The state is located in the extreme Northeastern corner of the country. It is situated between latitude 26° 30′ N and 29° 30 ‘ N and longitude 91° 30′ E and 97° 30’ E. It shares its border with the neighboring countries of Bhutan in the West, China (Tibet) in the North and Northeast, Myanmar in the East and Southeast and the Indian states of Assam and Nagaland in the South. Arunachal Pradesh is generally a hilly region.
The elevation of the hills ranges from 60 meters to over 7300 meters (GORICHEN peak in West Kameng). The territory falls in the outer Himalayas and Patkoi Ranges. It is endowed with wide topographical variations, vegetations and wild life along the greater part of the length of the territory the characteristics Siwalik type formation of Himalayan Mountains is native.
The definition of Siwalik ranges loses its typical character at the point of the valley head and is replaced by series of low hills with easier slopes. These hills gradually merge into the Patkoi hill ranges which separate India & Burma. The general tendency of the hills is found sloping towards the plains of Assam. These hill ridges of Arunachal Pradesh are situated in a very haphazard manner.
As soon as one ridge ends, the other starts either in opposite direction or parallel. At these intervals the wide and narrow valleys come into existence. Because of these hill ridges and the valleys, the surface of Arunachal Pradesh is found variegated almost everywhere which also results into numerous geographical isolation of places caused by various rivers and streams traversing the region and depositing the flowing detritus enroute in valley and again at the foothills
Topography
Topography wise Arunachal Pradesh has been divided into 4 distinct physiographical divisions:
- The Himalayan Ranges
- The Mishmi Hills
- The Naga-Patkoi Ranges and
- The Brahmaputra Plains
Arunachal Pradesh is the eastern stretch of the Himalayas and all the 3 sections of the Himalayas are represented here . Every division has a different geology and tectonic history
From the Brahmaputra Plains at an average elevation of 100m, the Himalayas attain an elevation of 7089m. The Himalayas can be further sub-divided into physio-tectonic Zones:
(i) The sub-Himalayan zone rising abruptly from the Brahmaputra Plains along a tectonic plane-the Foot Hill Fault. It is about 10 -20 km wide but narrows down to 1 -2 km in the Dibang valley.
(ii) The lesser or Lower Himalayas: Elevation 2500-4000 m and 80-90 km wide. Take a syntaxial bend towards SE to abut against Mishmi Hills. Its southern limit is defined by the Main Boundary Fault/Thrust.
(iii) The Greater or Higher Himalayas. A zone of very high relief with heights greater than 6000m having precipitous slopes and deep gorges. This zone is generally devoid of vegetation. Its southern limits are defined by the Main Central Thrust.
(iv) Trans-Himalayan Tibetan or the Thethyan zone: It is the northernmost zone which is about 30-40 km wide and of low relief (Elevation 3000 – 6000m). All important Passes are located in this zone .
The Eastern Himalayas that cover 18,518 sq. km area of East and West Siang districts is almost wholly a rugged mountainous terrain extending over the central region of Arunachal Pradesh. The region has been described as ‘an intricate labyrinth of precipitous, rocky and high hills and mountains, with beautiful green valley’s drained by innumerable rivulets and mighty rivers cascading down from upper elevations. The tangle of hills and mountains throwing a series of spurs and towering to majestic heights of eternal snow is the most imposing topographic feature of Siang.
