A History of Agriculture in India v1.c7: Chapter 1. Birth of the Indian Subcontinent
A History of Agriculture in India v1.c7
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- v1.c7.s1 References — 13 parts
Chapter 1. Birth of the Indian Subcontinent
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THE most outstanding fact about the physical geography of India is the natural division of the country with three distinct segments of totally dissimilar character: (i) the Himalayas, the great mountain system to the north, (ii) the Indo-Gangetic alluvial plain of northern India extending from the Punjab to Assam, and (iii) the Peninsula of the Deccan to the south of the Vindhyas—a solid stable block of the earth's crust, largely composed of some of the most ancient rocks, which the denudation of ages has carved into a number of mountain ranges, plateaus, valleys and plains.
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Ever since the Cambrian Period, the dawn of geological history, the peninsula has been a land area that has never been submerged under the sea. The mountains are not true mountains of upheaval, but are outstanding portions of the old plateau which have escaped weathering. The Western Ghats form the western edge, and the Eastern Ghats the eastern edge of the plateau, which slopes towards the east.
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Whereas the Himalayas and the Indo-Gangetic plain are comparatively young, the peninsula has a long geological history behind it. During the Triassic Period, there were three continents. Of these the North Atlantic continent comprising most of North America, and Angaraland comprising Russia, Siberia and China were in the north. To the south was the large continent of Gondwanaland, which comprised Peninsular India, Africa, South America, Australia and Antarctica. Separating the northern continents from the Gondwanaland was the Tethys Ocean, which covered northern India, Iran, Middle East and northern Africa.
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In the Jurassic Period, the Gondwanaland split into two land masses, the eastern comprising Peninsular India, Malagasy (Madagascar) and Australia and the western comprising Africa and South America.
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The Cretaceous Period began 110 million years ago and lasted for 50 million years. It was the age of armoured dinosaurs, birds, early mammals and primitive angiosperms.
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During the early part of the Cretaceous Period, the Tethys Ocean extended over wide areas. It spread over a great part of Europe. The southward extension of the Tethys flooded much of northern Africa with a series of embayments from Algeria to Egypt. A connection was established with the South Atlantic Ocean across the Gulf of Guinea through Algeria, Libya, the Sudan and Nigeria, converting the Sahara into a great island. In the eastern region, an arm of the Tethys spread over Iran, Pakistan, western India, and lower parts of the Narmada and the Tapti valleys; another arm invaded Assam, whereas embayments spreading northward from the Bay of Bengal merged with the arm of the Tethys in Assam. The sea at this time washed the east coast of India where the upper Gondwana deposits are locally associated with the marine fauna of the lower Cretaceous Age. The middle and the upper Cretaceous, especially in the Pondicherry-Tiruchirapalli sector, are mainly littoral. The fauna of this sector is similar to that of Malagasy (Madagascar) and South Africa and to that of the southern flank of the Assam range. Along the Narmada Valley on the west coast are some marine fossiliferous beds with fossils showing greater affinity with those of the Cretaceous of southern Arabia and Europe than with those of Assam and Tiruchirapalli regions. The dissimilarity indicates that there was still a sort of land barrier that separated the Bay of Bengal from the Arabian Sea. This land barrier has been called Lemuria, which included Peninsular India and Malagasy. The sea surrounding this long Indo-Malagasy island, with its dinosaur-infested forests, was in free communication with the Tethys to the north, as is shown by the presence of European species, or species with European relationships, in both the Narmada Valley and the Cretaceous of the east coast of India.
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The middle and the upper Cretaceous were periods of great marine transgression. A palaeogeographic map of the Gondwanaland during the middle and upper Cretaceous is shown in Fig. 1. While the uppermost Cretaceous beds were being deposited along the south-eastern coast of India, stupendous volcanic outbursts overwhelmed a vast area, comprising the present Gujarat, Maharashtra and Madhya Pradesh, the like of which are not known anywhere else in the world (Fig. 2). Several hundred thousand square kilometres were flooded by the outpourings of extremely mobile lava from fissures. The hills formed by the lava are in some places over 1,200 metres high and are known as the Deccan traps. They are peculiar in appearance, being frequently flat on the top and with steep sides so that they appear from a distance as gigantic steps and, therefore, are called traps, a name derived from the Swedish word, meaning a stair or step. The individual lava flows that make up the Deccan trap plateau vary greatly from a fraction of a metre to 36 metres in thickness. During the periods of quiescence that intervened between successive outbursts, lakes were formed, probably because the lava streams had blocked the rivers. In these lakes, fishes, frogs, small crustaceans, etc., flourished, whereas the flowering plants, including palms and other vegetation, grew on the adjoining land. In the marshy areas dinosaurs thrived, and on the beaches tortoises crawled. As time went on, the lakes were filled up with sediments washed down from the land. Then came another period of volcanic outbursts and lava flows when lakes were formed again, and plants and animals reappeared. Thus, volcanism and sedimentation were repeated many times until a great thickness of lavas and interbedded sediments, called the Intertrappeans, had accumulated, containing the petrified remains of organisms which bear [Figure: Cratonic areas Freshwater facies Mixed facies Marine facies 1,000 — 1 5,000 — 5 Isopach form lines in feet 10,000 — 10 1 foot = 0.3048 metre]
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Fig. 1. Palaeogeographic map of Gondwanaland in the middle and upper Cretaceous (After Ahmad)
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witness to the fauna and flora that existed during that period.
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The continent of Gondwanaland had split up into its integral parts towards the latter part of the upper Cretaceous. The drifting apart of the continents was facilitated by the great overflow of lavas, such as the Stromberg lavas of South Africa, the Serra Geral volcanics of South America and the Deccan traps of India. As a result of these earth movements, considerable parts of the marginal areas of the Gondwanaland broke off and sank into the oceans. The Tethys had already been shallowed in the upper Cretaceous. The intermittent mountain building continued throughout the Tertiary, as a consequence of which the great mountain systems, such as the Atlas, the Pyrenees, the Alps, the Caucasus, the Himalayas and the Malay Arc, were formed.
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The formation of the Deccan trap, to which reference has been made earlier, continued in the Tertiary Period. By far the greater part of western India is made up of the Deccan trap covering an area of 322,900 square kilometres. Originally it had a much greater area, probably covering 800,000 square kilometres; outlying patches of the trap occur in Sind, Kutch, Bihar, and the coastal areas of Andhra Pradesh. The formation of the Deccan trap began in the uppermost part of the middle Cretaceous and continued into the Eocene or even later.
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The crustal movements that gave birth to the Alps have also been responsible for the formation of the Himalayas. The movement that first outlined the belt along which the Himalayas were uplifted may have begun in the Cretaceous times, but such pre-Tertiary movement was probably mild, producing an incipient island arc, and there is little evidence to show that there was any real mountain-building movement before the middle of the Tertiary Period. The gigantic masses of sediments that had been accumulating upon the gradually sinking floor of the Tethys, since the upper Carboniferous, disturbed the gravitative equilibrium of the crust towards the end of the Eocene, when a series of intense mountain-building movements, separated by periods of some quiescence, started, resulting in the uplift of the Himalayas.
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The process was begun by the outpouring of the Deccan trap lava through numerous fissures during the late Cretaceous and the early Eocene. The abnormal tension of the earth's crust caused by the extraordinary sinking of the Tethys thoroughly upset the internal equilibrium of the earth. The equilibrium was adjusted only by the most violent changes starting with great outbursts of volcanism at many centres and ending in gigantic convulsions that forced up the deposits of the Tethys into a great mountain system. It is estimated that about 1,666,000 cubic kilometres of lava, which exceeds in bulk the entire Himalayas, was poured out from the bowels of the earth.
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The continued sinking of the Tethys is presumed to have been caused by crustal weakness. During the sinking of the seat, there was a lateral thrust which narrowed the basin. The narrowing of the basin upheaved the sediments, and surplus sedimentary materials were forced up into folds above the basin to form the Himalayan chain. The margin of the upheaved mass also yielded under strain, developing fractures in many places. As the sinking sea floor reached deep into the region of higher temperature and pressure, the lower layers of sediments were melted into "magma", which invaded the crumpled and crushed the overlying sediments as intrusions or as lava flows. These now form the central axis of the Himalayas. Some of the rocks have been squeezed up and out by the lateral forces from two sides and thrust into overturned folds and faults. Many of the overturned limbs are displaced as flat-lying folds known as nappes.
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INDIA AREA COVERED BY DECCAN TRAP
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Fig. 2. During the upper Cretaceous, stupendous volcanic outbursts overwhelmed a vast area in what is now Gujarat, Maharashtra and Madhya Pradesh. From the lava, the black cotton soil was formed over the ages The upheaval of the Himalayas has taken place in three major phases. The earliest phase was at the close of the Eocene, when the Eocene nummulitic limestone and the underlying older strata that had been accumulating on the floor of the Tethys since the upper Carboniferous were uplifted into ranges of considerable altitude (4,500-6,000 metres), found in Kashmir, Hundesh and in parts of eastern Tibet. With this uplift, all traces of the Tethys in the Himalayan region vanished. The next upheaval took place towards the close of the Miocene when the sediments deposited by rivers in estuaries along the flanks of the infant Himalayas were upraised. These now constitute the middle or lesser Himalayas. The last upheaval started after the Tertiary and continued into the sub-Recent through the Pleistocene. The movements involved the uppermost Siwalik freshwater sediments.
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It is now generally believed that the uplift of the Himalayan system of mountain ranges was due to movements of two solid continental masses on two sides of the Tethys, directed towards one another. The Central Asian continental mass, Angaraland, slowly moved from the north to the south under pressure from the floor of the Arctic Ocean, and the northern edge of the Indian continental mass, the Gondwanaland, became downwarped by the northward compressive force from the Indian Ocean. The Himalayan portion of the Tethys gradually shifted southward and became narrower, assuming its present trend in the early Eocene time. The presence of tongue-like projections of the Gondwanaland—one in the Kashmir-Hazara region (the Punjab wedge) and the other in the north-eastern extremity of Assam (the Assam wedge)—have moulded the pattern of the Himalayan chain. The effects of these two wedges can be clearly seen in any relief map of India. It will be seen that the Himalayan chain occurs as a huge arc between Nanga Parbat in the west and Namcha Barwa in the east. The convexity of the arc points south towards the Indian peninsula. Though geographically the Himalayas are considered to be limited between these two points, the Nanga Parbat and the Namcha Barwa, the rock formations seem to be suddenly folded round at these points and to turn southward in rather parallel ranges.
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Below the Himalayas are the Siwalik Hills, extending from Jammu in the west to Assam in the east. The Siwalik Hills are mainly river deposits of the middle Miocene to the lower Pleistocene Age, folded into arches (anticlines) and troughs (synclines). Many of the anticlines have been broken by faults—dislocations due to the slipping of the rocks along a plane of fracture (fault plane). The fault planes steeply sloping into the hills have given rise to steep scarps facing the plains.
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Immediately adjacent to and on the north of the Siwalik Hills lies the sub-Himalayan zone or lesser Himalayas, 65 to 80 kilometres wide and of an average altitude of about 3,000 metres. The rocks here are mostly non-fossiliferous.
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Farther north is the central Himalayan zone (the Great Himalayas) of high ranges with snow-clad peaks. It consists mainly of metamorphosed sedimentary rocks.
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The Indo-Gangetic plains, which lie at the foot of the Himalayas from Hazara to Assam, mark the side of a deep basin of estimated depth of 1,050 to 6,000 metres which resulted from the compression exerted on the peninsular margin against the advancing crustal waves from the north. The basin has been filled up with the river alluvium derived from the rising Himalayas as well as from the plateau on the south.
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Alberuni (Abu Raihan Muhammed bin Ahmed), a Central Asian scholar, with keen perception, came to northern India early in the eleventh century, and made a remarkable observation on the structure and formation of the Indo-Gangetic alluvium. "If you have seen the soil of India with your own eyes and meditate on its nature," wrote Alberuni, "if you consider the rounded stones found in the earth, however deeply you dig, stones that are of smaller size at greater distance from the mountains, and where the streams flow more slowly, stones that appear pulverized in the shape of sand where the streams begin to stagnate near their mouths and near the sea, if you consider all this, you could scarcely help thinking that India has once been a sea which by degrees has been filled up by the alluvium of the streams."¹
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¹ Sachau, E.C. *Alberuni's India*, Vol. 1, p. 198.
References
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Ahmad, F. *Palaeogeography of the Gondwana Period in Gondwanaland, with special reference to India and Australia, and its bearing on the theory of Continental Drift.* Mem. geol. Surv. India, 90 : 1-142, 1961
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Burrard, S.G., Hayden, H.H. and Heron, A.M. *Geography and Geology of the Himalaya and Tibet.* 2nd edn. Dehra Dun, 1934
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Krishnan, M.S. *Evolution of the Desert—Geographical History of Rajasthan and its Relation to Present-day Conditions.* Bull. natn. Inst. Sci. India, No. 1 : 19-31, 1952
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Krishnan, M.S. *Geology of India and Burma.* Higginbothams Ltd, Madras, 1956
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Pascoe, E.H. *A Manual of the Geology of India and Burma.* 3rd edn. I. Manager of Publications, Delhi, 1950
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Randhawa, M.S. and others, *Evolution of Life*, New Delhi, 1969
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Sachau, E.C. *Alberuni's India*, Vol. I, p. 198, Indian Reprint
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Sahni, B. *Recent Advances in Indian Palaeobotany.* Presidential Address, Botany Section, 25th Indian Science Congress, Jubilee Session, Calcutta, 1938
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Wadia, D.N. *The Tertiary geosyncline of north-west Punjab and the history of Quaternary earth-movements and drainage of the Gangetic trough.* Quart. J. geol. Soc. India, 4(3) : 69-96, 1932
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Wadia, D.N. *Geology of India.* 3rd edn (Revised). Macmillan & Co., Ltd, London, 1957
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Wadia, D.N. *The Himalaya Mountains—Their Age, Origin and Subcrustal Relations.* Meghnad Saha Lecture, 1964. National Institute of Sciences of India, New Delhi
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Wegener, A. *The Origin of the Continents and Oceans.* English translation by J.G.A. Skerl. E.P Dutton & Co., New York, 1924
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Zeuner, F.E. *The Pleistocene Period.* Royal Society, London, 1945