The Sivalik hills is a mountain range of the outer Himalayas also known as Manak Parbat in ancient times. Shivalik literally means 'tresses of Shiva’. This range is about 2,400 km (1,500 mi) long enclosing an area that starts almost from the Indus and ends close to the Brahmaputra, with a gap of about 90 kilometres (56 mi) between the Teesta and Raidak rivers in Assam. The width of the Shivalik hills varies from 10 to 50 km (6.2 to 31.1 mi), their average elevation is 1,500 to 2,000 m (4,900 to 6,600 ft).
Other spelling variations used include Shivalik and Siwalik, originating from the Hindi and Nepali word shiwālik parvat. Other names include Churia hills, Chure hills, and Margalla hills.
The Sivalik hills are the southernmost and geologically youngest east-west mountain chain of the Himalayas. The Siwaliks have many sub-ranges. They extend west from Arunachal Pradesh through Bhutan to West Bengal, and further westward through Nepal and Uttarakhand, continuing into Himachal Pradesh and Kashmir. The hills are cut through at wide intervals by numerous large rivers flowing south from the Himalayas. Smaller rivers without sources in the high Himalayas are more likely to detour around sub-ranges. Southern slopes have networks of small rills and channels, giving rise to ephemeral streams during the monsoon and into the post-monsoon season until groundwater supplies are depleted.
The Sivalik hills are chiefly composed of sandstone and conglomerate rock formations, which are the solidified detritus of the great range in their rear, but often poorly consolidated. The remnant magnetization of siltstones and sandstones suggests a depositional age of 16-5.2 million years with Karnali River exposing the oldest part of the Siwalik Group in Nepal.
They are bounded on the south by a fault system called the Main Frontal Thrust, with steeper slopes on that side. Below this, the coarse alluvial Bhabhar zone makes the transition to the nearly level plains. Rainfall, especially during the summer monsoon, percolates into the bhabar, then is forced to the surface by finer alluvial layers below it in a zone of springs and marshes along the northern edge of the Terai or plains. This wet zone was heavily malarial infested before DDT was used to suppress mosquitoes. It was left forested by official decree by Nepal's Rana rulers as a defensive perimeter called Char Kose Jhadi (four kos forest, one kos equalling about three km or two miles). Upslope, the permeable geology together with temperatures routinely exceeding 40° Celsius throughout April and May only supports a low, sparse, drought-tolerant scrub forest.
The Siwalik Hills are also among the richest fossil sites for large animals anywhere in Asia. The Hills had revealed that all kinds of animals lived there. They were early ancestors to the sloth bear, Sivatherium, an ancient giraffe, Colossochelys atlas, a giant tortoise amongst other creatures.
Low population densities in the Siwalik and along the steep southern slopes of the Mahabharat Range, plus virulent malaria in the damp forests on their fringes create a cultural, linguistic and political buffer zone between dense populations in the plains to the south and the "hills" beyond the Mahabharat escarpment, isolating the two populations from each other and enabling different evolutionary paths with respect to language, race and culture.
People of the Lepcha tribe inhabit the Sikkim and Darjeeling areas.