Khasi and Jaintia Hills, Garo Hills, Cachar (including the present North Cachar Hills) and Sylhet. The Present Mizoram, then know as Lusai Hills, was annexed by the British between 1891 and 1895 and in 1898 it was constitued into a district of Assam.

Like Kamarupa or Assam, Manipur was also an important area in the part. The epic Mahabharata mentions that Babruvahana, the son of the third Pandava, Arjuna, was born of a Manipuri princess. The Meiteis (the indigenous Hindu people of the Manipur plain ) claim their descent from Babruvahana, In the mediaeval period the Valley was deeply influenced by the Vaishnav religion cult of Bengal The old temples of Bishenpur (Bishnupur), Imphal and other places do substantiate this fact, However, in 1714 AD, Pamhabia a courageous Naga leader occupied Manipur and established his dynastic rule. On ascending the throne, he embraced Hinduism and took the name of Garibnawaz. After him the kingdom continued as an independent unit till 1891. During this period there were several invasions from the kingdom of Upper Burma, on the one hand and a lot of feuds and assassinations within the members of the royal family, on the other. The Britishers who established themselves firmly in Assam, took over Manipur in 1891 on the plea of murder of some top British officials who had gone there to quell a rebellion. Manipur was then made a princely state under the British rule. In 1949 it was made a Part-C state under independent India and subsequently in 1972 it became a full-fledged state.

Tripura also has a long political history of its own. The archeological evidences and old inscriptions reveal that a large part of Tripura, especially its south -western portion Samata, Vanga and Harikela , the old kingdoms that existed in ancient days. Subsequently, in the early part of the last millennium, Tripura came to be ruled by the Devas of Srihatta. When Srihatta finally fell into the hands of the Muslims in 1257 A.D., the chief of Tripura tribe rose to power in the hilly region of the state and established a dynastic rule . This dynasty of kings could extend their reign over the plains adjoining the hills. Further they accepted Hinduism, assumed the title of Manikya and claimed their descendence from Chandra. They ruled Tripura efficiently for several centuries. In 1761, British Government appointed a Political Agent in Tripura to protect their interest and to aadvise the Rajas. Thus Tripura became a princely state under the British. On October 15,1949, it was merged with the Indian Union as a Part-C state and on January 21,1972 it became a full- fledged state.

The foothill areas of the present Arunachal Pradesh inhabited by many culturally rich tribal groups since very old days, consisted of independent tribal territories. They used to maintain cordial relations with the people living in the plains. After the Britishers annexed Assam, they came in direct contact with the tribals of Arunachal Pradesh and foothill areas adjoining the Brahmaputra Valley came more or less under the control of the former from about 1875-76 with the introduction of the Inner Line Regulations. With the passage of time, the British administration penetrated deeper into what is today Arunachal Pradesh and the areas bordering Assam were brought under its control as Frontier Tracts and in 1914, the boundary between British India and Tibet was agreed upon along the snow-capped high Himalayan range from north-eastern Bhutan to the tri -junction of India, Tibet and Myanmar covering a distance of 1140 km. This international boundary is known as McMahon Line and was adopted at Simla Convention in 1914.

At the time of independence, Sylhet district, except the police station areas of Badarpur, Patharkandi, Karimganj and Ratabari, went to the then East Pakistan. Thus Assam lost an area of 18,969 km. retaining for itself 133165km. Further,on September 1, 1951, an area of 85 km. of Dewangiri (north of Nalbari district ) was seceded to Bhutan, thus further reducing the area of Assam to 133080 km. However, there were three distinct administrative units in North -East India at the time of transfer of power. They are Assam Manipur and Tripura . From 1921 Assam had remained a province administered by a Governor and after independence it was made a Group A state, The Frontiers Tracts to the north-east of Assam came to be administered as North East Frontier Agency (since 1951) by the Governor of Assam as the Agent of the President of India. Manipur and Tripura, which had been princely states were made Part-C states under Chief Commissioners. Later on these two were made Union Territories. Meanwhile, the regional social groups began to give vent to their aspirations for separate administrative units or states of their own and agitations to that end began. Naga hills was separated from Assam and made a separate state on December 1, 1963. Garo Hills and United Khasi and Jainta Hills were constituted into an autonomous state in the name of Meghalaya in 1970 and a full-fledged state in 1972. Similarly, Lusai Hills was made a Union Territory in the name of 1972 and in the same year North East Frontier Agency was also made so in the name of Arunachal Pradesh. These two Territories were raised to status of states in 1987.

Thus in the later part of the 1980 as many as seven full-fledged states emerged within the Union of India. Following are the primary details of the states of North East India.

North-East India,1991

Districts and Headquarters Of

North East India

 

ASSAM :

MANIPUR

MEGHALAYA

TRIPURA

 MIZORAM

North East India consists of there structural units. The core of the region is the Karbi-Meghalaya Plateaus, representing a part of the Gondwanaland or the Deccan Plateau. This oldest Pre-Cambrian part came into existence as a results of the igneous activities occurring at the time into cooling of the earth�s surface from is molten stage. The southern part of the Meghalaya Plateau, bordering the Cretaceous and Tertiary periods This is way the southern face of the Meghalaya Plateau is precipitous.

The Himalayan Mountains in the north and the hills found in the east and the south are the product of age-long sedimentation, folding and upliftment . All these hills and mountains were built up during the Tertiary period from the sediments deposited in the Tethys Sea that existed in the geological past around the Meghalaya-Karbi Plateau mentioned above.

The Brahmaputra Plain, as also the other smaller Plains have been built up a result of deposition of alluvial materials on the early Tertiary and Mesozoic bed rocks of the depressions that had come into existence in this region due to tectonic disturbances. Although the origin of the Brahmaputra trough is not clearly known, it is said to be the result of the down -buckling of the frontal part of the India Plate that has been pushing northward down the Central Asiatic Plate as per the Plate Tectonic Theory. The Barak Plain is another depositional Plain, that perhaps originated due to the pressure exerted again by the Indian Plate. Like the Brahmaputra plain it is now filled up with alluvial deposits laid on the Tertiary bed rock. The Tripura Plain is a piedmont one, originated as a result areas of erosional and depositional activities commonly active in the foothill areas of the humid tropical regions. The Manipur Plain, said to be a lacustrine plain, came into existence as a result of draining out of the water of the lake that existed at what is today Manipur Plain. The headward erosion of the Manipur River attacking the Southern rim of the lake from the Myanmar side perhaps succeeded in capturing the lake near Sugnu and drained it dry leaving aside only the deeper parts.

North East India is geologically a very complex region since it represents the dynamic frontal part of the India Plate on the one hand and the relatively stable central (Chinese ) Plate on the other. In between these two plates there lay the orogenically active bed if the Tethyan Geosyncline with its foredeep skirting the dynamic Indian Plate.

The Geological history of the region directly or indirectly involves almost all the major geological ages from the Pre-Cambrian to the Recent. The Shillong Plateau and the Karbi Hills being part of the Gondwanaland are Archaean (?) or Pre-Cambrian in age. In its northward movement a part if the Gondwana Massif (also termed as the Indian Plate), exerted pressure on the Tethyan belt, which, being pressed by the plate in the south and west and by the Chinese Plate (central and South -East Asiatic Plate )in the north and north-east, took a syntaxial bend along what is today Dibang-Lohit Hills. The Whole geosynclinal belt representing the Arunachal Himalaya and the eastern hills, from Dibang-Lohit to Mizoram and beyond , came into existence in the course of Himalayan orogeny in the late Tertiary and Quarternary periods. Thus, geological beds ranging from pre-Cambrian to Recent periods can be found scattered in the region.

The northern part of the Shillong Plateau and the western part of the Karbi Hills have been in existence ever since the Gondwanaland was formed containing highly metamorphosed gneissic complex. In some places these rocks, however, are overlain by a series of sedimentary rock deposited during the Pre-Cambrian age and are identified as Shillong Group of rocks. These rocks are not highly metamorphosed except around the granite plutons (where thermal metomorphism did take place). The rocks have lost their original elastic character. Further, in the southern of the Shillong Plateau there occurred eruptions in the Jurassic Period depositing a layer of basalt known as the Sylhet Trap. The southern part of the Shillong plateau and the eastern part of the Karbi Hills also underwent submarine condition in the Cretaceous and early Tertiary periods providing conditions congenial for the deposition of coal, limestone, shale and conglomerate.

Barring aside the Meghalaya Karbi Plateau, the rest of North East India is generally made up of rocks that were deposited in the Tethys Sea or Tethyan Geosyncline and the foredeep that skirted it (which is supposed to have given rise to the Brahmaputra Plain subsequently). Geologically the oldest rocks that are found in North East India in patches, outside the plateau, belongs to the Cretaceous Period in the form of limestone in the eastern part of Manipur. Following this, in the Eocene period there occurred extensive depositions identified in the region as the Disang/Jaintia Group of rocks. These are mostly shale, sandstone and conglomerate. But these also contain, in places, Limestone and coal as found in Bokajan area of Karbi Anglong, Garampani -Umrangsu area of North Cachar Hills and in the southern parts of Garo and Khasi hills . The next rock beds were deposited in the Oligo-Miocene periods which are known as the Barail Group . This group of rocks is important in that it is in this group that rich deposits of both coal and oil are laid. In general ,the beds laid down during this period are of sandstone, shale, siltstone, marl, conglomerate, etc. But in places like southern part of Tinsukia district, south-eastern parts of Dibrugarh and Jorhat districts, there are rich deposits of oil, as found in Digboi, Naharkatiya, Duliajan , Moran, Lakwa, Geleki, Rudrasagar and Borhola, and coal in Ledo-Lekhapani Jaipur, Naginimara, Tekok Parbat, etc. Overlying the Barail group of beds there are Surma and Tipam beds of the Miocene period. These again, generally contain siltstone, sandstone, conglomerate, shale and clay with occasional oil reserves locked in the sandstone beds, especially in Dibrugarh district. The deposits of this period are known as �Lower Siwalik� in Arunachal Pradesh. The deposits of the Mio-Pliocene are known as Namsang/ Dupi Tila Group and they again contain sandstone, clayey sandstone, clay and conglomerate. No coal or Limestone has so far been found in these Tertiary beds. By the Pliocene period perhaps the Tethys Sea, in this region became shallow and normally allowed the deposition of pebbles and sandy clay known as Dihing Group of sediments. These sediments are contemporaneous to the Upper Siwaliks of the Arunachal Himalaya. By the end of the Pliocene period most of the North East India rose up from the womb of the Tethyan Geosyncline in the long continued orogenic process, except; a few vestiges left in the form of isolated fresh water lakes in the Arunachal Himalayan region . It is in these lakes that the uppermost Siwalik beds are deposited in the Plio-Pleistocene times. In other low lying parts, which were above the sea level, alluvial deposits, known as the Upper Dihings, were laid down,giving rise to the high level older alluvium found in the foothills and local higher grounds , as in the eastern part of Sonitpur district( between Mijika and Brahmajan) and Golaghat district. The older alluvium contains clay, sand , shingle and pebble. In the Sub-Recent and Recent times newer low level alluviums, in the form of clay, sand and silt, were laid down. The deposition of these materials, in fact, are still continuing.

An important fact of the geology of North East India is the presence of a large number of thrusts and faults and upliftment of relatively older beds as a result of acute reverse faulting.

In the Arunachal Himalayan region one encounters successively older rocks as one moves northwards. The outermost hills here, skirting the Brahmaputra Plain, are made of Tertiary rocks. But as one moves to the middle part, one can find patches of Gondwana and Lower Palaeozoic deposits. Further north in the higher Himalayan region there are both early Palaeozoic and Pre-Cambrian deposits. This is because the thrusts and reverse faults acting from the relatively passive Central Asiatic (Chinese) Plate were more pronounced towards the north and these raised even the lower rocks up. While the younger upper rocks have meanwhile been removed by the processes of sub-aerial erosion the Lower, older rocks have come to be exposed. Although most of the faults in the Himalayan region are not evidently visible, the Main Boundary Fault, along with southern border is still traceable. It runs from WSW to ENE upto the Siang river. To the east of that i.e. in the Dibang Valley- Lohit- Changlang -Tirap region, the rock structures and the hill ranges take firstly a south-eastward and then south-westward syntaxial bend . It is in this zone that there lies the Mismi Thrust that runs from WNW to ESE.

In the eastern and southern parts of North East India, there lie parallel ranges trending NNE-SSW from Tirap District in Arunachal to Chimtuipui District of Mizoram. As one traverses eastward from the Indian side one can find the ranges gaining height, culminating at the Patkai- Arakan Yoma ranges , with an average height of 2000 m. The northern part of these ranges fall along the Indo -Myanmar boundary. What is interesting here again is the fact that the areas towards the higher eastern ranges are made of successively older rocks. For example, the western most hills of Nagaland and Manipur are made of Surma-Tipam (Miocene ) rocks, while the middle part has Barail; (Eo-Oligocene) rocks and the easternmost and highest ranges are made of Disang (Cretaceous-Eocene) deposits. This is again due to the presence of reverse faults, like Nimi Thrust, Phukpor Thrust, Zephehu Thrust, etc. acting from the Myanmar side and raising the lower beds up.

Apart from these thrusts and faults mentioned above, there is a series of parallel faults along the Assam -Nagaland border and along the southern margin of Meghalaya . The former group of faults cut through North Cachar Hills district. These faults of the Assam-Nagaland border are bound in the north by the SW-NE trending Naga Thrust and in the south by Disang Thrusts thus giving rise to a belt of schuppen in between. This belt of schuppen with their faults crss -crossing the oil bearing beds down below must have caused a great physical damage, for it is rarely that oil is found south of the Naga Thrust which runs south of Barhola , Geleki, Lakwa and Digboi. The Naga and Disang Thrusts running South-Westward meet near Haflong. The fault in the Southern part of Meghalaya, mentioned above, is known as the Dawki Fault. It runs westward from Haflong and separates the Shillong Plateau from the Sylhet Plain down south.

It is clear from the above accounts that three different sets of tectonic units with their differing geological history have formed the bases of physiographic features of North East India. The prevailing hot and humid tropical climate provides congenial conditions not only for vigorous chemical weathering but also has give rise to numerous turbulent streams and rivers. The emergent topographic features under such processes and agents is that (1) the Gondwana Massif has give rise to the Meghalaya Karbi Plateaus, (2) the folded sedimentary rocks have made up the hills, mountains, numerous valleys and gorges, and (3) the structural depressions have turned into Plains.

Physiographically the North-East can be divided into the following three divisions

The hard crystalline massif forming the core of the region is, in fact, an extension of the Deccan Plateau. The Latter extends underground from the Rajmahal Hills of Chotanagpur Plateau below Malda district of West Bengal and Rajshahi, Dinajpur and Rangpur districts of Bangladesh and appears in the North-East above the surface as Meghalaya Plateau and Karbi Hills . It is believed that a downward created the Malda-Rangpur gap which was subsequently filled up by the alluvial deposits of the Brahmaputra and the Ganges. However, the apparently detached Meghalaya and Karbi Plateaus are now about 402.2 km long from the Singimari river in the west to the Dhansiri river in the east. The unit�s average width is about 80 km. and covers an area of 32,829 km². The plateaus are high in the middle (attaining a height of 1961 m) and low towards the west and the east. Further, the plateaus can be divided into two units, viz the Meghalaya Plateau, comprising the Garo, Khasi and Jainta Hills and Karbi Plateau, comprising the Karbi and Rengma hills. The Karbi Plateau is almost isolated from the Meghalaya Plateau by age long erosion of the headstreams of the Kopili and Dhansiri rivers.

Although a part of the Deccan Plateau, the Meghalaya Karbi group of Plateaus are topographically different from the former in that they present more a hilly character with hills, ranges, deep valleys and gorges, while the Deccan Plateau has generally only an undulating ground and isolated monadnocks. The reasons for this difference are not far to seek. Firstly, the hot and humid tropical climate of the region as stated above, favours vigorous chemical weathering and has given rise to rivers and streams. Secondly, the Plateau is skirted by very deep Brahmaputra Plain to the north and Barak-Surma Plain to the south. Thus the local base level of erosion being very low the rivers and streams of the plateau run turbulently, with vigorous down cutting and forming deep valleys and gorges along their courses and leaving the harder parts standing as high hills with steep slopes. Thirdly, being located on a tectonically sensitive zone (at the front of the Indian Plate) and near the seismically unstable Eastern Himalayan zone, the Karbi-Meghalaya Plateau party or in toto experiences uplift and subsidence. These factors have contributed to the present hilly physiography of the Plateaus.

The Meghalaya Plateau is traditionally divided into Garo, Khasi and Jaintia Hills. Garo Hills in the west is a relatively low and more dissected part. The area of this part is about 7769.9 km². Although its average height is 900 m, the Garo Plateau rises to a distinct east-west range known as the Tura Range or Kylas Range just south of Tura. This range is higher than 1100 m and reaches its highest point at Mount Nokrek (1529 m ) Another relatively low range, running north south between Maheshkhola and Adabari separates the western, more dissected part of the plateau from the central higher part. Rainfall being high , the whole plateau is criss-crossed by numerous streams. Of these Simsang, Krishnai, Dudhnoi are the main.

To the east of the Garo Hills lie the higher Khasi Hills. This is the highest part of the Meghalaya Plateau with an average height of about 1066 m. However, its central upland covering an area of 1250 km². is still higher and reaches a general height of 1400m. There runs in this highland an east west range known as the Shillong Range with its highest Shillong Peak reaching 1961m. The Khasi Hills descends through three discernible steps to the Brahmaputra Plain in the north. As one moves from Shillong to Guwahati one encounters the first plunge of about 400 m near Umiam river(Barapani). The second platform, which start at Umiam, is located at a height of about 1000m and is about 30 km wide. The second plunge starts at about 7 km north of Nangpo and descend for another 400 m to Umpling. This (third ) Platfrom is again about 30 km. wide and has a few minor steps between Umpling and Byrnihat. This platfrom stands at about 205 m. a. s. l. and finally descends to the Brahamaputra Plain between Jorabat and Khanapara.

Towards the south the Central upland descends by about 150m-200 m. to the Cherrapunji Platfrom. Several structural platforms made of sandstone and limestone of the late Mesozoic period with steps in between can be found in this area. The presence of structural platforms and steps has given rise to numerous waterfalls large (like Mausmai) and small and cascades with dancing and foaming water.

The easternmost part of the Meghalaya Plateau is the Jaintia Hills It consists of Jaintia Hills district of Meghalaya and Hamren Sub -Division of Karbi Anglong district and is bounded in the east by the Kapili river flowing to the north and the Lubha river flowing to the South. This part of the Plateau slopes down from Khasi Hills from 1200m to about 500 m in the east, the average height being 900 m. Its area is about 3790 km². In the western side the Jaintia Hills is continuous with the sescond Platform of the Khasi hills and merges with the young folded hill of North Cachar district in the east. Toward north the Plateau is highly dissected by the headstreams of the Kapili and several plain embayments are found to enter into it. As in Khasi Hills . the southern part of Jaintia Hills also has several structural and erosional platforms that descend steeply to the Sylhet Plain.

The Karbi Plateau which is the easternmost part of the north -eastward psrojection of the Gondwanaland, lies almost detached from Patkai -Meghalaya Plateau due to headward erosion of the Kapili and Dhansiri rivers and their tributaries. It is pear -shaped and has an area of about 7000 km². To its south lie young folded rasnges of North Cachar Hills and to the north Brahmaputra river. Being eroded by rivers which descend to the surrounding lowlands on all sides, the plateau has developed a radial drainage pattern. However, its central part still remains high with the Rengma range reigning the east-central landscape. The highest peak Dambukso (1363 m ) lies in this region. The important river that come out of the region are Jamuna, Dikharu, Missa, Diphalu, Kaliyani, Nambar, etc. The eastern part of the plateau is made of Upper Mesozoic sedimentary rocks like limestone, sandstone, shale etc. which provide raw material for the cement industry at Bokajan. Karbi Plateau has its underground extension north -eastward upto Negheriting with occasional hillocks rising amidst flat alluvial grounds.

The second major physiographic unit of NE India, consists of as stated above, hills and mountains . This unit stretches from the Bhutan Himalaya covering the northern part of the region and then turning southward covers its eastern and southern parts. Starting from the Dibang Valley and Lohit region of Arunachal, the eastern hills cover south-eastern Arunachal, Nagaland, North Cachar Hills of Assam, a major part of Manipur , Mizoram and the eastern part of Tripura. The whole unit may be divided into two sub- units viz (A) the northern mountaineous part of Arunachal Himalaya and (B) the eastern and southern Parkai- Purvachal Hills. Because of their topography, alignment and location. these two sub-units may be considered separately.

This portion is an integral part of the Eastern Himalaya It stretches from the Bhutan -Arunachal border , demarcated by the Jia Dhansiri river and the Arkhala Range in the west to the Siang River in the east . Structurally it continues north -eastward to Namcha Barwa Peak (7755 m. ) in Tibet and eastward beyond Siang -Dihong River to Mismi Hills and then takes a southward turn through a syntaxial bend to from the purvachal ranges.

The Arunachal Himalaya rises steeply from the Brahmaputra plain and its successive ranges attain gradually higher altitude northwards, ultimately culminating in the snow -clad Great Himalaya Range lying along the India China border. The descent to the Tibetan Plateau of China in the north is , however , gradual . The ranges normally run in the south west north-east direction . Namcha Barwa, lying near the southward bend of the Tsangpo in Tibet , is the highest peak in the Eastern Himalaya . Within Auranachal Himalaya , however, highest peak is Kangto (7590m. ). It is on the Indo-China border lying north of west Kameng district. Other important peaks are Kula Kaangeri (7544m), Sum Lari (7314.2m), etc. There are a few glaciers and glacial lakes in this area . The region immediately south of the Great Himalaya not only supports such glaciers and glacial lakes but also has a fairly dry periglacial landscape with moraines, eskers and drumlins. Glacial and fluvioglacial deposits are so spread out in the area that it assumes a Plateau topography.

The portion to the south of this region is covered by the ranges of the Lesser Himalaya. Unlike in the western Himalaya, the Lesser Himalayan ranges of Arunachal do not have any definite alignment. Further, unlike the western Himalaya this region does not have an Outer Himalayan range made of Siwalik rocks. The Siwalik rocks do exist along the juncture of the Brahmaputra Plain and the hilly terrain, but they do not give rise to a definite range. Barring aside the Great Himalayan range and its glaciated piedmont zone, the rest of the region consists of Lesser Himalayan ranges. These hills, especially those of the northern part, are made of rocks deposited in the mid-Miocene and lower -Pliocene periods. But their inner parts contain hard and crystalline metamorphic rocks . Of course, some of the hills towards the south are entirely made of Sedimentary rocks. The outer most part of the Arunachal Himalaya is made of Cretaceous sandstone with Gondwana deposits occasionally occurring here and there.

The hills rise steeply from the Brahmaputra Plain to an altitude of 800m first and then 1200m. Further north the Lesser Himalayas attain a height from 3000 m to 4000m. A large number of streams come down from the Great Himalaya with torrential force. A few , like the Subansiri (Tsari Chu), originate even beyond the Great Himalayas in Tibet. Most of their important head streams flow from the north -west to south -east and turn southward near the region bordering the Brahmaputra Plain. These streams have created numerous deep gorges and narrow V - shaped valleys . It is only in the middle part that limited flat portions like Apatani Plateau, (or Zino Plateau : 585 km². ) and Tenga Valley can be seen. The torrential streams carry down enormous quantities of sediments of silt, sand pebbles and boulders which are deposited at the points of debouchment onto the Plain, creating alluvial fans and cones which in turn give rise to a piedmint appron or the Bhabar zone. The rivers on encountering the plain begin to braid.

The important rivers of this region from the west are the headstreams of the Manas (nyamjang and Tawang ), which drain the Tawang area, Kameng (Bichom) and its tributary Tenga, headstreams of Dikrong and Ranga Nadi, Khru (Kurung), Kamla and Subansiri. Khru is a tributary to Kamla in its turn is a tributary to Subansiri. In the east, there is Siyom which is a tributary to Siang. The latter is the traditional eastern most boundary of the Arunachal Himalaya.

Arunachal Himalaya is covered by dense vegetation to a height of about 4000 m. The southern part bordering the Brahmaputra Plain receives heavy rainfall providing conditions congenial for evergreen, semi-evergreen and deciduous forests of multi -storied physiognomy.

It should be noted that the division of the hills and mountains of North -East India into the Arunachal Himalaya and Patkai -Purvachal Hills is rather traditional and arbitrary. Geologically and structurally the Himalayas continue beyond the Siang gorge and take a syntaxial bend at the Dibang (Sikang ) Valley- Luhit hills and then run southward.

The portion of the Arunachal Pradesh, east of the Siang river is like a mountain knot, extensive and high, giving out ranges to the west, south and north-east The Great Himalaya here continues north eastward at a height of about 4000m to 5000m through Yonggyap (3962m). and Kangri Karpo La (6100m) and runs upto Trakge La (5800m) in China. The extensive hills of the Sikang -Lohit knot corresponding the Lesser Himalayas are generally steep and high. Noi Aisong (4130m), Diphu (4500 m), Dapha Bum (4578m) Kumjawng ( 2930m) , Hpungan (3072 m), Chankam (2800 m) are some of the important ranges and passes here . The region in the north also holds a relatively wide intermontane basin called Tarwan Valley, several glaciers and a glacial lake named Yegrong Tso, The important rivers are Dibang or Sikang and Luhit which drain the whole area. The Dibang presents an ideal centripetal drainage with its tributaries like Ahui ,Emra, Adzon, Aropo, Amili etc. meeting at Anelin region. The Luhit comes from China where it is known as Rongto Chhu. In Luhit district it is called Tellu in the upper part and Luhit downstream of Brahmakunda or Parasuramkunda.

From the Sikang-Luhit knot the ranges move parallely towards S.S.W through Changlang and Tirap district of Arunachal Pradesh and enter Nagaland. These parallel range are higher eastward attaining maximum height at the Patkai Bum bordering Myanmar. In Nagaland the ranges fan out to the west while continuing southward to Manipur , Mizoram and Tripura. While fanning out at Nagaland , the hills extend upto North Cachar Hills of Assam aand the relatively high Barail Range of them runs NE-SW to reach as far south-west as Jaintia Hills, acting as the divide of the Brahmaputra and Barak catchments areas.

All these ranges, raised during the late Himalayan orogeny, are made of Tertiary sandstone, mudstone, shale and occasional Limestone. Although the region is generally above 950 m of mean sea level, some of its ranges attain a height of more then 3000 m. The ranges generally slope down steeply to the western side and gently to the eastern side. The highest range, i.e. Patkai Bum has a height of 2000 m -3000 m. It is a synclinal range, and made of hard sandstone. Its highest peak is Saramati (3826 m. ) lying at India (Nagaland) -Myanmar border. The Barail Range which is generally above 1600 m. high, has many peaks above 2,000m. It reaches its maximum height at Japvo (3016 m).

Towards south in Manipur, the ranges, especially the branches of patkai Bum and Barail. lie fairly high turning north Manipur into an area of headstreams for a vast region. Mount Tenipu (2994 m), Siroi (2568 m), Khayangbung (2833 m) Leikot (2832 m), etc. all located in northern Manipur, are some of the highest peaks of Manipur. In between the parallel ranges of Manipur there lies the pearl-shaped Manipur Valley which was perhaps a huge lake in the geological past. While this basin is located at 700 m a.s.l, the surrounding hills are much higher.

The hills ranges of Manipur run further south to enter Myanmar as Chin Hills and Mizoram. In Mizoram the ranges in the east along the Myanmar border are on the average 2000m high with the highest peak, Blue Mountain or Phawngpui reaching 2154 m. In between the parallel ranges there are deep gorges, rendering east-west movement particularly difficult in Mizoram. The ranges often assume hog-back shape with asymmetrical slopes. As one moves westward the ranges successively lose height.. The average height of the ranges of western Mizoram is about 1000 m, the highest peaks in this region not exceeding 1600 m. The hills are made of sandstone, shale, mudstone and slate. Mizoram has a large number of north south flowing rivers. Among the north flowing ones Tuivai, Sonai, Dhaleswari and Longai and among the south-flowing ones Kaladon are the main. The south-flowing rivers through their erosional and depositional activities have succeeded in creating relatively Plain areas in the south western part of Mizoram bordering Bangladesh and Myanmar.

The ranges continue to occur retaining their north-south alignment in Tripura also. However, they are generally low and the intervening valleys wide. In fact, there are seven parallel ranges like Jampai Tlang Sakhan Tlang, Lanstara Range, Atharamura Range, etc, starting from the Mizoram border with each of the intervening valleys being 16-18 km. wide. The western part of Tripura is plain but it is dotted with isolated hillocks (Tilas )as in the Barak Plain.

On crossing international boundary the Patkai -Purvachal ranges take the name of the Chin Hills first and then Arakan Yoma, which progress southward and plunge into the Bay of Bengal as Andaman and Nicobar Islands.

Although the Patkai -Purvachal ranges running north -south give rise to deep, narrow gorges, there are occasional east-west valleys and pass, dissecting the ranges. Some such transverse gaps like Hukawng Pass, Pangsu pass, Tamu-More Pass provide traditional passages between upper Myanmar and India.

The hills and mountains of North -East India are seismically unstable and said to be still rising, Earthquakes occur frequently. There are many faults and thrusts in the region as mentioned earlier. Of them Naga Thrust, Haflong-Disang Thrust and Dawki Thrust are the main.

C. The Plains :

1. Apart from plateaus, hills and mountains, North -East India has four Plains . Of these the Brahmaputra Plain (area 54315 km.²) is Large and the other three , namely, Barak Plain (6962 km^.2) Manipur Plain (1843 km² ) and Tripura Plain (3500 km^.2) are small. But each of then is of immense human importance . As each of these plains are physically separate, they may be discussed individually a follows :

The Brahmaputra Plain :

This plain is surrounded by the Arunachal Himalaya in the north, Patkai Bum and Arunachal hills in the east and Naga hills, Karbi plateau and Meghalaya plateau hills in the south . It is open to and Joined with the Ganga plain in the west. It is , therefore, often referred to as the easternmost part of the Indus-Ganga-Brahmaputra Plain . From the north east to the west it is about 720 km. long and from the north to the south it is on the average 90 km. wide.

The origin of the trough on which the plain has been built up is not clearly known. Geologists, however, believe that the trough was created at the time of building of the Himalayas. While the Himalaya were built up due to the pressure exerted on the bed of the Tethys Sea by the north-eastward migrating Gondwana (Deccan) landmass, there developed a sag in between the southern rim of the Tethys Sea and the northern frontal part of the Deccan landmass. A sag so developed is referred to as a foredeep. The rivers that came down from the rising Himalayas in the north and the hills and plateaus of the south, in course of time, filled up the trough to give rise to the plain. The sedimentary deposits in the plain are on the average 1500 m. thick and range between Disang series (upper Cretaceous - Eocene ) at the bottom to Dihing series (Plio- Pleistocene ) at the top . These are , however , overlain by the alluvial deposits of the Recent and Sub- Recent times.

A study of the shape of the plain reveals that although it is originally of depositional origin, it has attained its present shape and size due degradational activities also. The original foredeep was filled up by the deposits carried by the Brahmaputra and its numerous tributaries . But these rivers have also substantially levelled down the hills and plateaus at the points of their debouchment onto the plain by degradational and aggradational activities. Here, degradation is due to steep slope of the hills and aggradation is for the presence of the plain which reduces the velocity of the water and hence its carrying capacity. Thus plain embayment projecting into the hills are often found along the courses of larger tributaries of the Brahmaputra. In the North-East the Dihang, Dibang, Kundil, Luhit and Noa-Dihing have created the vast Pasighat-Sadia-Khamti Plain . Along the Himalayan foothills such plain embayments have been created by Subansiri, Jia Bharali, Barnadi, Pagladia, Manas and jointly by Ai , Champamati and Saralbhanga. In the Patkai-Changlang region the Burhi-Dihing and its tributaries, Tirap and Dapha have created the Margherita -Ledo -Dapha Valley. Here three river terraces lying at 83.3 m (east of Miao), 53.3 m (east of Namchik ) and 46.6 m. (east of Margherita) can still be seen . Similarly , Dhansiri and Daiyang have created the Barpathar -Dimapur Plain between Karbi Plateau and the Naga hills . The Kapili and Jamuna river again have created the Jamunamukh -Lumding Plain between Karbi Plateau and Jaintia Plateau. The Digaru, Kulsi, Dudhnoi and Krishnai have also created similar embayments of smaller dimensions. Thus the present shape and size of the Brahmaputra Plain are the result of both aggradational and degradational activities.

The Plain is relatively wide in its upper part reaching a width of about 90 km. between the Himalayas and Naga hills . The Karbi Plateau, projecting northward upto the bank of the Brahmaputra, restricts the width of the plain near Burhapahar to 45 km. But it becomes wider westward with the Nagaon Plain lying to the west of Karbi Plateau. However, the width of the main plain becomes narrow (70 km. ) again near Guwahati with the Meghalaya Plateau skirting the Brahmaputra. Thereafter, the plain widens as the Meghalaya Plateau recedes southward.

An interesting feature of the plain is the presence of isolated hillocks on either bank of the Brahmaputra . In the east, presence of such hillocks start from Negheriting . Then there are Biswanath,(North Bank), Kamakhya (SB) ,Bhumuragiri(NB), Singari (NB), Mayang (SB), Kurua (NB), Chunchali (SB), Chatrachal (SB), Nilachal (SB), etc. upto Guwahati. Similar hillocks Chander Dinga (N.B.) Hatimura (SB), Nagarbera (SB), Srisurya (SB), Pancharatna (SB), Jogighopa (NB), Kamakhya (EB) etc. occur upto Mankachar . Sometimes such hillocks may be found many kilometres north of the Brahmaputra amid the north-bank alluvial plain. Baramboi in Kamrup district and Baukumari -Rajsula in Bongaigaon district are such hillocks. It may be noted that these are all outliers of the Meghalaya-Karbi Plateau and made of hard pre-Cambrian granite and granitic -gueiss. These stand evidence to the fact that the Gondwana Platfrom (Indian Plate) extends underground farther north and north -east and these hillocks prop up amidst alluvial cover.

The gradient of the plain is extremely low. In the North-East, the Pasighat-Sadiya Plain lies at a height of 130m.while about 700km west near Dhubri, the altitude is 28m. Thus, average gradient of the plain is only 14cm. Per km. While the Sadiya plain is located at 130m, Dibrugarh,96km downstream, has an altitude of 103.6m, Guwahati(480 km downstream) stands at 50.5m, Goalpara fartherwest 36.9m and Dhubri 27.8m. There is a regional variation in the gradient. It is 0.62 m/km between Kobo and Dibrugarh ,0.17m/km between Dibrugarh and Nemati, 0.13m/km between Nemati and Guwahati and 0.094m/km between Guwahati and Dhubri. This facts clearly indicate the possibility of flood hazards in the plain.

The Brahmaputra has a large number of tributaries in the Plain . Of these Subansiri, Ranga Nai, Dikrong, Buroi, Bargang, Jia Bharali, Gabharu, Jia Dhansiri, Na Noi, Pagladia , Champamati, Saralbhanga and Sonkosh in the north bank and Dibru, Burhi Dihing, Disang Dikhou, Jhanji, Teok, Bhogodoi, Kakadanga, Dhansiri, Kalang-Kopili, Kulsi, Dudhnoi, Krishnai Jinary and Jinjiram in the south bank are the main. The north -bank rivers debouch onto the Plain from the Himalayas and hence they not only braid but also change their course frequently. The south bank rivers, on the hand , come out from the lower hills and eroded plateaus and meander over the plain. Both the sets of rivers however, give rise to the ox-bow in their abandoned courses and meet the Brahmaputra sub-parallely due to the presence of incipient levee on either bank of the master river.

The extremely low gradient of the plain from the east to the west, its flatness and the hills and plateau in the north and south and the presence of higher grounds on the banks of the Brahmaputra render the plain ill drained.. Besides, the earthquakes, great and small, occurring frequently cause subsidence and upheaval locally. The tributaries, especially the ones coming from the Himalayas, depositing enormous quantities of sediments, often raise their own beds and consequently often in the rainy season their water overflows the banks. All these contribute to the presence of numerous marshes, swamps and lakes in the plain.

Although the plain is by and large flat, it has some geomorphological variations. Along the northern margin of the plain skirting the Himalayan foothills there is an alluvial fans and diluvial appron resulting from coalescence of alluvial fans and cones stated earlier . The ground here contains unsorted silt , sand, pebbles, and boulders. The zone rises relatively steeply to the hills. This strips of land occurring all along the foothills with an average width of about 5 km. is known as Bhabar zone. This is normally densely forested . The soil consisting mostly of sand and pebble, the zone is pervious and does not contain surface water. Along the southern border of the Bhabar zone there likes a flat ground, all along the north -bank plain, from the east to the west known as Tarai Zone. The water that percolates down in the Bhabar zone reappears in this flat zone and hence this zone remains damp and sometimes swampy. In fact, many small streams originate in this zone criss -crossing the plain lying to the south. Tarai Zone is abut 10 km. wide and is normally covered with tall grasses. To the south of the Tarai belt there lies a belt of relatively high and compact ground of about 15-20 km width. It is this zone on which human settlement is extremely dense and along which pass the trunk roads and railway lines linking the not infrequent towns and commercial centres founded here. This zone therefore, may be called as the built up zone. The built -up zone merges towards the south with the Brahmaputra flood -plain which is about 8-10 km. wide from the right bank of the Brahmaputra. The flood-plain is not uniform in its width. At places, especially where the outliers of the Meghalaya Karbi Plateau are present in the from of hills and hillocks on the bank of the Brahmaputra, the flood -plain is found to be absent. Biswanath , Tezpur, Singari, Uttar Guwahati -Amingaon, Jogihopa, etc. are such areas. But in some places, were the tributaries join together to find their way to the Brahmaputra, the flood plains become extensive. The southern parts of the Dhemaji, Dhakuhana, Lakhimpur, Darrang Nalbari and Barpeta districts and Bilasipara and Dhubri sub-divisions are covered by extensive flood plains. This Brahmaputra is an extensively braided river and has numerous riverine islands locally known as Chars and Chaparis, small and large and temporary, semipermanent and permanent. The permanent ones are even inhabited by the immigrant settlers. During the summer rains these settlements virtually remain cut off from the rest of the plain. In the mid-eastern part there is a riverine island called Majuli (800 km.) which is claimed to be largest riverine island in the world. This however , came into existence due to southward migration of the Brahmaputra, enjoining the lower course of the Burhi Dihing. The south-bank plain is extensive only in the eastern part to the east of Karbi Plateau and in the Kalong-Kapili plain immediately to the west of the same plateau. From the western border of Marigaon district the south-bank Plain is generally narrow. However , the Brahmaputra flood -plains exist in this bank also except at Dibrugarh area, Chandrapur- Guwahati area, and Agia -Goalpara area. Thus the Brahmaputra river itself and its adjoining north -bank and south-bank flood-plains together form an extensive low lying geomorphic zone containing tall grasses as found in Kaziranga, Laokhown, Pabitara, pabha, Orang, etc. wild life sanctuaries. Beyond the flood plain in the south-bank there is again a relatively high built up zone with dense settlement, roads, railways, towns and commercial centres as in the north bank zone . In the eastern part and in Kalong -Kapili plain this zone is wide and extends upto the foothill zone. But in the western part in Kamrup, Goalpara and southern part of Dhubri districts, it is restricted in its spread . Unlike in the north-bank the southern foothill zone bordering the hills and plateaus do not have distinct Tarai and Bhabar strips. There are occasional swamps, marshes and low hillocks in the western part from Lumding region and high grounds supporting tea -gardens and low hillocks in the eastern part east of Dhansari.

The Barak Plain :

The Barak Plain is located in the southern part of Assam encircled on the north by the North Cachar hills, on the east by the Manipur hills and on the south by the Mizoram hills. To the west the plain merges with Sylhet Plains of Bangladesh. The plain is, in fact the headward piedmont part of the Sylhet (Surma-Kushiyara) Plain. The Plain is horse -shoe shaped with 85 km of east-west extension and 70 km north-south extension near Bangladesh border. The plain also includes the Jiriban Sub-Division of Manipur which lies at its apex in the east. The total area of the Barak plain is 6962km². The plain is low lying, its height being 75m near its apex near Jiribam which slopes down slowly to 73 m near Silchar and further to 51 m near Karimganj.

The hill ranges of both North Cachar in the north and Mizoram in the south are aligned north south. Hence some detached hillock in the same alignment as the above can be seen lying scattered in the plain. These are locally known as �tilas� . They are mostly made of rocks of the Tipam (miocene) series. The middle part of the plain, along which the Barak finds its sluggish way westward, is so lowlying that the plain assumes a bowl shape . A large number of tributaries from the North Cachar hills and Mizoram hills join it rendering the middle part susceptible to frequent floods. Of these rivers. Jiri, Labak, Madhura, Dalu, Jatinga and Larang in the north bank and Sonai , Katakhal, Dhaleswari, Singla and Langai in the south bank are the main. There are numerous ox -bow lakes and swamps on either side of the extremely meandering course of the Barak.

The three foothill margins of the plain have steep slopes and hillocks. These were earlier covered by thick forests like North Cachar Reserve Forest, and Barail R,F, in the north and upper Jiri R.F. and Lower Jiri R.F., Katakhal R.F, Singla R.F. and Badshahtila R.F. in the south bordering Mizoram. A large number of tea -gardens of Cachar is located on the higher grounds bordering the foothills.

Amidst the extensive hills and ranges of Manipur, there lies a wheat shaped plain of immense human importance. Thus intermontane plain is about 70 km long from kanglatongbi in the north to Sugnu in the south. It maximum width at the middle is about 40 km. but as it narrows down to the north and south, the average width works out to be 30 km The plain has an area of about 1843 km² .It is said to be of lacustrine origin. The geologists believe that there was an intermontane depression filled by water at what is today the Manipur plain. A headstream of Myittha (a tributary of the Chindwin ) through its headward erosion evacuated the water near Sugnu, leaving the deeper parts of the lake floor as smaller lakes and exposing the higher parts bare. The rivers like Nambul, Thoubal, Iril, Imphal , etc. that subsequently flowed over the floor started spreading alluvium , giving rise to the present plain. The plain slopes gradually down from 838m in the north to 793.5m in the south at a gradient of 75 cm/ km. The Loktak and other smaller lakes lying in the southern half of the plain are said to be the remaining vestiges of the original lake.

The kuga, Imphal, Nambul, Iril, Thoubal etc. rivers meet one another south of Imphal city in the low lying zone near Loktak and take the name of Turel Achaauba or Manipur river and gets out of the plain southward at Sugnu gap. The river is connected with the lake by a channel called Kordark. The channel discharges river water into the lake in winter and takes out lake water in summer.

Manipur plain has a large number of isolated hillocks scattered here and there. Of the hillocks Singmeirrong, Langthaabaan, Waithau, Langathel, etc. are the main. Even within the Loktak Lake there is a hillock that stands out as an island, supporting a few fishing villages sand attracting tourists for its scenic beauty. Besides , there are river terraces in the plain lying in the from of gravel patches beside the river banks , especially near the foothill margins. Such gravel patches can be seen near Sekmai, Kangla, Tonghi, Bisenpur, Thoubal, Sugnu and Chakpikarong. Their presence indicates minor upliftments of the plain in the recent geological period.

The average slope of the plain is not high (75 m/ km ), further, the southern part is very low and full of lakes and marshes. After heavy and continuous rainfall in the surrounding hills , the enormous quantity of water carried down by the otherwise shallow rivers, overflow the banks and cause flood, especially in the southern part of plain.

The Tripura Plain is a piedmont one, lying at the foot of the north-south trending hill ranges extended from Mizoram. The plain has been built up in the western part of Tripura, over an area of about 3500 km due to erosion caused by the headstreams of Manu and Khowai in the north, Titas and Gumti in the west and Muhari and Fenny in the south. This piedmont plains merges with the greater Bangladessh plain in the three ssides mentioned , while the eastern part of the state is covered by seven parallel ranges with relatively narrow valley plains in between. The Tripura Plain covering west and South Tripura districts slopes down west and southward from the Atharimura Range at a fairly high gradient providing little scope for flood to occur. As any other piedmont plain, it is the product of both degradational and aggradational activities. While the Tertiary hill ranges have been eroded by the hill streams. the sediments so collected have been spread over the surrounding lowlying areas. It is in this process that the plain has been built up . Below the alluvial cover the plain contains Surma, Tipam and Dupi Tila deposits of sandstone and clay . The Tipam sandstones here bear rich reserves of natural gas and is said to be potentially rich in oil deposits also.

As in the Barak plain , it has a large number of north-south trending isolated hillocks (tilas ) standing amidst alluvial deposits . As the plain is an agglomeration erosional flats, worn -down hillocks piedmont terraces and depositional grounds over structural synclines, it is not dead flat.

Important Rivers of NE India

Dihang

Dibang

Lohit

Na-Dihing

Dibru

Burhi Dihing

Disang

Dikhou

Janji

Teok

Kakadonga

Dhansiri

Daiyang(A)

Kolong

Zumki

Jamuna

Kapili

Daiyang(B)

Barapani

Kiling

Digaru

Manas

Ai

Champamati

Saral Bhanga

Sonkosh

Godadhar

Gangadhar

Kherkatia-Luhit

Barak

Kulsi

Dudhnoi

Krishnai

Jinjirang

Jiadhol

Subansiri

Ranganadi

Dikrang

Buroi

Bargang

Jia Bharali

Gabharu

Panchnoi

Jia Dhansiri

Barnadi

Puthimari

Baralia

Pagladia

Kaldia

Pahumara

Beki

Manu

Gomti

Tuitan Pui

Kaladan

Makru

Imphal

Iril

Thoubal

Manipur

Jiri

Jatinga

Sonai

Katakhal

Dhaleswari

Longai

Lubha

Jadukata

Someswari

Brahmaputra

The North East India falls within the tropical monsoon climate with an average rainfall of 200 cm. It may be noted that the region contains the rainiest locality if the world, that is the Cherrapunji -Mawsynram area. The region has as many as seven sets of river basins. The seven sets of river basins. The seven sets of rivers setting up their drainage systems are the Brahmaputra in the north, Barak- Surma- Meghna in the mid -south, Chindwin through Tizu, Yu and Manipur rivers in the east and Kaladan, Karnaphuli, Gumti and Fenny in the south and south-west. All these river are international in their extent and go out either to Bangladesh or to Myanmar.

There rivers and their basins are discussed below:

Brahmaputra Basin : The Brahmaputra is one of the largest rivers in the world . It is 2880 km long from its source to is its mouth. Out of this 1700 km. falls in Tibet, 920 km in India (Arunachal and Assam) and the remaining 260 km in Bangladesh. The entire basin of the Brahmaputra inside and outside India is about 928,000 km². Within North-East India its basin covers an area of 165,000 km² covering whole of Arunachal Pradesh and major parts of Assam Nagaland and Meghalaya, accounting for 70% of the total area of the region.

Source : Because of its vastness and mighty nature, the Brahmaputra has always evoked human interest. There are mythological tales about its genesis and source both in India and in Tibet. The Padma Purana, the Kalika Purana and Tibetan tales give different mythological versions of its origin and source and these become parts of traditional folk beliefs of the Indians and Tibetans. It was believed that the Brahmaputra originated from Manasorwar Lake in Tibet lying between the Himalayas and the Kailash Range . It is also believed that it came out of Brahmaputra or Parasuramkunda, a small lake created by Luhit river at its point in debouchment of Arunachal Pradesh upstream of Tezu . It is significant to note that the Brahmaputra is often referred to as Luhit by the Assamese people . This indicates the fact that the people of the region perhaps took Luhit to be the main headstream of the Brahmaputra . Scientific investigations have revealed that neither of the traditional beliefs is correct. The Tsangpo, which is the headstream of the Brahmaputra in Tibet does not originate in Manasorwar, nor the Luhit which passes though the Brahmakunda is the actual head stream of Brahmaputra, although it (Luhit) it one of the headstreams. The three headstreams, Dihang, Dibang and Luhit meet in Sadiya Sub-Division of Tinsukia district, east of Kobo and it is from this confluence that the river takes the mighty form and the name Brahmaputra.

Recent scientific explorations have revealed that the Brahmaputra does not originate from the Manasorwar . In fact, its source is separated from the lake Manosorwar by a pass called Maryum La. The rover originates from a point near 31^o 30� N lat and 82^o0 E long. The place, locally known as Tamahok Khambala Chorten, lies to the south of Lake Konggya. There is a glacier called Chemayungdung in Tamehok Khambab Chorten, from which the main headstream of the Brahmaputra comes out. This stream is known as the Matsang or Tamchuk Khambala in its upper region. After this the river flows eastward in the name of Tsangpo for a distance of about 1700 km parallel to the Himalayas . In this region the Tsangpo flows over the Tibetan Plateau at an altitude of 3600m. "Tsangpo" is a Tibetan word meaning purifier. The river has several important tributaries in the Tibetan region. Of these Raga Tsangpo, Kyichu, Griamd Chu, etc. in the left bank and Ngang Chu in the right bank are the important ones.

After flowing for long 1700 km. the river takes a southward and south- westward turn like a hair -pin in between the Gyala Peri and Namcha Barwa and enters into Arunachal Pradesh at Kepang La above Tuting. Here its bed is at a height of about 3600 m and is full of cascades and cataracts. As the river enters into Arunachal Pradesh near Tuting , it takes the name of Siang in the northern part and Dihang in the southern part and flows across the high mountainous region for 200 km through deep gorges, water falls and cataracts. It finally reaches Pasighat at the southern foothills of the Himalayas, at an altitude of 150 m and then takes the south -eastward course and enters Assam. On entering Assam, the Dihang meets with the Dibang and the Luhit, coming from the north -east and east respectively , and takes the name of the Brahmaputra and begins to flow towards the south-west . Its south-westward course in the eastern part of Assam, westward course ( from about Dhansirimukh) in the western part, and southward course beyond Dhubri, together add to a total length of about 720 km within the state. As it enters into Bangladesh it is known as Jamuna and flows in that name upto Goalundo, where it meets with the Ganges and takes the name of Padma. In Bangladesh, Jamuna has a Branch called Purana (Old ) Brahmaputra, that runs though Mymensingh district and meets the Kalni (Lower courses of the Barak-Surma -Kusiyara ) near Bhairab Bazar. From the confluence of the Kalni and Purana Brahmaputra, the course downward is known as Meghna. The Meghna and the Padma meet at a place about 140 km north of the Bay of Bengal coast and give out distributaries forming huge deltas. From this confluence the vast sheet of water flowing into the Bay of Bengal is known as the Meghna again. The Brahmaputra itself is about 1280 km long from its confluence with the Dibang and Luhit upto to the deltas. The Brahmaputra is a highly braided river, especially in its middle as Chaparis and Chars. Besides, there is Majuli, 800 km², the largest riverine island in the world, which came into existence as a result of the southward migration and bifurcation of the river. While the smaller northern branch of the Brahmaputra here is known as the Kherkaatia-Lohit Suti, the larger southern branch retains its soriginal name (sometimes also called locally as the Dihing ). With in the Indian territory the Brahmaputra has a stretch of 920 km from Tuting to the south of Dhubri.

The Brahmaputra has as many as 109 tributary systems within Assam itself. Some of these tributary system themselves are large., each with several large sub-tributaries. Of such rivers, the important north-bank ones are Jia Dhol, Subansiri, Ranga Nadi, Dikrang, Buroi , Bar Ganga Baranadi, Pagladi , Chaulkhowa, Beki, Manas, Ai, Champamati, Gauranga, Saralbhanga , Gangadhar and Sankosh. Of the south bank tributaries, Noa-Dihing meeting Luhit, Dibru, Burhi Dihing, Disgarn, Kulsi , Dudhnoi, Krishnai, Jinari and Jinjiram are importan. Apart from the Kherkatia Lohit channel which branches out from the Brahmaputra in the right bank opposite Dihingmukh, the Brahmaputra gives out another channel in left bank at Arikatimukh, west of Kaziranga. The branch is known as Kalang It Passes through the Nagaon-Morigaon Plain, meets with the Kapili and Digaru and again falls into the Brahmaputra at Kazalimukh, east of Chandrapur.

The Genesis and Development of the Brahmaputra : As there were controversies about its genesis. The differences of opinion arose as to how could the Brahmaputra, originating in the relatively low Tibetan Plateau, come to the lower Indian Plain having crossed the very high Himalayan Ranges. According to Dr. GE. Pilgrim there was in the Tertiary Period, only one drainage system in North India adjoining the Himalayas, in place of the there (Brahmaputra, Ganga and Sindhu) systems as at present. The master stream of this system of the geological past was named as the Siwalik River by Pilgrim and as the Indo Brahm by E.H. Pascoe. Pascoe believed that in the early Tertiary era (Eocene Period) the Arabian sea extended to the present day Afghanistan through the state of Sindh, Pakisthan and a branch of it extended eastward for a short distance. The Indo Brahm river, originating in the present -day North- East India, flowed westward and fell into the eastward branch of the extended portion of the Arabian Sea. The Indo-Brahm was fed by numerous tributaries coming down both from the rising Himalayas and the Peninsular block. According to Pascoe this river used to carry enormous quantity of deposit which were deposited in the present-day Siwalik (southern most Himalayan) region. Subsequently, the upheavals that took place during and immediately after the formation of the Siwalik range broke down the river system into there systems. There was, according to Pascoe, perhaps a SW-NE upheaval from Delhi to Shimla, as there was a N-S down warping in the northern part of the present day Bangladesh and West Bengal. The result of these tectonic disturbances was that the eastern part of Indo Brahm River started flowing to the Bay of Bengal through the down-warp and became the Brahmaputra system. The westernmost part, west of the Delhi- Shimla upwarp, became separated from the north -Indian Part of the river and kept flowing to the Arabian Sea in the name of the Indus System, while the middle portion (east of the upwarp) started flowing east to the Bengal down-warp to became the Ganges System.

It is also that on the northern side of the Himalaya, there was a palaeo river that used to flow from the east to the west along what is today identified as Tsangpo -Sutlej- Gartang-Indus-Gilgit-Oxus furrow. This river called as the Tibetan River, used to fall at the Aral Sea along the present course of the Oxus. But more active rivers from the Indian side could cut bank to its course to capture it at several places in its upper course and drain its water to them. A headstream of the Brahmaputra thus perhaps cut across the Himalayan ranges, reached the Tsangpo part of the Tibetan River and started draining that part to the low -lying Brahmaputra. That is how Brahmaputra now has come to have its source in Tibet.

The Barak Basin : The river Barak originates from the hill complex near Mao at the border of Nagaland and Manipur, south-east of Japvo peak. It runs westward for some distance forming the boundary of Nagaland and Manipur and then suddenly turns southward and flows through Manipur until it reaches Tipaimukh at the South-western corner of that state. It then takes a sharp northward turn, forms firstly the boundary of Manipur and Mizoram and then Manipur and Cachar district of Assam. Thus, flowing northward for about 60 km, it again sharply turns westward at Jirimukh and flows through Caachar Plain sluggisly . In the western part of the Cachar plain the river gives out two branches near Bhanga. While the northern branch is known as the Surma, the southern branch is called the Kusiyara. Both the branches fall into Kalni in Bangladesh. The total length of the Barak is about 900 km. of this about 525 km. falls within North East India.

The drainage basin of Barak within North East India is about 52000 km.². It covers the western part of Manipur, southern parts of North Cachar Hills and Meghalaya and the northern parts of Mizoram and Tripura. Its important right-bank tributaries include Makru and Jiri in Manipur and Labak, Madhura , Dalu, Jatinga and Larang in the Barak Plain. The major left-bank tributaries are Irang and Tuivai(Tipai) in Manipur and Sonai, Rukni, Katakhal, Dhaleswari, Singla and Langai in Cachar Plain. Besides several small tributaries from northern Tripura fall into its Kusiyara branch through Sylhet Plain of Bangladesh. Similarly, Lubha. Kangs, Jadukata and Someswari from southern Meghalaya join its northern branch i. e. Surma in Sylhet.

The river Barak used to be an important water-way in the past. Even today it is used by country boats for trade and commerce, sometimes deep into Manipur and Mizoram.

The Chindwin Drainage Basin : Although the main river Chindwin falls within the territory of Myanmar, a large number of its headstreams drain the eastern part of North East India especially the region bordering that country. The three important tributaries. through which this drainage has been set up are the Tizu, Yu and Manipur River. Tizu drainage the South-eastern part of Nagaland and north-eastern part of Manipur and then falls into the Chindwin. The Yu also drains the eastern margin of Manipur and falls into the Chindwin. The Manipur River sub-basin is more important in that it drains the central part of Manipur including the famous Manipur Valley. The tributaries Imphal, Iril and Thoubal which originates at the central Manipur hills flow south and meet one another just south of Imphal city. The combined water courses takes the name of Tuvel Achouba or Manipur River or Imphal River and flows by the east side of Loktak Lake out of North East India to Myanmar. In Myanmar it falls into Myttha, which, in tur falls into Chindwin. Near Loktak, it is joined to the lake by the nullah called Kordark. The area in North East India under the drainage basin of Chindwin is about 24000 km².

The Kaladan Drainage Basin : The river Kaladan has set up its drainage basin over the southern and south-eastern parts of Mizoram. It has a drainage basin of about 7000 km² in that state. This river is also known as Chimtuipui Its important tributary is Mangpui Lui. In the central eastern region of Mizoram, its tributary, Tuichong, drains a part of Champhai sub-division. The river Chimtupui flows to Myanmar across the southern boundary of Mizoram and falls into the Bay of Bengal in the name of Kaladan.

The Karnaphuli Basin : Although Karnaphuli is an important river of the southeastern part of Bangladesh, all of its headstreams line in the western margin of Mizoram. The important headstreams of this river are Phairang. Tuiching, Thega, Deh and Tuilianpui. They meet one another near Demagiri and flow southwest to Bangladesh in the name of Karnaphuli to fall into the Bay of Bengal near Chittagong. The drainage area of this river within North East India is about 4000 km² .

The Minor Basins of the Fenny and Gumti drain the southern part of Tripura. The two basins together comprise at area of about 4000 km² while the Gumti river drains the south-central part of the state and flows westward to fall into the Meghna, the Fenny river drains the southernmost part and falls independently into the Bay of Bengal.