The geological records of the world are classified on the basis of Geological scale devised in Europe On the basis of it the geological history of the earth is divided into five eras (Neozoic, Cainozoic, Mesozoic, Palaeozoic and Proterozoic), four epochs (Quaternany, Tertiary, Secondary and Primary), and sixteen periods (Holocene, Pleistocene, Pliocene, Miocene, Oligocene, Eocene, Paleocene, Cretaceous, Jurassic, Triassic, Permian, Carboniferous, Devonian, Silurian, Ordovician and Cambrian).
Since the geological records of India do not fully conform to the Europe Sir T. Holland of the G.S.I, has suggested four geological eras (Aryan, Dravidian, Purana and Archaean) for the geological history of India on the basis of major unconformities between them. Thus we have Achaean era corresponding to the Azoic or Achaean era of the western scholars which is separated from the Purana or Proterozoic era by Eparchaean unconformity. Dravidian era stretches from Cambrian to the Middle Carboniferous periods and has a shorter duration than the Palaeozoic era of the West.
The Aryan era runs from the Upper Carboniferous to the recent periods and has still longer span of time covering whole of Mesozoic and Cainozoic eras and a part of the Palaeozoic era of the western classification.
The geological history of India is complex as well as varied. It begins with the first formation of the earth’s crust, first deposit of sedimentary rocks, first progeny and extends up to the recent laying down of alluvial deposits. Many of these rock formations occur in juxtaposed or superimposed position and have been subjected to intense folding and faulting activities.
The Achaeans including the Dharwars and the associated granites and gneisses, which cover about two-thirds of the Peninsular India, are the earliest rock formations found in the country. These include rocks like schist’s, gneisses, granites, and charnockites and form the basement complex around and upon which the subsequent rock systems have been laid down. The Cuddapahs, Vindhyans, Gondwanas and the Deccan Trap which occupy remaining one-third of the peninsular surface succeeded the Achaeans.
The folding and upliftment of the Himalayas started during the mid Miocene period and continued in phases up to the Pleistocene period. This includes huge quantity of marine sediments along with igneous and metamorphic rocks from the floor of the Tethys. The Northern Plains (including coastal plains) came into existence during and after the Pleistocene period and largely consist of alluvial deposits. The basement complex of the peninsular rocks underlies the Great Plains and even perhaps the Himalayas.
In short the entire geological history which laid to the formation of the present land mass of India may be summarised under following major geological phases (Singh, 1971, pp.3-4):
1. The first phase is characterised by the cooling and solidification of the earth’s crust during the pre-Cambrian era (600 million years ago) is represented by the exposure of the Achaean gneisses and granites especially on the Peninsula. Minor igneous activities, subsequent metamorphism and crumpling along with the folding of the Aravalli mountains represent the main activities.
2. The leveling of the undulations and crushing and crumpling of the sediments of the Dharwarian group (Bijawars) mark the second phase. The igneous activities and intrusions imparted it the character of mixed sedimentary.
3. The calcareous and arenaceous deposits corresponding to humid and semi-arid climatic regimes in the Cuddapah and Vindhyan basins bordering or lying within the ancient landmass and its upliftment during the Cambrian period (500 million years ago) characterise the third phase.
4. The permo-Carboniferous glaciations and extensive glaciofluvial deposition in the depressions and their subsequent sag faulting mark the fourth phase of the formation of Godwin system of rocks (270 million years ago) which preserve 95% of coal resources of the country.
5. This phase is witnessed by the major geological event of the fracturing of the super continent of Gondwanaland and drifting of the peninsular mass towards north (mid Mesozoic, 200 million years ago). This led to further uplift of the Vindhyan sediments and formation of the Western Ghats.
6. Cretaceous lava flow covering more than 5 laky sq. km. of surface in the western part of the Peninsular India and formation of the Deccan Trap.
7. Tertiary progeny due to the collision of the Indian Plate (Gondwanaland) with the Asian Plate (Angaraland) and formation of three successive ranges of the Himalayas-(a) Himadri or Greater Himalayas during Oligocene (25-40 million years ago), (b) Himanchal or Lesser Himalayas during mid- Miocene (14 million years ago), and (c) Siwalik or Outer Himalayas during post-Pliocene (750 thousand years ago) periods-by the folding of the Tethys sediments. Contemporaneous had been the formation of the Indo-Gang trough during this phase.
8. Alleviation and sedimentation of the Indo- Gang trough during Pliocene-Holocene period by glacis-fluvial deposits.
9. Down warping of the Rajmahal-Garo gap or the Malda Gap (Pleistocene period) and upheaval of the Indo-Ganga divide (Potwar Plateau) which disrupted the old drainage channel (the Indo-Brahma or Siwalik river) and led to the evolution of the present drainage pattern of the Great Plains of India. Other major derangements may be the formation of the Narmada-Tapti troughs and the foundering of the West Coast.
The underlying Peninsular block has given semicircular or actuate shape to the Himalayan ranges with syntaxial bends around Salt Range in the northwest and Nacho Birwa in the north-cast. The channel of the Gang has also negotiated with the shape of the peninsular foreland and ‘has perhaps reached its limit of southward shift, washing at places the old bed rocks.’
On the basis of above lathe logical, sedimentation and tectonic history the whole land mass of India can be divided into three major structural units. These are also called the triple tectonic divisions of India.
1. The Peninsular Block-the triangular plateau of the Peninsula India (i.e. the Deccan south of the Vindhyans), with the island of Sri Lanka.
2. The Young Folded Mountain Belt-The mountainous region of the Himalayas which borders India to the west, north and east, including the countries of Afghanistan, Baluchistan (Pakistan), and the hill tracts of Myanmar as the extra Peninsula.
3. The Indo-Ganga Trough-the great Indo- Ganga Plain of Punjab, Uttar Pradesh. Bihar and Bengal, separating the two former areas and extending from the valley of the Indus in Sind (Pakistan) to that of the Brahmaputra in Assam including the country of Bangladesh.