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The Siwalik Hills (also spelled Shiwalik, Shivalik, or Sivalik) are the southernmost and geologically youngest foothills running parallel to the main Himalayas. The Siwalik is a relatively low-altitude mountain range cresting at 900 to 1,200 meters. They extend 1,600 km from the Teesta River in Sikkim, westward through Nepal and Uttarakhand, continuing into Jammu and Kashmir. The Mohan Pass is the principal pass accessing the Siwalik Hills from Saharanpur in Uttar Pradesh to Dehra and the hill station of Mussoorie in Uttarakhand. Eastward they are cut through at 50-80 km. intervals by rivers flowing south from the Himalaya and higher foothills.

The Siwalik Hills are chiefly composed of sandstone and conglomerate formations which are the solidified detritus of the great range in their rear, but often poorly consolidated. They are bounded on the south by a fault system called the Main Frontal Thrust, with steeper slopes on that side. Sivapithecus (a kind of ape, formerly known as Ramapithecus) is among many fossil finds in the Siwalik region.

Immediately south of these low mountains is a permeable alluvial zone called "bhahar" where rains (mostly during the summer monsoon) percolate downslope to a zone of marshes and springs. Although any lowland area south of the Middle Hills is now called "Terai", the word apparently originated as a Persian loanword meant "damp" or "marshland" applied only to this wet zone. It was heavily malarial before DDT was used to suppress mosquitos and was left forested by official decree in Nepal to magnify this effect as a defensive perimeter.

North of the Siwalik belt lies a higher range of foothills, the 1,500-3,000 meter Mahabharat Lekh(Range) also known as the Lesser Himalaya or the Middle Himalaya. In some places the two ranges are adjacent, in others valleys 10-20 km wide separate them. These valleys are called Duns or Doons in India (for example the Doon Valley) which includes Dehradun, as also Patli Dun and Kothri Dun, both in Corbett National Park in Uttarakhand, and also Pinjore Dun in Himachal Pradesh. In Nepal, they are called "Inner Terai" and these include Chitwan, Dang, Deukhuri and Surkhet.

The permeable sediments and poorly-developed soils of the Siwalik hills do not retain water between storms and are unsuited to agriculture. They are lightly populated by a few tribal groups, especially the Van Gujjars, or Gujjars, that follow a quasi-pastoral livestock-dependent lifestlyle and are responsible for heavy deforestation and denudation in many parts, given that the size of their herds has dramatically outgrown the ecosystems' capacity to sustain them. 76.80.26.121 22:43, 8 April 2007 (UTC)

• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.

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Geography of South Asia
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