The present shape of India is the cumulative result of the geological evolution of millions of years. In order to understand the relief of India, it is necessary to have a brief geological background.
India can be broadly divided into 3 divisions:
(A) Peninsular Plateau
(B) The Himalayas and the associated mountains
(C) The Indus-Ganga-Brahamputra Plain.
A brief description of the above will throw light on the changing evolutionary stages through which India of today has passed.
(a) Peninsular plateau
An almost Stable Block. This block raised more than 500 million years ago and since then it has never been submerged under ocean.
It is a rigid and inflexible type of landmass. Many compression forces forced the Himalayas to be uplifted but these forces had little effect on this block. Only once did the organic forces raise a few parts of this block about 350-500 million years ago. In one such incident, the geosynclines! Sediment rose up to form the Ravalli Mountains and possibly the Vmdhyan Range. Another incidence took place in the same age when Nallarnalai Hills and Veliconda Hills were uplifted and arc found sprawling between the Krishna and the Penner rivers. Both of these are now in old stage with rounded tops and smoothened wavelikc form.
Some people call the peninsular plateau a horst which is an uplifted block parallel to a fault. In fact it did not rise but appears uplifted because of the subsidence in the fault.
The Effect of Tensional Forces.
It also does not mean that it remained unaffected by the end genetic forces which produced tensional forces. This made some parts of the peninsular plateau rise but this had only marginal effect on the peninsula which remained more or less stable.
These tensional forces, however, raised and rejuvenated some parts of the plateau. This is why some parts appear young in the otherwise old monotonous landscape.
Such upliftment 70-150 million years ago created the Palni Hills and Nilgiri Hills but subsidence produced trough faults which are now occupied by the Damodar, the Mahanadi, the Godavari, the Narmada and the Tapti. The Makran Coast in Pakistan is also the product of trough faulting.
The formation of Arabian Sea and Volcanic Eruption.
During the times when the Himalayas were rising two very important incidences took place:
(a) The formation of the Arabian Sea.
A large sea area west of the Western Ghats subsided and a big sea, now called the Arabian Sea, was formed. This subsidence caused the Western Ghats to look like mountains.
Some scholars think that the whole of the Peninsula tilted towards the west and this made Western Ghats rise up into a mountain. Whatever be the facts, the incidence took place in Pliocene epoch (12-5 million years ago).
(b) The volcanic eruption.
In the north-western part of this peninsula, the crevasses emitted (60-70 million years ago) so much lava (basalt) that layers of lava spread horizontally on an area 5, 00,000 sq km. The lava is 600 to 1500 metres deep but at certain areas it is 3,000 metres deep.
The black soil produced on this lava region is very fertile for the cultivation of cotton. The area is also called Deccan Trap.
The Peninsular Plateau is a tabular upland. During the Pre-Cambrian era the rocks in the ocean bed rose to form the peninsula.
These rocks are the most ancient rocks of the world. Most of these rocks arc crystalline. Due to the action of faults, subsidence, etc., this peninsula appears divided into many parts.
(b) The himalayas and the related mountains
The Birth of Tethys.
Up to Mesozoic era, in the area north of the peninsula there was a narrow, shallow and elongated sea. Its bottom was weak. Rivers continued to fill this sea for a very long period.
The bottom under the weight of deposition continued subsiding with almost the same rate at which the deposition was taking place. As a result, the surface remained almost unchanged. This depression is referred to as the geosynclines of Tethys Sea.
To the south of the Tethys geosynclines there was the Peninsula and to the north of it was a block made up of hard and ancient rocks. This block is called the Angaraland.
The Angaraland due to tectonic forces shifted towards the south and pressed the sedimentary rocks of the Tethys. As a result the sedimentary rocks were uplifted and spread in the form of a mountain.
During the process, the south block- the Gondwanaland, remained almost unmoved. This fold mountain is called the Himalayas.
Some scholars believe that the thrust came pot only from the north but the Peninsula also moved towards north to fold the sediments of the Tethys into the Himalayas.
The curvature of the Himalayas and the turnings of the western and eastern ends appear to point to the unmoved situation of the Peninsula. It appears that the rising folded layers were blocked at the Peninsula in the middle but the eastern and western arms turned round to become north-south.
It is not only the Peninsula but its northern ranges also played the role of obstructing the thrust from the north. These peninsular mountains arc the Meghalaya Hills and Rapnahal Hills in the north-cast and the Ravalli Mountains in the north-west.
The Himalayan formation did not take place in one go. It took 5-6 million years and the formation remained slow. This organic movement can be divided roughly into the following three stages:
(a) First Uplift.
This stage of Himalayan progeny took place in Oligocene epoch (35 million years ago). During this epoch the Central Axis of the Himalayas was uplifted. This consisted of sedimentary and crystalline rocks.
(b) Second Uplift.
This upliftment took place in Miocene (15-25 million years ago). The sedimentary rocks of Postwar basin (Pakistan) were folded.
(c) Third Stage.
This stage is post-Pliocene. The uplift took place about 10 million years ago. The Swales were formed. Scholars think that this movement is still continuing and this part of the Himalayas is still rising.
Karakoram Mountains appear continuous with the Himalayas in the north but this mountain had formed well earlier in the Cretacious period of the Mesozoic era (130 million years ago).
(c) The ganga-sindh-bramamputra plain
This plain is situated between the Himalayas in the north and the Peninsula in the south. The major rivers of this plain are the Sitidh, the Ganga and the Brahamputra.
There are mainly three causes of its origin – (*) the formation of the Himalayas (it) the subsidence of some parts of the northern peninsula and (Hi) the filling up of the plain with the sediments brought by the rivers.
(i) Formation of the Himalayas.
In the Tertiary period when the sediments of the Tethys were uplifted by the compressional forces in the form of the Himalays, a foredeep was formed to the south of the Himalayas.
The bottom of the foredeep is the continental shelf of the Tethys. The sea was filled with the sediment of estuaries. The sea sediment of the lagoons, which were numerous, covered this shelf.
(ii) The Subsidence of the Continental Shelf.
When the thrust of the organic forces from the north pressed the Tethys in the south, the Tethys sediment in turn pressed against the peninsular plateau but the latter remained stationary.
The result was that the soft sediment rose up in the form of a mountain. It also does not mean that the Peninsula remained wholly unaffected. Some parts of the northern Peninsula subsided as a result of this movement.
For example, the portion situated between the Rapnahal Hills and the Garo Hills subsided.
It is because of the formation of an cast- west fault. However, the gap between the two hills was later on filled with the alluvial sediment brought by rivers.
Another proof of the subsidence is the formation of a rift valley in which the Damodar flows.
(iii) The Filling of the Plain.
The rivers flowing down the Himalayas in the north and those flowing down the Peninsula deposited their sediment on the shelf of the Tethys.
This aggradation turned the Tethys depression into a plain. Sir Burard thinks that there was a rift valley in this area which was formed due to faulting process.
Whatever be the fact, it cannot be denied that this plain is still being filled. The Sunderban delta is extending into the Bay of Bengal.
The filling of the Indo-Ganga trough is not of equal depth. The older view that the depth of the sediment is about 4,500 metres is challenged by Glennie, E.A. who has proved on the basis of gravity anomaly readings that the depth is around 2,000 metres only. However, the shallower area is found between Rajmahal Hills and Shillong Plateau.