The Purvanchal Hills, or the Eastern Hills, constitute a major physiographic sub-division of India running along the country’s north-eastern border. Structurally, they represent the sharp southward syntaxial bend (orogenic bending) of the Himalayan mountain system beyond the Dihang gorge in Arunachal Pradesh. Geologically, these hills are composed of strong Tertiary sandstones, quartzites, shales, and silts rather than the older crystalline basement rocks of the Peninsular Shield. They form a series of parallel ridges and valleys characterized by anticlinal ridges and synclinal valleys, covered with dense tropical evergreen forests.
| Parameter | Purvanchal / Eastern Hills | Main Himalayan System |
| Orogenic Alignment | North-South longitudinal alignment. | East-West latitudinal alignment. |
| Average Elevation | Relatively low relief, ranging between 500 meters and 3,000 meters. | High relief, ranging between 3,000 meters and over 8,000 meters. |
| Geological Composition | Predominantly Tertiary sedimentary formations (Barail, Surma, Tipam series). | Complex metamorphic, granitic, and sedimentary sequences. |
| Tectonic Activity | Strike-slip faulting and reverse faulting under Indo-Myanmar plate convergence. | Heavy thrust tectonics (MCT, MBT, MFT) under Indo-Eurasian collision. |
Major Mountain Ranges and Orographic Subdivisions
The Purvanchal system is segmented into distinct regional ranges across the northeastern states, each possessing specific geomorphic identities.
Patkai Bum Range
- Geographic Extension: Forms the international boundary between Arunachal Pradesh (India) and Sagaing Region (Myanmar).
- Key Features: It is the northernmost range of the Purvanchal system. The terrain is characterized by conical peaks, steep slopes, and deep incised valleys. It acts as an orographic barrier to the South-West Monsoon, receiving intense precipitation.
Naga Hills
- Geographic Extension: Situated south of Patkai Bum, running through the state of Nagaland and extending into Myanmar.
- Highest Peak: Mount Saramati (3,826 m), which remains snow-capped during winter and serves as a natural border marker between India and Myanmar.
- Drainage Partition: Functions as a water divide between the Brahmaputra river basin (via Doyang and Dhansiri rivers) and the Chindwin/Irrawaddy river basin.
Manipur Hills
- Geographic Extension: Occupies the central part of the Purvanchal system within the state of Manipur.
- Topographic Anomaly: The highest elevations reach up to 2,500 meters. The central portion features a unique intermontane structural basin containing Lokkat Lake. It is bordered by the Chin Hills to the south.
Mizo (Lushai) Hills
- Geographic Extension: Situated in the southernmost part of the Purvanchal system within Mizoram and parts of Tripura.
- Highest Peak: Phawngpui, also known as the Blue Mountain (2,157 m), which is a highly biodiverse national park area.
- Geomorphic Texture: Characterized by a “Cuesta” topography with steep western slopes and gentle eastern dip-slopes.
Barail Range
- Geographic Extension: Connects the Purvanchal system with the Meghalaya (Shillong) Plateau, running through Assam, Nagaland, and Manipur.
- Structural Role: It forms a continuous mountain wall separating the Brahmaputra Valley from the Barak Valley.
Tripura Hills
- Geographic Extension: A series of five low-lying parallel north-south tracking ridges located in Tripura.
- Key Ridges: Comprises the Jampui, Sakhan, Langtarai, Atharamura, and Baramura ridges, decreasing in height from east to west toward the Bangladesh plains.
High-Altitude Passes and Strategic Communication Corridors
The rugged terrain along the Indo-Myanmar border is traversed by historical and strategic passes that facilitate regional connectivity.
Pangsau Pass
Located at an altitude of 1,136 m in the Patkai Bum range, it connects Ledo in Assam with Myitkyina in Myanmar. It is historically famous as the route of the Stillwell Road (Ledo Road) built during World War II.
Diphu Pass
Situated near the tri-junction of India (Arunachal Pradesh), China, and Myanmar, providing a strategic approach route over the eastern Himalayas.
Kumjawng Pass
An important border pass in the Patkai hills facilitating traditional cross-border movement between eastern Arunachal Pradesh and Myanmar.
Tuipang Corridor
A crucial geographical transit point in southern Mizoram that anchors multi-modal connectivity projects directed toward Southeast Asia.
Drainage Networks, Lacustrine Systems, and Hydrography
The hydrology of Purvanchal is split between the Brahmaputra, Meghna, and Chindwin-Irrawaddy river basins, exhibiting antecedent and dendritic drainage patterns.
The Barak River System
Originates in the Manipur Hills, flows through Mizoram and Assam, and enters Bangladesh as the Surma and Kushiyara rivers to join the Meghna. It drains the southern slopes of the Barail Range and forms the economic lifeline of southern Assam.
Chindwin Tributaries
The eastern slopes of the Naga and Manipur hills are drained by rivers like the Tizu and Manipur River, which flow eastward into Myanmar to join the Chindwin River, a major tributary of the Irrawaddy.
Loktak Lake and Phumdis
Located in the Manipur intermontane basin, it is the largest freshwater lake in Northeast India and a designated Ramsar site. It is globally unique for its Phumdis—heterogeneous masses of vegetation, soil, and organic matter in various stages of decomposition that float on the water surface.
Keibul Lamjao National Park
The only floating national park in the world, located in the southeastern zone of Loktak Lake. It forms the last natural habitat of the critically endangered Sangai (brow-antlered deer), also known as the dancing deer of Manipur.
Geological Exclusions: The Meghalaya Plateau
A critical distinction for civil services aspirants is that the Garo, Khasi, and Jaintia hills of Meghalaya are not part of the Purvanchal or Himalayan system. Geologically, the Meghalaya Plateau (Shillong Plateau) is an detached northern outlier of the ancient Peninsular Shield of India. It was separated from the Chotanagpur Plateau by the tectonic subsidence of the Malda Gap (Garo-Rajmahal Gap) during the Tertiary period, which was subsequently filled by alluvial deposits from the Ganga and Brahmaputra rivers.
Soil Profiles, Ecological Biodiversity, and Anthropological Matrix
The unique geographical alignment and bioclimatic conditions of the Eastern Hills support distinct ecological and cultural landscapes.
Soil Typology
The region is dominated by Red Loamy soils and Laterite soils. Due to high rainfall and steep slopes, these soils are heavily leached, acidic in nature, deficient in phosphate and lime, but rich in nitrogen and organic humus content.
Jhum Cultivation (Slash-and-Burn)
The traditional agricultural practice of shifting cultivation, locally known as Jhuming, is widespread among the indigenous tribes. It has led to extensive soil erosion, loss of forest canopy, and fragmentation of critical wildlife corridors.
Faunal Endemism
Purvanchal is a major component of the Indo-Burma Biodiversity Hotspot. It hosts rare mammalian species such as the Hoolock Gibbon (India’s only ape), Clouded Leopard, Marbled Cat, Capped Langur, Malayan Sun Bear, and the Red Goral.
Tribal Demographics and Seventh Schedule Exemptions
The region is home to distinct ethnic communities belonging to the Mongoloid race. Tribes such as the Mishmi, Patkai Nagas, Konyaks, Kukis, Mizos, and Chins populate these ranges. Autonomous District Councils (ADCs) constituted under the Sixth Schedule of the Constitution govern parts of Assam, Mizoram, and Tripura to protect tribal customary laws and land rights.
Last Modified: June 3, 2026