Like history of human kinds, we can divide the geological history into eras and periods. Eras are the primary interval of time and periods the Secondary intervals. Rocks formed during an era are called group and those during a period are called system.
Geology is the science which concerns with the origin, composition and history of the earth. The geological history of India is both complex and varied. It is possible to find in the country rock formations of rock strata ranging from Pre-Cambrian to recent times many of which occur in juxtaposed or superposed position.
The peninsular massif is the core around and upon which different acts of geological drama were staged and all have left their imprints in some form or the other.
The flat summits, entrenched valley,, the dykes, folds and faults all speak of the alternating periods of disturbances and in- activities responsible for the present configuration of the country. Broadly speaking the major geological phases responsible for the making of India can be enumerated thus.
India’s Geological Eras:
The standard geological eras are:
(i) The Pre-Cambrian (over 570 million years old)
(ii) The Paleozoic (245-570 million years old)
(iii) The Mesozoic (66-245 million years old)
(iv) The Cenozoic (66 million years old to the present).
The Indian geological eras are:
(i) The Archean or Early Pre-Cambrian
(ii) The Purana or Late Pre-Cambrian
(iii) The Dravidian or Paleozoic (400-570 million years old)
(iv) The Aryan (400 million years old to the present)
The Pre-Cambrian derives its name for Wales in the United Kingdom. The periods got their names from places where rock formations of that period were formed. The Pre-Cambrians do not contain fossils of plants and animals. The Paleozoic have the fossils of very early lives, the Mesozoic have middle lives and the Cenozoic recent lives.
The Pre-Cambrian ERA:
(i) Archean System:
The Archean of India corresponds to the first half of the Pre-cambrian era. The Archean system contains the first formed rocks of the earth formed at the time of cooling and solidification of the upper crust of the earth’s surface in the pre-cambrian era (about 4,000 million years ago).
The gneisses and schists of the Archaean System form together with the Dharwars the oldest rock systems of India. The schists mostly crystalline include mica, hornblende, talc, chlorite, epidote, sillimanite and graphite schists.
Basic trapdykes of dioritic or doleritic or ultrabasic composition cut through the gneisses and schists at a number of places. The Archaeans evince a high degree of metamorphism. They are all azoic, thoroughly crystalline, extremely contorted and faulted, often formed as plutonic intrusions and generally have a well-defined foliated structure.
The crystalline metamorphosed sediments and gneisses of the Archaean System form about two-thirds of the Peninsular surface, and also occur in several localities in the Himalayas. Bengal Gneiss, along with the Khondalites of Orissa, the Closepet, Champion and Peninsular Gneiss of the Peninsula and the gneisses of Jharkhand and Madhya Pradesh, is one of the important groups of the Archaeans.
The third important group of the Archaeans is the charnockites which occur widely in Tamil Nadu forming the Nilgiri, Palni and Shevaroy hills. The bulk of the higher ranges of the Himalayas forming the central zone is formed of crystalline and metamorphic rock like granites, granulites, gneisses, phyllites and schists. The rocks have no marks of fossils.
This system derives its name from the rocks first studied in Dharwar district of Karnataka.
(ii) The Dharwar System:
The Dharwars include some highly metamorphosed rocks of both igneous and sedimentary origin. Hornblende, Chlorite and Granulite represent the igneous types; whereas the various schists like mica-schist, talc-schist, chlorite-schist, quartzites and ferruginous quartzites are important rocks with sedimentary origin. These rocks are often highly metalliferous containing ores of iron, manganese, copper, lead and gold.
These rocks occur in the Dharwar-Bellary-Mysore belt of Karnataka; in the Aravallis between Jaipur and Palanpur, in Rewa, Balaghat and Jabalpur districts of Madhya Pradesh; Nagpur district of Maharashtra; Ranchi, Hazaribagh of Jharkhand; Sundargarh and Keonjhar districts of Orissa; and Ladakh and Zaskar Ranges of Jammu and Kashmir and Himachal Pradesh.
Purana or Late Cambrian ERA:
(i) The Cuddapah System:
The Cuddapah System is composed of a number of parallel-series of ancient sedimentary strata of great thickness. They rest with a great unconformity on the Dharwars and Archaean gneisses and schists and underlie the Vindhayan System of Madhya Pradesh.
The most extensive occurrence of the Cuddapah is found in the Cuddapah followed by Kurnool district of Andhra Pradesh. The Chhattisgarh, Singhbhum district of Jharkhand, Kalahandi and Keonjhar districts of Orissa and the Aravalli Range of Rajasthan show isolated exposure of Cuddapah rocks. These rocks contain ores of iron, manganese, copper, cobalt and nickel, and deposits of barytes, asbestos.
(ii) The Vindhayan System:
Mainly composed of undisturbed sandstones, shales and limestones reaching a thickness of up to 4,000 metres, the Vindhyan System occupies about one lakh km2 area, stretching from Sasaram and Rohtas in western Bihar to Chittorgarh in Rajasthan. It covers large areas of Madhya Pradesh, Chhattisgarh, Uttar Pradesh and Rajasthan. The Bundelkhand gneisses mark a gap in this belt while a large area of these rocks is covered by the Deccan Trap.
The Vindhayan system of rocks stands over the Cuddapah rocks. The Vindhayans are divided into Lower and Upper divisions.The lower Vindhayans as (13,00-1,100 mya) is marine in origin mostly calcareous in nature and shows tectonic deformation by folding movements, the upper Vindhayans systems (1000-600 mya) is fluviate in origin and is gently lying in undisturbed horizontal strata.
The upper Vindhayan bed enclose two diamond bearing horizon, from which Panna and Golconda diamonds have been mined. The Vindhayan system on the whole is devoid of metalliferous minerals but provides large quantities of excellent and durable free stones, flag stones, ornamental stones, limestone and some coal.
Dravidian Rock System (Palaeozoic):
The rocks of the Dravidian system came into being about 600-300 million years ago. Most rocks of this system are found in extra peninsular region. The rocks belonging to the Dravidian system contains abundant fossils which help in determining correctly the age of the rocks and make correlation of rocks possible over distant areas. The rocks of Cambarian, Ordovician, Silurian, Devorian and Carboniferous periods are included in the Dravidian system.
The Cambarian Rocks (600 mya):
They are best developed in the north western region. In the Spiti valley of Himachal Pradesh, there is an extensive form known as the “Haimanta System”. It consists of slates, quartizites and dolomities. In the salt range on the Indo-Pak border, the Cambarian is represented by 900 meters of t’ossiliferous Sandstone, Shales and dolomities known as saline series.
The Ordovician Rocks (500 mya):
They overlie the Haimanta system in all parts of the Spiti in the form of a thick series underlain by conglomerates. They are also present in the Lidar valley of Kashmir and in the Kumaon region.
The Silurian Rocks (440 mya):
In the Spiti valley, the Silurian rocks are in continuation with the Ordovicians. Lime and shales of the Kumaon region belong to the Silurian period.
The Devonian Rocks (400 mya):
These are devoid of any fossil .cinains. These rocks have definitely been identified in the Muth quarzite of Spiti and Kumaon, on the flanks of Lidar anticline and in the Haridwar district of Uttarakhand.
The Carboniferous Rocks (350 mya):
It comprises mainly of limestone, shale and quartzite. These rocks are generally divided into:
(a) Upper Carboniferous:
These rocks are made of limestone and dolomite.
(b) Middle Carboniferous:
These rocks are found in Spiti Valley, Kashmir, and Shimla and in the Eastern Himalayas.
(c) Lower Carboniferous:
It includes Pir Panjal trap and some rocks of the Kumaon region.
Coal formation started in the carboniferous age, carboniferous in geology means “coal bearing”.
The Aryan Rock System:
Aryan system comprises the rock formation ranging from the “Upper Carboniferous to Recent”. They are fairly preserved in the Peninsular India and are found in perfect sequence in the Himalayan region along the entire northern border.
It derives its name from the kingdom of the Gonds, the most primitive people living in the Andhra Pradesh. They are of continental origin, fluviatile and lacustrive deposits. The main regions are Damodar valley in Jharkhand, Mahanadi river valley in Chhattisgarh and Orissa, in the southern past of Madhya Pradesh.
It is also found in Kashmir, Darjeeling and Sikkim. Economically, the Gondwana rocks are the most important in India containing about 98 per cent of her coal reserves. They have rich depisits of iron ore, copper, uranium and antimony.
It means three-fold (280-285 mya) is almost unknown in the peninsula but are found over extensive areas from Hazara to Nepal. Impressive sections of the system are exposed on the South flank of the Great Himalaya range from Kashmir to Byans in Eastern Kumaon.
Jurassic System (180 mya):
It overlies the Truassic, covering wide areas in Tibet, South Ladakh, Spiti, Nepal and Bhutan where linestone occurs to a depth of 600-900. There is 190 km long and 64 km wide area in Kuchchh which is covered by rocks of the Jurassic System.
The Cretaecous System:
It is one of the best developed Marine System of India showing a variety of rocks, deposited in the land, sea estuaries and lakes. No other system is so widely distributed in India as the Creataceous system is, both in the Peninsular and the Extra-peninsular parts. It includes Sandstones, quartzites, limestone and shales.
The Deccan trap:
From the end of Cretaceous till the beginning of the Eocene, stupendous volcanic outburst overwhelmed a vast area of the Peninsular. These volcanic deposits have flat top and steep sides so that they appear as gigantic step. The process of denudation over a long period has reduced the Deccan Trap to almost half of its original size and the present Deccan Trap covers about 5 lakh sq. km. mainly in pasts of Kuchchh, Saurashtra, and Maharashtra, the Malwa plateau and Northern Karnataka.
The rocks of the Tertiary System were formed from Eocene to Foliocene about 60 to 7 million years ago. It is the most significant period in the India’s geological history because the Himalayas were born and India’s present form came into being in this period. It is also called the “Age of the Mammals”. It has three sub divisions:
(a) The Eocene System (60 mya):
It is mainly found in Jammu and Kashmir, Himachal Pradesh, Rajasthan, Gujarat and in the North-Eastern part of India. Eocene is represented by limestones and coat-bearing Sand stones in Jaintia series in the southern and eastern parts of the Meghalaya plateau.
(b) Oligocene and Lower Miocene System:
It is very poorly developed in India. It is estimated that during a part of this period of the Tertiary outcrops suffered considerable denudation which resulted in the removal of rocks belonging to this system. Rocks of the Oligocene age are found in the greater part of the Barail series of Assam where they are overlain with a masked unconformity by lower Miocene rocks.
(c) The Shiwalik System (14 to 0.2 mya):
(Middle-Miocene to lower Pleistocene) It takes its name from Shiwalik hills between rivers Ganga and Yamuna, where they were first known to science. It is found all along the length of Shiwalik Hills. Sandstones, grits, conglomerates, clays and silts comprise the rocks of this system.
It is also found in Assam, Kuchchh and Saurashtra area of the Gujarat. The great bulk of the Shiwalik formation is non-fossiliferous, but in certain areas some formations are highly fossiliferous. The variety of fossils shows wide range of environment-from humid to aridity.
(d) The Pleistocene and Recent Quaternary:
It is a brief period of nearly one million years and is said to have just begun. It has two divisions and younger division is called Recent. It started about 12,000 years ago, since the withdrawal of of the last glaciation. Pleistocene age is divided into four glacial and three interglacial periods.
The first glacial and Inter glacial periods belong to the lower pleistocene. The second belongs to middle pleistocene. The Karewas of Kashmir are from this glacial period. (The flat topped terraces of the Kashmir Valley, and on the flanks of the Pir Panjal consisting of clay, sands, and silts together are called Karewas in Kashmiri language) The upper pleistocene include the third and fourth glaciation and the intervening third inter glacial period.
The fossiferous clays, sands and gravels of the upper Satluj and the alluvial deposits in the valleys of Tapi, Godavari and Krishna are also of the Pleistocene Age. The older alluvian called bhangar is of the middle or Upper Pleistocene Age. The newer alluvium called Khadar belongs to the Upper Pleistocene Age.
Geological History of India:
The present day three-fold physiographic divisions of India have evolved through a long geological history. A brief account of each division is given below.
(I) The Peninsular Plateau:
The southern part of India consists of an old plateau which is known as the Southern Plateau because it is surrounded by sea on three sides. Geologists believe that it is the oldest landform of the Indian sub-continent and is just one of the several plates of the earth’s crust.
It is known as Indian Plate. During pre-Cambrian era, there was a large depression in which sediment was deposited and a block of crustal rocks known as the Peninsular Plateau came out of this depression and never submerged again. This table land has behaved as a rigid and inflexible block throughout its geological history and is often compared to a ‘horst’.
The first major event in the structural history of the peninsular block was in Vidhyan or Palaeozoic period. During this period, there was a geosyncline. The strata of this geosyncline were tightly folded with great vigour and the Aravalis were formed. The present Aravalis are much denuded and are much reduced in size. It is a typical example of a residual mountain. Nallamalai range also came into being at the same time.
Accumulation of tension in the earth resulted in the faulting and fracturing of the crust associated with vertical movements (uplift and subsidence). With every uplift or subsidence the topography underwent refreshed erosional process.
Palani Hills and Nilgiri Hills are the result of uplifting process, while the fault valleys of the Godavari, the Mahanadi, the Narmada, the Tapi and the Damodar came into existence due to subsidence process. The Malabar Coast and the Mekran Coast are also the result of subsidence process.
At the time of uplift of the Himalayas, two important events took place. As a result of first event, a volcanic eruption occurred in the north-western part of the plateau leading to the formation of the horizontally arranged beds of the Deccan Lava.
In the second event, the western flank of the plateau subsided and the Indian Ocean advanced landwards leading to the formation of the Arabian Sea. The foundering of this flank has given to the Western Ghats a sharp contrast in relief and a prominence of a mountain.
It is now commonly accepted view that the Arabian Sea came into being in the Pliocene or late Pliestocene period. The eastern coast and the adjoining continental shelf furnish contrary evidence. There is not much change in the coastline since the upper Palaeozoic Age.
(II) The Himalayas:
The Himalayan mountain ranges are much younger as compared to the Peninsular Plateau and are called young fold mountains
Several scholars have expressed their views regarding the origin of the Himalayas. The prominent among them are O.H.K. Spate, D.N. Wadia, M.S. Krishnan, S. Burrard, E.H. Pasco, G.E. Pilgrim, de Terra, T.T. Paterson, T. Hagen, Auden, A. Heim and A. Gansser, Wager and a host of others.
There is almost a complete unanimity that the Himalayan Mountains have come out of a great geosynchine called the Tethys Sea and that the uplift has taken place in different phases. But divergent views have been expressed regarding the process and time of uplift as well as the forces responsible for such a vast scale uplift. The consensus which has emerged from the views of different scholars is reproduced as under:
About 120 million years ago, the arrangement of continents and oceans was quite different from what it is today. There used to be a vast shallow sea, known as the Tethys Sea lying between the Angaraland in the north and the Gondwanaland in the south.
Sediments were brought by rivers from these land masses and deposited at the bed of the sea. These sediments were subjected to powerful compression, either because of the southward movement of the Angaraland or due to the northward movement of the Gondwanaland.
Majority of the scientists believe that it is the northward movement of the Gondwanaland which caused compression in the sediments at the floor of the Tethys Sea. In any case, whether Angaraland moved southwards or Gondwanaland moved northwards or both moved towards each other, the net result would be the same; the sediment in the Tethys Sea was squeezed and crushed, and a series of folds were formed one behind the other giving birth to the highest relief features on the earth the Himalayas.
This tertiary mountain building phase is generally known as Alpine because Alps Mountains of Europe were also formed almost at the same time. The curved shape of the Himalayas convex to the south is attributed to the maximum push offered at two ends of the Indian Peninsula during its northward drift.
In the norhtwest it was done by the Aravalis and in the north-east by the Assam ranges, both acting as two extended arms pushing out the extremities, while the central area sagged giving the arcuate shape to the Himalayas. The recent studies have shown that India is moving northwards at the rate of about five cms. Per year and crashing into rest of the Asia, buckling the Himalayas between Angaraland and Gondwanaland.
The diastrophic movements which helped in the formation of the Himalayas started in the late Cretaceous times and continued through the Eocene, Middle Miocene, Pliocene to the lower Pliocene and finally into the upper Pleistocene to sub-Recent times. There are evidences to show that the process of uplift of the Himalayas is not yet complete and they are still rising.
The heights of various places as determined by trigonometrical methods indicate that the Himalayas continue to rise till date. According to the estimates made by Godwin Austen, the average elevation of the Himalayas was 2440 m. above the sea level about a million years ago which has now risen to 3050 m.
The Mahabharat range is still in a state of rigorous uplift. Some of the fossil formations found in the Shivalik hills are also available in the Tibet plateau. It indicates that the past climate of the Tibet plateau was somewhat similar to the climate of the Shiwalik hills and that the elevation of Tibet plateau was almost the same as that of the present Shivalik hills and the plateau has since risen to its present elevation.
The desiccation of the lakes of Tibet within recent or even historical times provides roof of the further uplift of the Himalayas. Surrounding these lakes, the sand and gravel terraces at higher levels, sometimes 60-100 metres above the present water level, are seen which prove that the water stood at a much higher level till recent times.
This could be possible only in the event of uplift of the region. The frequent occurrence of earthquakes in the Himalayan region shows that the Himalayas have not yet attained the isostatic equilibrium and they still continue to rise further.
The Himalayan Rivers are still in their youthful stage and have been rejuvenated in recent times. This is another proof of rising trend in the Himalayas. The present rate of uplift of the Himalayas has been calculated at 5 to 10 cm. per year.
(III) Evolution of Surface Features:
The regular extension of the Sunderban Delta indicates that the process of plain formation is still continuing. The Sunderbans is estimated to be about 4110 km2 of which about 1,700 km2 is occupied by water bodies in forms of river. The Sunderbans was originally measured (about 200 year ago) to be about 16,700 km2. Now it has dwindled into about 1/3 of the original size. The sunderbans floor varies from 0.9 m to 211m about sea level.
Unifying Role of Geological Processes:
The above description reveals that though the three geomorphological units are quite different from one another, yet they are not mutually exclusive and their similarities are no less important. Following three points will explain it:
It is generally believed that the Himalayas are young and the peninsular plateau represents senile topography. But there is no dearth of senile traits in the Himalayas and youthful features in the peninsular plateau.
The structural evidences show that the peninsular block played an important role in the emergence of the Himalayas in Tertiary Age. The edge of Indian plate and relief features of Shillong (Meghalaya) Plateau, the Aravalis and the Kirana Hills near Chenab played an important role in the formation of north-western and north-eastern extremities of the Himalayas.
The rock strata found in the. Himalayan ranges are similar to those found in the Peninsular Plateau. The huge accumulation of sediments in the Great Plains has been derived both from the Himalayan ranges and the Peninsular Plateau.
From the above description, it is clear that the three macro level physiographic divisions of the Indian sub-continent are the result of interaction and interdependence upon one another. They are unifying in character and they present a look of integrated whole, though they are varying in their heights, nature and mode of formation.