Complex sentence: Terai, extended from east to west, is made up of alluvial soil, to the west of Koshi River, in between Mahabharat and Churia, there elongates a valley called Inner Terai.
Split into simpler sentences: Terai, extended from east to west, is made up of alluvial soil. To the west of Koshi River, in between Mahabharat and Churia, there elongates a valley called Inner Terai.