225. Physiographic Division

Physiographic division begins with the relation between structure, process and stage of landform development. India is not a single relief surface: the Northern and North-eastern Mountains, Northern Plain, Peninsular Plateau, Indian Desert, Coastal Plains and Islands are six macro units that carry different rock ages, slopes, drainage patterns and resource bases. The Himalaya is young, high and tectonically active; the Peninsular block is older, harder and more stable; the Indo-Ganga-Brahmaputra plain is a deep alluvial trough filled by river sediments. The exact phrase Greater-Lesser-Shiwalik Himalayan divisions belongs to the mountain unit, while Peninsular Plateau: Central Highlands and Deccan Plateau belongs to the shield unit. Rajasthan shows why this frame matters locally: the Aravalli old fold mountain system is part of the old shield margin, and the Thar Desert of western Rajasthan overlays the same broad block west of the Aravalli. In one state, rocky uplands, sandy plains, inland drainage and plateau margins appear together, so a national map becomes easier to read through Rajasthan's Aravalli-Thar contrast. The same macro frame also keeps causes and consequences together. Mountain relief controls snow-fed and rain-fed drainage; plains store new and old alluvium; plateau surfaces expose minerals and resistant rocks; deserts show wind work, saline basins and water deficit; coasts show marine submergence or emergence; islands preserve submarine, volcanic and coral histories. Rajasthan's position is unusually instructive because the state sits where the old shield, an eroded fold range and a hot desert meet. A single west-east cross-section across Rajasthan moves from dunes and playa-like salt depressions through the Aravalli divide toward more river-linked plains. That cross-section makes the national division more than a memorised label set. It also shows why relief, drainage and soil must be read together whenever a map places mountains, plains and deserts side by side.