Geology

Though various parts of India evolved during a long geological history, India is composed of three geological units:

• The Peninsular Plateaus
• The Himalayan mountains, and
• The Indo-Ganga-Brahamputra plain, between the above two

The Peninsular Plateau

The southern part of India consists of an old plateau which is known as the Southern Plateau. This is also called the Peninsular 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 crystal rocks known as the Peninsular Plateau came out of this depression and never submerged again. is table-land has behaved as a rigid and in flexible block throughout its geological history and is often compared to a ‘horst’.

The peninsula received some lateral thrusts and orogenic (mountain building) forces, without affecting the original basement. It is considered as a part of Indo-Australian plate that has been subjected to vertical movement and block faulting. River valleys of the Narmada, the Tapi, and the Satpura block mountains are some of the outstanding examples of such movements. This peninsula contains mostly residual m ountains. The Aravali hills, the Nallamala hills, the Javadi hills, Velicondo hills, the Palkonda range and the Mahendragiri hills etc. are some of the residual hills in the peninsular plateau. The river valleys here are shallow with low gradients.

Most geologists believe that the Indian peninsula, a part of the global Gondwanaland (continent), dried northwards and striking with the Central Asiatic plate was raised up to form the high Himalayas out of the Tethys sea.

The Himalayas

The Himalayan mountain ranges are much younger as compared to the Peninsular Plateau and are called the Young Fold Mountains. Till Mesozoic era (about 70 million years ago) the entire Himalayan area as well as the plain area remained under sea-water in a geosyncline of Tethys Sea as sediments. To the north and south of the Tethys Sea were the landmasses of Angaraland and Gondwanaland respectively. The sediment in the Tethys Sea was deposited by the rivers owing into it from two landmasses. As a result of northward drift of the Peninsular Plateau in the last stage of mountain-building process, the sediments of the Tethys Sea were folded and the present Himalayas were formed.

Himalayan mountain-building process is now better explained with the help of theory of Plate Tectonics. This theory has replaced the former geosynclinal the”” theory of mountain-building. The entire crust of the earth has been divided into a number of plates. The collision of the plates leads to the building of stresses within the plates and the continental rocks above, resulting in folding, faulting and igneous activity. According to the Plate Tectonic theory, ‘The Himalayan ranges were formed when the Indian Plate was driven northwards and pushed beneath the Eurasian Plate. With the advance of the Indian Plate towards the north, the Tethys started contracting about 65-70 million years ago. About 30-60 million years ago, the two plates came closer and the Tethys Sea crust began to fracture into thrust edges. About 20-30 million years ago, the Himayalan ranges started emerging.’

The Himalayan mountain ranges are still in their youthful stage which is evident from the presence of landforms like gorges, V-shaped valleys, rapids, falls etc.

Indo-Ganga-Brahamputra Plain

In between the Himalayas in the north and the Peninsular Plateau in the south, lies the great plain of India popularly known as the Indo-Ganga-Brahamputra plain. It is believed that there was a large depression caused due to Himalayan uplift out the Tethys Sea and the subsidence of the northern of the peninsular plateau. Rivers owing into this depression from the mountains of the north and the plateau of the south deposited sediment in this depression and in course of time it turned into a at land of alluvium deposits.

Physiography

Physiography is the study of formation and development of surface features of land like mountains, plateaus and plains. It is the outcome of structure, process and the stage of development. India is a vast country with mountains, plateaus and plains. In the north are the Himalayas ‘ the abode of snow. In the south there are peninsular plateaus. Between the Himalayas and the peninsular plateaus, lie vast Indo-Gangetic plains. A rough estimate made by the Census of India in 1951, shows that of the total land area, 10.7 per cent is more than 2,135 metre above sea level and is mountainous, 18.6 per cent is hilly area (305 to 2135 metre), 27.7 per cent is plateau (305 to 915 metre) and the remaining 43 per cent is plain area.

India is generally divided into following relief divisions.

• The Himalayan Mountain Complex
• The Indus-Ganga-Brahmaputra Plain
• The Peninsular Plateau
• The Coastal Plains
• The Islands

1. The Himalayan Mountain Complex

The chain of Himalayan mountain ranges stretches on the northern borders of India in an east-west direction. These mountain ranges run for about 2,400 kilometres from the Indus river in west to Brahmaputra river in the east. The average width of the Himalayas is 160 to 400 km. In fact, the Himalayan mountain is not just one range but a group of three mountain ranges running almost parallel to one another all along its longitudinal axis. A brief description of these mountain ranges is given below:

Great Himalayas or the Inner Himalayas

This is the highest mountain range of the Himalayan system. This always remains snow-covered and is generally referred to as ‘Himadri’. Its average altitude exceeds 6,000 metres and its average width is 25 kilometres. Almost all the important peaks are located in this range. Some of the loy peaks are as under: