The Himalayas Mountain belt to the North and the Nagalushai mountains in the east are the regions of mountain-building movement which affected the relief of the earth in the last phase of its physical history, it is generally held to have been formed in the Tertiary era. Because of their sharp and striking contrasts in altitude the Himalayan relief features are described as youthful.
The Peninsular Plateau, on the other hand, is an old mass of the Earth’s crust worn down by continual erosion. As a consequence, the Plateau has acquired the look of old age. It has a characteristically senile topography and has existed since the Pre-Cambrian era, 600 million years ago.
In between the two main physiographic units lies the Northern Plains which marks an initial marine depression filled by deposits brought down by the rivers over the ages. The filling has been done so uniformly that the plain gives an impression of a flat surface, though it is not so.
Besides mainland, Indian Territory also extends into the Arabian Sea and the Bay of Bengal in the form of the Lakshwadeep and the Andaman and Nicobar Islands, respectively. The Lakshadweep meaning a hundred thousand islands is a group of 36 coral islands in the Arabian Sea and none of them is more than a couple of square kilometres in area.
The Andamans and Nicobars also known as the Bay Islands are also a cluster of Islands stretched almost in a line. There are as many as 200 islands in the Andaman groups alone extending for 350 kilometres. There are 19 islands in the Nicobar group. Some of the Islands extend from 60 to 100 fairly large and more numerous than the Lakshawdeeps. Some of the islands are of volcanic origin. Barren Island is an active volcano.
The Lakshdaweeps extend just to the north of the Andaman and Nicobar are remnants of the sub-merged mountain range, which was an extension of the Arakan Mountains of Myanmar and continued through the islands of Java and Sumatra in Indonesia.
These islands formed of coral deposits are called atolls which is originally derived from the Malayalam word ‘Atolu’.
The seven physiographic units described above may be called macro units. Each macro unit can be further divided into smaller second order units. These second order units can further be subdivided into third order or micro units. These units provide as a base for the study of physiography, climate, vegetation, soil, agriculture, industries, population etc.
The senile and the youthful features in the peninsular block and the young-folded mountains of the north are not mutually exclusive. Their physical history reveals that there have been deep-rooted interactions between the two units. Their structural characteristics, and their mode of building, as brought out by the tectonic details, furnish evidence of their mutual interdependence and borrowings.
The outlying fragments of the peninsular block, such as noticed in the Shillong Plateau, the Aravalis and the Kirana Hills near the Chenab in the Punjab, played a very important role in defining the trend lines of the Himalayan ranges.
The sediments embedded in the rocks of the Himalayan ranges have similarities to the rock strata found on the peninsular block. The huge accumulations of the sediments forming the surface of the Great Plain, lying between the two main physiographic divisions have been contributed by both of them.