Siwalik ranges are the southern-most hills of the Himalayas, foothills of the mighty Himalayas. They mostly maintain a regular course from the River Yamuna to River Ravi on the south of the Western Himalaya.
The ancients called them Upgiri. It is 600 meters to 1500 meters high with a width ranging from 15 km to 50 km.
On the north, the Siwalik descends gently to flat-floored structural valley called dun. Dun is, in fact, longitudinal depression filled up by recent gravels derived from the Antagiri. It is the Lake (locally called Tal) Region of the Himalayas. The examples are Nainital Lake and Bhimtal Lake. However, on the southern slopes the region has steep scrap.