The Sivalik Hills, also known as Churia Hills, are a mountain range of the outer Himalayas that stretches from the Indus River about 2,400 km (1,500 mi) eastwards close to the Brahmaputra River, through parts of both Nepal and India. It is 10–50 km (6.2–31.1 mi) wide with an average altitude of 1,500–2,000 m (4,900–6,600 ft). Between the Teesta and Raidāk Rivers in Assam is a gap of about 90 km (56 mi). In some Sanskrit texts, the region is called Manak Parbat. Sivalik literally means 'tresses of Shiva’.
Geologically, the Sivalik Hills belong to the Tertiary deposits of the outer Himalayas. They are chiefly composed of sandstone and conglomerate rock formations, which are the solidified detritus of the Himalayas to their north; they are poorly consolidated. The remnant magnetization of siltstones and sandstones indicates that they were deposited 16–5.2 million years ago. In Nepal, the Karnali River exposes the oldest part of the Shivalik Hills.
They are the southernmost and geologically youngest east-west mountain chain of the Himalayas. They have many subranges and extend west from Arunachal Pradesh through Bhutan to West Bengal, and further westward through Nepal (here known as Churia Hills) 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.
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 Bhabar 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.
North of the Sivalik Hills, the 1,500– to 3,000-meter Lesser Himalayas, also known as the Mahabharat Range, rise steeply along fault lines. In many places, the two ranges are adjacent, but in other places, structural valleys 10–20 km wide separate them.
The Sivalik 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; and Megalochelys atlas, a giant tortoise named the Sivaliks giant tortoise; amongst other creatures.
The remains of the Lower Paleolithic (around 500,000 to 125,000 BP) Soanian culture have been found in the Sivalik region. Contemporary to the Acheulean, the Soanian culture is named after the Soan Valley in the Shivalik Hills of Pakistan. The bearers of this culture were Homo erectus.
Low population densities in the Sivalik Hills and along the steep southern slopes of the Lower Himalayan 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.
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