<P> 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 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 . </P> <P> They are the southernmost and geologically youngest east - west mountain chain of the Himalayas . They have many sub-ranges and 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 . </P> <P> 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 . </P> <P> North of the Sivalik Hills the 1,500--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 . </P>

The name given to the area at the foothills of the shiwalik range