India - Structure and Physiography

The present relief and geomorphological configuration of India are the outcome of complex endogenetic and exogenetic forces operating over millions of years. Geologists and geomorphologists classify India into three distinct macro-geological units based on structural history, rock composition, and tectonic deformation:

The Northern Mountains form an unbroken, convex-southward arc extending over 2,400 km from Nanga Parbat (8,126 m) in Jammu & Kashmir in the west to Namcha Barwa (7,782 m) in Tibet/Arunachal Pradesh in the east. Its width varies from 400 km in Kashmir to 150 km in Arunachal Pradesh.

Beyond the Dihang gorge in Arunachal Pradesh, the Himalayas take a sharp southward hairpin bend (syntaxial bend) and form the Purvanchal Hills along the Myanmar border: Patkai Bum, Naga Hills (Saramati Peak, 3,826 m), Manipur Hills, and Mizo (Lushai) Hills (Blue Mountain, 2,157 m).

The Great Plains of India span over 3,200 km from the Indus delta to the mouth of the Ganga-Brahmaputra, with an average width of 150 to 300 km, covering an area of approximately 7.8 lakh sq km. Formed by thousands of years of deposition by the Indus, Ganga, Brahmaputra, and their tributaries, it represents one of the largest and most fertile continuous alluvial plains on Earth.

• 1. The Bhabar (ভাবর):
  A narrow belt of 8 to 10 km width running parallel to the Shiwalik foothills. Rivers descending from the steep slopes deposit coarse unassorted pebbles, gravels, and boulders. Due to extremely high porosity, all small streams disappear underground, creating dry surface riverbeds in the dry season.
• 2. The Tarai (তরাই):
  A 10 to 20 km wide zone lying immediately south of the Bhabar. Here, the underground streams of Bhabar re-emerge at the surface, turning the terrain into an ill-drained, marshy, waterlogged, and densely forested tract. Highly prone to malaria, now largely cleared for sugar cane, rice, and wheat cultivation in Punjab, Haryana, and Uttarakhand.
• 3. The Bhangar (ভাঙ্গর):
  The older alluvium of the plains lying above the reach of regular annual river floods, forming mature river terrace benches. It contains calcareous concentrations and clayey nodules known locally as Kankar (কাঁকর).
• 4. The Khadar (খাদার):
  The newer, younger alluvium deposited annually on low-lying active floodplains by recurrent monsoon floods. Highly porous, clayey-loamy, remarkably fertile, and replenished without requiring chemical fertilizers; known locally as Bet in Punjab.

1. Punjab-Haryana Plain: Drained by the five tributaries of the Indus (Sutlej, Beas, Ravi, Chenab, Jhelum), dominated by flat interfluvial tracts known as Doabs (Bari, Rechna, Chaj, Sindh Sagar Doabs).
2. Ganga Plain: Largest section (~3.75 lakh sq km) spanning UP, Bihar, and West Bengal; subdivided into Upper Ganga Plain, Middle Ganga Plain, and Lower Ganga Plain (including the mangrove delta of the Sundarbans).
3. Brahmaputra Plain (Assam Valley): Aggradational low-gradient valley between the Himalayas and Meghalaya plateau; marked by braided channels and Majuli (the world's largest river island).

Rising to an average elevation of 600 to 900 meters, the Peninsular Plateau is a massive triangular tableland bounded by the Aravalli Range in the northwest, the Rajmahal Hills in the northeast, the Western Ghats (Sahyadris) in the west, the Eastern Ghats in the east, and Kanyakumari in the south.

• The Central Highlands:
  • Malwa Plateau: Gently undulating basaltic plateau dissected by the Chambal, Kali Sindh, and Parbati rivers.
  • Bundelkhand and Baghelkhand: Ancient granitic and gneissic uplands characterized by deep ravines.
  • Chota Nagpur Plateau: Centered in Jharkhand, western West Bengal, and northern Odisha; the mineral heartland of India (coal, iron ore, mica); drained by the Damodar River in a structural rift valley; highest point is Parasnath Hill (1,365 m).
• The Deccan Plateau:
  • Deccan Traps (Maharashtra Plateau): Formed by fissure volcanic lava sheets over 2,000 meters thick; characterized by step-like terrace topography ('trap' means stairs in Swedish) and fertile black regur soil.
  • Karnataka (Mysore) Plateau: Divided into two distinct physiographic zones: Malnad (hilly, forested, dissected Western Ghats flank) and Maidan (rolling, open, gently undulating eastern plains); contains the Baba Budan iron ore hills (Mullayanagiri, 1,930 m).
  • Telangana Plateau: Semi-arid plateau underlain by Archaean crystalline rocks.

Flanking the Peninsular plateau on either side are two narrow to broad strips of coastal lowlands:

• Western Coastal Plain (Submerged Coastline):
  Narrow (10 to 50 km wide) strip wedged between the Western Ghats and Arabian Sea. Subdivided into:
  • Kachchh & Kathiawar Coast: Salt flats of the Rann and volcanic Gir hills.
  • Konkan Coast: Maharashtra and Goa coastline; highly indented, rocky cliffs, and natural harbors.
  • Kannada (Canara) Coast: Karnataka coastline.
  • Malabar Coast: Kerala coastline; famous for parallel sandspits enclosing calm brackish lagoons and backwaters known as Kayals (e.g., Vembanad Lake, the longest lake in India, and Ashtamudi Lake).
• Eastern Coastal Plain (Emergent Coastline):
  Much broader (80 to 150 km wide) strip lying between the Eastern Ghats and Bay of Bengal. Characterized by extensive alluvial deltas built by the Mahanadi, Godavari, Krishna, and Kaveri rivers. Subdivided into the Northern Circars (Odisha & Andhra Pradesh) and the Coromandel Coast (Tamil Nadu). Contains major coastal lagoons: Chilika Lake (Odisha - largest brackish lagoon in India) and Pulicat Lake.

Lying northwest of the ancient Aravalli Range, the Thar Desert covers western Rajasthan and parts of Gujarat:

• Arid Landforms: Shifting crescent-shaped sand dunes (Barchans), longitudinal dunes (Seif), and bare rocky pediments.
• Internal Drainage: Rivers are seasonal and ephemeral, vanishing into sand (e.g., the Luni River). Contains saline playa lakes known as Dhands (e.g., Sambhar Lake, Didwana).

West Bengal exhibits remarkable physiographic variation within its 88,752 sq km expanse, categorized into three distinct physiographic provinces:

Occupying the northernmost districts of Darjeeling and Kalimpong, this region forms part of the Eastern Himalayas. Deeply dissected by the Teesta River gorge, it is subdivided into:

• West of the Teesta: Contains the prominent Singalila Range along the Nepal border and the Darjeeling Range:
  • Sandakphu (3,636 m): The highest mountain peak in the state of West Bengal, offering panoramic views of Mount Everest and Kanchenjunga.
  • Phalut (3,600 m): Tri-junction peak of Nepal, Sikkim, and West Bengal.
  • Sabargram (3,543 m) and Tonglu (3,036 m).
  • Tiger Hill (2,573 m) and Ghum (2,258 m), the highest railway station in India.
• East of the Teesta: Comprises the Kalimpong hills (Durpin Dara, Deolo Peak - 1,704 m) and the sub-Himalayan Buxa-Jainti ranges along the Bhutan frontier.

Forms the eastern undulating fringe of the Chota Nagpur Plateau, covering the entire district of Purulia and western sectors of Bankura, Paschim Medinipur, Jhargram, and Birbhum:

• Erosional Topography: Ancient Archaean granitic and gneissic rocks worn down by prolonged subaerial erosion into a rolling peneplain.
• Monadnocks & Residual Hills:
  • Ayodhya Hills (Purulia): Highest summit is Gorgaburu (677 m), the highest elevation in the plateau fringe of West Bengal.
  • Panchet Hill (Purulia) and Baghhmundi Hills.
  • Biharinath (452 m) and Susunia (442 m) in Bankura district.
  • Mama Bhagne Pahar in Birbhum district.

Covering over 80% of West Bengal, the plains are divided into five distinct sub-units:

1. Terai and Dooars Plain: Sub-Himalayan piedmont gravelly alluvium in Jalpaiguri, Alipurduar, and northern Cooch Behar; drained by the Torsa, Jaldhaka, and Teesta.
2. North Bengal Plains: Older alluvium tracts known as Barind (বরেন্দ্রভূমি) in Malda and Dakshin Dinajpur, and fertile low-lying Tal and Diara lands.
3. Rarh Plain (রাঢ় অঞ্চল): Gently undulating shelf of lateritic red soil lying between the western plateau and the Bhagirathi river (Murshidabad, Birbhum, Bardhaman, Bankura).
4. Coastal Sand Dune Plain: Narrow marine beach and sand dune tract in Purba Medinipur (Digha, Contai).
5. Deltaic Plain: Split into Moribund Delta (Nadia/Murshidabad), Mature Delta, and Active Delta (the Sundarbans, featuring tidal mudflats, estuaries, and dense halophytic mangrove forests).