The southern most range of Himalayas is known as Siwalik. The Himadri and Himachal ranges of the Himalayas have been formed much before the formation of Siwalik range. The rivers rising in the Himadri and Himachal ranges brought gravel, sand and mud along with them, which was deposited in the rapidly shrinking Tethys Sea.
In course of time, the earth movements caused folding of these relatively fresh deposits of sediments, giving rise to the least consolidated Siwalik range.
The average height of the Siwalik range is very low, about 600 metres only. There are some broad valleys in between the Himachal and the Siwalik ranges. These valleys are known as ‘duns’. Dehradun valley is one of the best examples.