Some 200 million years ago, Pangea, the super continent began to break into various fragments of which Indian subcontinent was one. This fragment moved northwards and collided with northern fragment, also known as Laurasia. As a consequence of this massive collision, the Himalayan mountain range were created. For the last 40 million years or so, the Indian subcontinent has been pushing itself against the Tibetan plateau thereby causing the Himalayas to grow further up (Molnar, 1986).

(Snow capped Himalayan peaks as seen from Nameri)

According to the physiographic characteristic and stage of evolution, the Himalayan mountain ranges are divided into five parallel zones viz., I) Trans-Himalaya, II) Great Himalaya, III) Lesser Himalaya, IV) Siwaliks and v) Bhabar-Tarai tract (Tiwari & Joshi, 1997).

Considering the location of NTR along the foothills of Arunachal Pradesh, it may be inferred that NTR lies at the junction between Siwaliks of Arunachal Pradesh and the Bhabar- Tarai tract. The Siwalik formation of Arunachal Pradesh is similar to the ones found in the Darjeeling-Kalimpong, Sikkim and Bhutan. These formations contain gravels made up of gneiss, quartzites and sandstones. Within the reserve, Siwalik formations are noticeable mainly in the northern parts of the reserve. Towards southern side of NTR as we move from the north to the south, at lower elevations the Bhabar-Tarai tract becomes the prominent formation.