The Hills and Mountain Ranges of Northeast India have two different geological and physiographical units. In the northeastern part of the region hills and ranges is part of the Himalayas Mountain. On the other hand, the southwestern part of the region's hills and ranges is the production of the Peninsular Plateau.
Northeast India is a place of heavy rainfall, high relative humidity, and high-temperature cause rapid weathering of rocks. As the weathering of surface rocks of the hills and mountains proceeds, the sheet flood caused by heavy rainfall removes the weathered materials and carries them down to the headward steams of the rivers. The rapidly flowing rivers then carry the sediments, big and small, down to the foothills and plains.
Northeast India located in the extreme northeast corner of the Indian subcontinent with diverse physiography and relief features. In general, the region may be divided into three physiographical regions that are Plateau region, Plain regions and Mountain region. Because of its diversity of physiography the region endowed with rich and varied biophysical environments bounded by hills and mountains on its three sides except a narrow gap in the west.
The Purvanchal Range or Eastern Mountain is a sub-mountain range of Himalaya. “ Purvanchal ” as the name itself suggests, it is the eastern frontier region of India is surrounded by Bangladesh in the south-west, Myanmar in the south-east and china in the north-east. Its north-western limit, for a major part, follows the boundaries of Meghalaya-Mikir region, Assam valley and Eastern Himalaya and the plain areas of Tripura and Cachar are the continuations of Surma valley.