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On the basis of origin and structure, the Indian sub-continent can be divided into five physiographic units: 1. The Himalayas 2. The Peninsular Region 3. The Indus-Ganga-Brahmaputra Plain 4. The Coastal Plains 5. The Islands.
Unit # 1. The Himalayas:
The Himalayas consist of a series of parallel mountain ranges that stretch in a north-west to south-east direction. It has been recognised as a natural guard on the north of the sub-continent. The total length of the Himalayas is about 2415 kilometres and is considered to have consisted of four main sections separated by the gorges of different rivers. The Himalayas is the highest mountain range in the world.
Origin:
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The Himalayan Mountain ranges were formed by the sediment collected on the bed of the Tethys Sea for millions of years. The sediments might have been derived from the two stable land masses—Angara land to the north and peninsular landmass to the south.
Geologists believe that the lateral force came from the northern Angaraland, and the sedimentary strata resting on the Tethys floor were tightly squeezed against the southern rigid mass of peninsular India, commonly known as Gondwanaland.
As a result, the Himalayas originated as folded mountains. The Himalayas follow a general west to east direction exactly in conformity with the alignment of the Tethys Sea. The other mountains like Aravalli, Rajmahal and Meghalaya originated from the Gondwanaland, and are situated approximately in the north-south direction.
Geological studies reveal that the Himalayas came into the present stage as results of the development during three geological phases or epoch. During the first phase, the middle Himalayan range was raised during the Oligocene Epoch or era. This range is formed of the old crystalline and old sedimentary rocks. The squeezing of the Himalayas for the second epoch took place during Miocene era.
During this period the sediments of the Potwar Basin (now in Pakistan) were raised. The squeezing of the Himalayas for the third time took place during the later part of the Pliocene era. The Siwaliks were formed during this time.
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Many geologists believe that the third phase of the squeezing of the Himalayas is still continuing while some other geologists believe that there might have been another epoch much earlier to the three mentioned above during Cretaceous era when the Karakoram mountain ranges were raised.
An examination of the location and direction of the north-western part and the part located in the Indo-Myanmar border of the Himalayas, clearly indicates that the lateral pressures were exerted against the Gondwana landmass from the north-west and from the eastern side of the sub-continent.
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Again some other geologists and earth scientists offer an opposite view against the one mentioned above. They believe that the lateral pressures were exerted by the Gondwana landmass against the Angaraland. That is, the pressure came from south towards north. International debates are going on till now in support of and against the two schools of thought. However, the first hypothesis is found to be more acceptable.
Whatever may be the direction of the compressional forces, one thing is clear that the Himalayas are one of the most young folded mountain ranges in the world. One of several major evidences is the occurrence of the sediments of Lake Kashmir at heights of 1500 to 1800 metres near the Pir Panjal range.
The occurrences of the tilted beds of the lake deposits of Kashmir are called the Karewas. This clearly suggests that the Himalayas were in the process of uplift as late as the Pliocene era. Fossils of some mammals of Post—Tertiary era have been also found in the foothills of the sub-Himalayan region. Further, the deposits near Chandigarh area clearly suggest that the last phase of Himalayan uplift took place when early man had already settled in the region.
However, one thing should be remembered; and that is the present reliefs of the Himalayas are very complex, and many of them might have been created by the erosional activities on the landmass long before it had risen from the floor of the Tethys Sea.
Relief Features:
The relief features of the Himalayas exhibit striking youthfulness. A complex nature of folding and deformation of the strata has taken place in this region. The mountains are of tectonic origin, with deep river valleys, gorges, and waterfalls.
On the basis of relief features, the Himalayas are divided in two ways—from west to east, and from south to north. Four main sections have been identified in west to east direction. They are separated by the deep gorges of the different rivers.
Although, there are many local names for these four sections, they are generally known as:
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1. The Punjab Himalayas extending over a distance of 560 km between the Indus and the Sutlej.
2. The Kumaon Himalayas between the Sutlej and the Kali stretching over a distance of 320 km.
3. The Nepal Himalayas between the Kali and the Tista stretching over a distance of 800 km.
4. The Assam Himalayas extending over a distance of 720 km between the Tista and the Tsangpo-Brahmaputra in the east.
Similarly, the Himalayan mountain chain, when arranged in longitudinal direction, is found to consist of three main series of parallel ranges. They are separated by Longitudinal Valleys of tectonic origin.
From the Indus-Ganga-Brahmaputra plain in the south towards the north, the three ranges are identified as:
(a) The Siwalik or Sub or Outer Himalayas
(b) The Mid or Lesser Himalayas
(c) The Great or Inner Himalayas
The existence of the undulating erosional plains of Aksai Chain, Deosai, Depsang and Lingzi Tang at high altitude within the Himalayas indicates that these plains were formed due to erosional activities much before the formation of the Himalayas. On the other hand, there are many small longitudinal valleys. They are of tectonic origin and they are known as Duns. Dehra Dun, Kothri Dun, Path Dun and the Vale of Kashmir are some of the famous examples.
The physiographic complexity of the Himalayas is found to have increased in altitude from the outer towards the inner Himalayas. The average height of the great Himalayas is 6091 metres. Almost all the world famous peaks are located here.
Of them, Everest (8848 metres), Kanchanjunga (8598 metres), Dhawalgiri (8126 metres), Nanga Parbat (8125 metres), Nanda Devi (7817 metres) and Namcha Barwa (7756 metres) are important. The average width of the middle or lower Himalayas is about 80 km, and average height is 4500 metres. The average width of the outer or sub-Himalayas is only 10-50 km and average height between 900-1200 metres.
In the north-west, the Himalayas meet with the Karakoram, the Hindukush, Kun Lun, Tien Shan, Altay, and ultimately merge with the Pamir Knot. The general west-to-east direction has taken turn towards south, and then south-west at its west and east extremities near the Indus and the Dihang gorges. These bends of the Himalayas are very sharp and resemble a hair pin.
The southern slopes of the mountains of the middle and the inner Himalayas are very steep, and the northern slopes are—relatively gentle. Similarly, the altitude of the eastern Himalayas is found to have increased very rapidly from the southern plain. That is why the world’s highest peaks like Everest and Kanchanjunga are situated very close to the plains of Bihar and Bengal.
On the other hand, the altitudes of the western Himalayas have increased in a step-like manner through a series of low ranges. Here, the sub-Himalayan hills of Jammu and Kashmir are in the first step, followed by Pir Panjal and Dhaoladhar in the second step, and the great Himalayas, north Kashmir and the Zaskar Ranges in the third and last step.
On the eastern part of the Himalayas, Patkai, Naga, and Mizo hills are scattered towards the south, and ultimately mixed up with the Arakan Yoma of Myanmar (Burma) to reach the sea. The Barail ranges, part of North Cachar Hills are located to the south-west of Nag a Hills.
To cross the Himalayas from south to north is extremely difficult. It can be crossed only through some mountain passes. Some of the important mountain passes like Zozi-La and Shipki-La in Laddakh region connect Laddakh with Tibet; Sasar pass connects Leh of Kashmir with the Yarkhand of Karakoram range in China; Jelep-La connects Darjeeling with Lhasa of Tibet through Chumbi valley; and Tezu, An and Taungup connect Arunachal Pradesh with China and Myanmar.
On the basis of geological structure and age of the rocks, the Himalayas again can be divided into four zones.
They are as under:
1. Tibet Zone:
It is situated on the northern side of the Great Himalayan region. Tibet zone is mainly composed of fossilized sedimentary rocks of Pleistocene era.
2. Himalayan Zone:
This zone includes both the Greater and the Lower Himalayas. It is mainly composed of crystalline granite and metamorphic rocks, like schist. The folds of this region are nearly isoclinal, and the rock strata are displaced horizontally.
3. Nappe Zone:
This zone includes Kashmir, Himachal and Garhwal Himalayas. In this zone, huge quantities of old rock strata are forced to be displaced by the process of over folding and thrust faulting, and they are placed over the newer rock strata.
4. Sub or Outer Himalayan Zone:
This zone is composed of the rocks formed during the Tertiary era. It is believed that this zone is formed mainly of the sediments derived from the main Himalayan region to the north and the Gondwana landmass to the south.
In addition to the above major divisions, there are numerous smaller relief features created by the erosional and depositional activities of the glaciers. Boulder clay moraines, U-shaped valleys, hanging valleys and morainic terraces are some of the important examples.
The study of the relief features of the Himalayas has given rise to the view that it underwent different phases of glaciations followed by warmer periods during which water-actions were more predominant.
Unit # 2. The Peninsular Region:
Origin:
The great peninsular plateau of India is one of the oldest landmasses in the world. It is composed of old crystal rocks of Pre-Cambrian age, while the north-western portion is formed of horizontally arranged volcanic lavas.
The plateau extends from the plains of UP, and Bihar towards south, and encompasses the whole of the Peninsula. The general elevation is between 600-900 metres. As the area remained above sea level, since the Cambrian period and never submerged again.
It has great significance in the evolution of the relief features.
The first major structural evolution took place during Palaeozoic era, when the rock strata of the geosyncline of the Aravalli region were pressed to be folded, and the Aravalli Mountain Range was formed. However, the Aravalli of today is the worn-out portion only.
The Annamallai Range is also believed to have been formed during the same time. It is also believed that the peninsular plateau has not been subjected to any other compressional force except the one mentioned above.
However, the plateau has experienced a number of vertical movements (upward or downward), as a result, the plateau is full of several faults and fractures. Every uplift or subsidence has resulted in renewed actions of erosion by which youthful relief features have developed over the old features. One of such newer uplift can be seen in the Nilgiri Hills. The faults of the Narmada and Tapti rivers are the evidences of subsidence of the crust.
Two events of great geological significance took place over the plateau region at the time of the uplifting of the Himalayas. First, the north-western part of the plateau was formed by the horizontally layered lava that came out of vigorous volcanic eruptions. Second, the subsidence of the western part of the plateau to become the floor of the Arabian Ocean today led to the advancement of the sea water landward.
Relief Features:
The great Indian peninsular plateau is flanked by the Aravalli, the Rajmahal and the Khasi and Jayantia Hills in the northern outskirts, while some portion of the plateau is buried deep under the sediments of the Indus-Ganga-Brahmaputra plain. The plateau is sloping towards the north and the east. The slope in the western side is much steeper, and that is why the western coastal plain is relatively narrow.
The relief features of the plateau are of complex nature. They are resulted by the actions of upward and downward movements of the plateau and modified mainly by the actions of running water. They now exhibit the mixture of complex geomorphological diversities. Many fault troughs and fault scarps can be seen on the northern portion, specially between the Thar Desert and the Rajmahal Hills.
The other features like the old residual hills, eroded plain of Chotanagpur plateau, dissected plain of Bundelkhand, trough valleys of the Narmada, the Tapti and the Damodar, the broad shallow valleys of the Godavari and the Kaveri and the steep sided Nilgiri, the Annamallai, Palni and Cardamom represent distinct stages of evolution of the plateau.
Even though the relief features of the peninsular plateau are of complex nature, they can be broadly divided into seven major groups, viz:
1. The Deccan Lava Plateau
2. The Malwa Plateau
3. The Aravallies
4. The Trough Region
5. The River Basins
6. The Coastal Plains and
7. The Southern Granitic Plateau.
The Deccan Lava Plateau is almost plain and is inclined towards east and south-east. There are several small basins, and they are separated by the low hills. The plateau surface is made up of the horizontally placed lava beds deposited during the last phase of volcanic eruptions. The Vidarva plain, the Godavari and the Virna basins are separated by low hills like Ajanta, Balaghat, etc.
To the east of Nagpur, the Deccan Lava Plateau is marked by the Wainganga valley and the Upper Mahanadi Basin of Chattisgarh region. From the hilly tracts of Orissa, the Chattisgarh region rises very gently towards the Bastar Hills and has remarkable uniformity. However, the gneissic Telengana Region of the south has erosional surfaces.
The relief features of the Eastern Ghats Mountain are not as impressive as that of the Western Ghat. Its height is also relatively less. The Eastern Ghat is surrounded by Narnamalais, Velikondas, Palkondas and the Pachmalais. These hills are referred to as Northern Hills in the north, Cuddapah Ranges in the middle, and the Tamil Nadu Hills in the south. No part of the Eastern Ghat has elevations more than 900 metres and the mountain is highly dissected.
To the north and north-west of the Deccan Lava Plateau, lies the Malwa Plateau. The plateau is relatively open and is drained by the Upper Chambal and Betwa rivers. The plateau has been the centre of human agglomeration as its western part has the Aravallies and the eastern Bundelkhand is not very suitable for human habitation. The plateau is dominated by the Vindhyan scarps. It consists of several erosional surfaces of low elevations. The average height of the surface is below 600 metres.
The Aravalli Range is located to the north-western part of the plateau. It is one of the oldest folded mountains in the world. It is deeply worn-out and eroded by the running water and forms a complex of hill ranges. The Aravallies lie between Delhi and Gujarat having a north-east and south-west direction. However, the main range is in Rajasthan, having imposing relief features. The Thar desert lies to the west of the Aravallies.
Further to the east, the peninsular region gives way to the Shillong Plateau. As the rainfall is highest in the world over the plateau, it is highly dissected and densly covered with forest. The average altitude is between 1300-1800 metres.
The highest point is the Shillong peak (1800 metres). The plateau has a sharp escarpment on the southern slope facing the Surma Valley. It is believed that the Shillong Plateau is an extension of the peninsular block having the connecting rock strata buried deep under the Ganga-Brahmaputra alluvium.
To the south, the lava formation gives way to the granites and gneisses of the Kamataka plateau. The plateau has two main subdivisions: Malnad and Maidan. The average altitude of the plateau is between 500-800 metres. The Malnad portion is full of hills and thick forests. The highest hill range is the Baba Budhan Hill tract.
On the other hand, the Maidan portion is open and relatively plain. The reliefs of the Kamataka and Deccan Plateaus extend upto Kanya Kumari, and are dependent on the Western Ghat Range. The average altitude of the Western Ghat is between 900-1100 metres. The lavas of the Ghat region are completely dissected.
However, small granite hills are seen near Goa. Here, the height of the Ghat is not more than 900 metres. However, it rises again towards south in the Nilgiris. The Palghat Gap is believed to be an abandoned valley of some unknown neighbour.
Unit # 3. The Indus-Ganga-Brahmaputra Plain:
Origin:
The vast plain of India, between the Himalayas to the north and the peninsular plateau to the south is formed by the Indus-Ganga-Brahmaputra Rivers. The surface of the plain has resulted by the works of Himalayan Rivers.
The infilling is not uniform, but has the striking differences in depth of alluvium between the Indus and the Ganga basins. The maximum depth of alluvium has been recorded to be about 2000 metres. The plain has longitudinal diameter of about 3200 kilometres; with varying width of 150-300 kilometres. It is narrowest in Assam with only 40-100 kilometres width.
Among the three plains, the Indus plain is more extensive; it covers a distance of 960 kilometres. The Ganga Plain, on the other hand, extends for only 430 kilometres from the Rajmahal hills to the Sundarban delta.
As the Ganga and its tributaries flow through the areas of heavier rainfall, the amount of sediments carried by them is much more than that of the Indus, which flows through the drier part of the country. As a result, there is no uniformity in the thickness of alluvium and, naturally the Indus basin is much shallower than the Ganga basin.
Relief Features:
Even though many geologists think that the plain is featureless, it can be observed that there are many distinct geomorphic features present in the plains also. It may not be as distinct and magnificent as that of the Himalayas but the plain has its own diversities of relief.
The Indus-Ganga-Brahmaputra Plain is recognised as consisting of four distinct divisions.
They are:
(a) Bhabar Belt
(b) Terai Belt
(c) Bhangar Belt
(d) Khadar Belt.
The Bhabar Belt of the plain is situated along the foot of the Himalayas. The belt is made up of pebbles, and boulders carried down by the numerous streams flowing from the Himalayas. The porosity of this belt is so high that all the streams flow under ground. The Bhabar is generally narrow, only 8-16 kilometres wide, and the pebbles are arranged in parallel direction to the slope of the river beds.
Next to Bhabar, lies the Terai Belt. This belt is composed of newer alluvium of relatively smaller sizes. All the underground streams reappear to the surface. The belt is excessively damp, covered with thick forest and home of variety of wild life. Terai belt receives heavy rainfall throughout the year.
The third zone is known as the Bhangar Belt. It is composed of the older alluvium and forms the alluvial terrace above the floodplain. The Bhangar in the deltaic plain of the Ganga is arranged into a low upland covered with laterite deposits. Lastly, the Khadar Belt is made up of the newer alluvium of the flood zones.
In addition to the above, two distinct depositional features created by the numerous streams coming down the Himalayas can be identified.
They are called:
(a) Alluvial cone and
(b) Inter cone.
The alluvial cones are formed at the foot of the mountains by the streams. Their vertices point toward the mountain side and the bases extend towards the plains. The alluvial cones have convex surfaces. The Inter cones have reverse plain as their edges are concave.
Most of the cones formed by the Himalayan Rivers are simple. However, in course of time some of them became composite. For example, the plains of Bihar, Gondak, Kosi, and Mahananda—Tista are formed as composite cones.
From Punjab in the west to Assam in the east, the Indus-Ganga-Brahmaputra plain can be divided into a number of segments on the basis of the differences in local relief.
(a) The Punjab Plain:
The Punjab plain is formed by the sediments carried by the five rivers—Jhelum, Ravi, Beas, Sutlej and Chenab—flowing through the region. The whole plain is the combination of five doabs which are filled up by the sediments to form a homogeneous plain.
The doabs from east to west are:
1. Jallandhar doab – between the Beas and the Sutlej
2. Badi doab – between the Ravi and the Chenab
3. Rechana doab – between the Chenab and the Jhelum
4. Chaj doab – between the Chenab and the Jhelum.
5. Sind Sagar doab – between the Jhelum and the Indus.
The eastern part of the Punjab plain is relatively flat, but the northern part is extensively eroded by the numerous streams coming down the Siwalik Hills. The network of such streams is locally known as chos. The erosion by the chos is quite common in Hoshiarpur District of Punjab.
(b) The Ganga Plain:
The Ganga Plain of Uttar Pradesh and Bihar is the combination of the Ganga—Yamuna doab and Mithila plain. The area is formed out of the alluviums and debris carried in by the Ganga, Yamuna, Ghaghra and their tributaries. The Bhabar and the Terai belts are well developed. However, most of the terai forests are now cleared and the lands are put to agricultural uses.
The total area of the Ganga plain is approximately 3.57 thousand square kilometres and the thickness of the alluvium in certain parts is upto 1000-2000 metres. The plain gradually slopes to the south-east and is formed of the old Bhangar alluvium, while the lowlands are formed of the newer Khadar alluvium. The relief of the Ganga plain from Uttar Pradesh to the delta region is more or less uniform.
(c) The Deltaic Plain:
Most part of the deltaic plain of Bengal is now in Bangladesh. The whole deltaic plain is composed of old and new alluvium mud and is full of marshy areas. To the north, lies the Ganga-Brahmaputra Doab region and the Duars of the terai belt. The Meghna and the Surma valleys lie to the east and the peninsular plateau to the west.
(d) The Brahmaputra Plain:
The Brahmaputra plain extends from Sadiya in the east to Dhubri in the west. Its length is nearly 720 kilometres, and it has a width of only 100 kilometres or less. The plain is formed of the alluvium carried by the Brahmaputra and its tributaries.
The plain is gradually sloping from the east towards west. The major portion of the Surma valley, located to the south of Meghalaya plateau is now in Bangladesh. The small part of the Surma valley that falls within India is called the Barak Valley.
The Indus—Ganga—Brahmaputra Plain is the home of millions of Indians. It is the centre of the Aryan civilization and was the administrative headquarters of many of the kings, including the Mughals.
Unit # 4. The Coastal Plains:
The coastal plains of India extend from the delta regions of west Bengal to the Run of kutch in Gujrat. The coastal plains are located along the seaward borders of the peninsular plateau. Due to various physiographic and climatic conditions, there exists striking differences between the east and west coastal plains.
The west coast is much narrow, except of course to the north in Gujrat and to the south of Karwar. The coastal plains are made up of newer alluvium and occasionally interspersed by the hilly terrain. The coastal plains to the west of Western Ghat consist of three different smaller plains.
They are:
(i) Konkan coastal plain in the north
(ii) Keral coastal plain in the south
(iii) Goa coastal plain in the middle.
The konkan coastal plain is 50-80 kilometres wide and generally flat. The Goa coast has estuarine formations and the Kerela coast is marked by beautiful lagoons.
The eastern coastal plain, in contrast to the western coastal plain, is wide and has well developed delta formation at the mouth of the major rivers. The east coast is known by the name Coromandal coast. The Andhra and Tamil Nadu portion of the coast is 80-100 kilometres wide, and is bordered by the sharply rising Eastern Ghats. Thickness of alluvium, especially in the delta regions, is quite high.
Unit # 5. The Islands:
The Andaman and Nikobar of the bay of Bengal are the two major islandic groups of India. There are 200 Islands in the Andamans group alone, and some of them are of volcanic origin. The Andamans cover an area of 6340 sq. kilometres and they are known by two sub groups- Great Andaman and Little Andaman.
The Great Andaman Islands are hilly. There are 19 Islands in the Nicobar group. They are relatively plain and thirteen are inhabited. The islands extend over nearly 350 kilometres.
Lakshadweep group is the major island group of the Arabian sea. They are formed of the coral deposites located at a distance of 230 km to 480 km off Kerela Coast. There are no other major islands within Indian Territory, as Sri Lanka and Maldives constitute independent territories.
