Himalayas separate plain of Indian subcontinent from the Tibetan Plateau and are formed by folding of Tethys sea sediments. These mountain ranges run for about 2400 kilometers from Indus River in west to Brahmaputra River in the east. The average width of the Himalayas is 160 to 400 km.
Himalayas can be divided into following four divisions from north to south
- The Trans Himalayas
Trans Himalayas are about 40 km wide with an average height of 5500-6000 m. They contain sediment of Tethys Sea. The rocks contain fossil bearing marine sediments. Material composition of the rocks is that of tertiary granite.
- The Greater Himalayas The Greater Himalayas are about 25 km wide with an average height of 6100 meters. Major peaks of this region are Mt. Everest, Kanchenjunga, Nanga Parbat etc. They are composed of crystalline, igneous and metamorphic rocks. They receive less rainfall and have little forest area.
- The Lesser Himalayas The width of lesser Himalayas is about 80 km with an average height of 1300-4600 m. It consists unfossiliferous sediments and metamorphosed crystalline. The main rocks are slate, limestone and quartzite. The region is prone to heavy rainfall and deforestation.
- The Shiwalik or the outer Himalayas The width of Shiwalik varies from 8 km in the east to 45 km in the west with an average elevation of about 900-1500 meter above sea level. Between the Shiwalik and lesser Himalayas are longitudinal valleys called Doons for example, Dehradun. They are mainly composed of sandstones, sand rocks, limestone etc.