The Himalayas appear to plunge rather abruptly on their south into Indo-Gangetic Plain, there being no transition belt between the plains and the Himalaya except the narrow pebbly zone known as Bhabar, being a series of coalesced alluvial fans and cones. On the north the limits are not so well marked because probably of the adjoining Tibetan median mass. But Mt. Kailas, the Great Mansarowar and other adjoining physical features belong to the Himalaya. They find reference in Kalidas’s works and were well known to the Indians of his time and earlier inhabitants of India.
According to Wadia the western limit of the Himalayas is the bend of the Indus and the eastern one is marked by a similar turning in the course of the Brahmaputra. But others consider the Himalayas extending from Baluchistan to some ranges in Burma.
According to Krishnan, “the Himalayas extend with a smooth sweep from Assam to Kashmir”. Here we may confine our attention only to that part of the Himalayas which is in India, i.e., in the whole of Kashmir, Himachal Pradesh, Uttar Pradesh, Sikkim and West Bengal and Arunachal.
From the viewpoint geomorphology, seismicity and tectonics the Himalayas form one of the most active parts of the earth. The Himalayas are considered to be one of the youngest mountains of the world. While the Alps are 17 million years old, the major part of the Himalayas is believed to be 10 million years old. Yet they are free from volcanism, which characterizes most of the Tertiary Fold Mountains. The Himalayas, most probably, are a region of ‘convergence’ of tectonic plates, which has resulted in the shortening of the crust here during Cretaceous—Tertiary to the tune of 200 km.
There are root-like projections and down-saggings in the Mid-Cretaceous sedimentary rocks along the Upper Indus, which is believed to be a ‘suture line’. This suture (the word meaning the surgical mark of a wound) represents the line where the rupture (represented by basic volcanic eruptions) was closed and where the subduction, i.e., going down of the rocks occurred.
Probably a major part of the stupendous height of the Himalayas is due to isostatic uplift in response to the erosion of vast material.
The Himalaya became probably as high as now in early Pliocene (c. 10-15 million years ago). This conclusion is based on the size of pebbles in the present Bhabar, which are equal to the size of pebbles in Siwalik conglomerates (early Pliocene in age). As the size of foothill pebbles is related to the height of mountains, the conclusion is arrived at that the Himalayas had become as high in early Pliocene as now.
The Himalayas, the folded Tertiary mountains, are backed in the north by the largest and the highest ‘median mass’ in the world. This highest mountain of the world is more geometrically and symmetrically shaped than the other great mountains like the Alps, the Rockies and Andes.
As regards structural complexity, they are intermediate between the most complicated structure represented by the Alps and the moderately complicated structure represented by the Appalachians. Locally various structural types are found. Simple Jura type of structure occurs in the Spiti region as well as in the south in the open symmetrical folds of several areas of the Siwaliks.