Physical geography of India (relief, drainage)
India's physiographic divisions and drainage systems: the Himalayas, Northern Plains, Peninsular Plateau, coasts and islands, plus Himalayan vs Peninsular rivers.
The Six Macro-Divisions
Indian physiography is conventionally divided into six macro-units, a classification embedded in NCERT (Class XI, India: Physical Environment) and reproduced in every standard text: the Northern and North-eastern Mountains, the Northern Plains, the Peninsular Plateau, the Indian Desert, the Coastal Plains, and the Islands. These divisions reflect distinct geological histories. The Peninsular block is a fragment of ancient Gondwanaland, composed of Archaean gneisses and granites over 2,500 million years old and therefore among Earth's most stable landmasses (a shield). The Himalayas, by contrast, are young, fold mountains raised by the collision of the Indian and Eurasian plates that began roughly 50 million years ago after the Tethys Sea closed.
The Himalayas
The Himalayas run about 2,400 km from the Indus gorge in the west to the Brahmaputra gorge in the east, in three longitudinal ranges. The Greater Himalayas (Himadri) carry the loftiest peaks—Kanchenjunga (8,586 m, India's highest), Nanda Devi (7,816 m)—and the perennial snowline. The Lesser Himalayas (Himachal) hold the famed hill stations and valleys: Kashmir, Kangra, Kullu; the Pir Panjal is its longest range. The Outer Himalayas (Shiwaliks) are the youngest, built of unconsolidated sediments, enclosing longitudinal valleys called Duns (Dehra Dun, Kotli Dun). Regionally the system is split by rivers into the Punjab, Kumaon, Nepal and Assam Himalayas. The Purvanchal—Patkai Bum, Naga, Manipur and Mizo hills—forms the eastern extension curving southward.
The Northern Plains and the Peninsula
The Northern Plains were formed by the alluvial deposition of the Indus, Ganga and Brahmaputra, infilling a foredeep south of the rising Himalayas. They are subdivided into Bhabar (porous gravel belt at the foothills where streams disappear), Tarai (re-emergent marshy zone), Bhangar (older alluvium with calcareous kankar nodules) and Khadar (newer, fertile flood-plain alluvium). The Peninsular Plateau comprises the Central Highlands and the Deccan Plateau, bounded by the Aravalis (NW), Vindhya–Satpura ranges, and the Eastern and Western Ghats. The Western Ghats (Sahyadris) are continuous, rising 900–1,600 m, with Anaimudi (2,695 m) in the Anaimalai Hills being the highest peak of Peninsular India; the Eastern Ghats are discontinuous and dissected by rivers. The Deccan Trap of the northwest is basaltic, formed by Cretaceous–Eocene volcanic eruptions (~65 million years ago), weathering into black regur soil. The Indian Desert (Thar) lies west of the Aravalis, while the coastal plains and the Andaman–Nicobar (volcanic/tectonic) and Lakshadweep (coral) islands complete the framework.
