Physiography of India

India's land looks like a patchwork, but geographers divide it into six physiographic divisions: the Northern Mountains (Himalaya), the Northern Plains, the Peninsular Plateau, the Indian (Thar) Desert, the Coastal Plains, and the Islands. A very useful thought to keep in mind: the Himalaya is the youngest and the Peninsular Plateau the oldest part of India. The plateau is a stable piece of the ancient Gondwana landmass; the Himalaya rose only when the Indian plate pushed into Asia and the sea between them, the Tethys, was squeezed up into mountains.

The Himalaya is a great arc about 2,400 km long. Its width is about 400 km in Kashmir and about 150 km in Arunachal Pradesh. It stretches from the Indus gorge (near Nanga Parbat) in the west to the Brahmaputra gorge (near Namcha Barwa) in the east. From north to south it is built of several parallel ranges, each younger and lower than the one behind it: the Trans-Himalaya, the Greater Himalaya (Himadri), the Lesser Himalaya (Himachal) and the Outer Himalaya (Shiwaliks). In the east, the hill ranges that swing south along the Myanmar border are called the Purvanchal.

Geographers also divide the Himalaya from west to east into regions named after the rivers that bound them. This is the Burrard's regional division: the Punjab Himalaya (between the Indus and the Sutlej), the Kumaon Himalaya (Sutlej to Kali), the Nepal Himalaya (Kali to Teesta) and the Assam Himalaya (Teesta to Dihang). East of the Dihang the Himalaya bends south into the Purvanchal hills: Patkai, Naga, Manipur and Mizo hills.

The Duns (also called Dwars in the east) are flat, wide valleys between the Lesser Himalaya and the Shiwaliks, filled with gravel and river deposits. The best-known are Dehradun (the largest), Kotli Dun and Patli Dun. The Himalayan valleys between the ranges, such as the Kashmir Valley (between the Pir Panjal and the Zaskar/Great Himalaya, drained by the Jhelum), the Kangra and Kullu valleys in Himachal Pradesh, are fertile and densely populated.

Peaks, passes and glaciers are classic fact-lists, so we will learn them as tables with a reason for each. The Himalaya holds nine of the world's ten highest peaks (the other, K2, is in the Karakoram, which is also part of the great Himalayan-Karakoram system). Most of the giants lie in Nepal and Tibet, so only a few of them are in India. The question is therefore always 'highest in the world', 'highest in India' or 'highest in a state', and each has a different answer.

Now the passes. A pass is a natural gap in a mountain range where roads and trade routes cross. The Himalayan passes matter because they connect India with Tibet and Central Asia, and because they close in winter with snow. Learn each one with its state and the places it links.

The Himalaya also stores fresh water as ice. The Siachen Glacier in the Karakoram (about 76 km long) is the longest glacier in the Karakoram and one of the longest outside the polar regions. The Gangotri Glacier in Uttarakhand gives birth to the Bhagirathi, the source stream of the Ganga. The Zemu Glacier (Kangchenjunga, Sikkim) is the largest glacier in the eastern Himalaya. Glaciers feed the great rivers, and when they shrink, they add to floods now and shortages later. Most Himalayan lakes (such as Tso Moriri, Pangong and Gurudongmar) are glacial or tectonic in origin.

Between the Himalaya in the north and the Peninsular Plateau in the south lies a vast, flat belt: the Northern Plains, also called the Indo-Gangetic Plain. It was once a deep trough in front of the rising Himalaya. For millions of years the Indus, Ganga and Brahmaputra and their tributaries filled that trough with alluvium (silt, sand and clay) brought down from the mountains, which is why the plain is flat, fertile and deep. It is about 2,400 km long, 240 to 320 km wide, and roughly 7 lakh sq km in area.

Think of the plain as a conveyor belt of soil. Rivers carry silt from the Himalaya and drop it when they reach the flat land and slow down. Near the mountains the heavy gravel falls first (Bhabar), then the water seeps out as marshy springs (Terai), then come the old plains (Bhangar) and finally the flood plains that get new silt every year (Khadar). So the plain is arranged in strips running parallel to the Himalaya: coarse to fine, old to new, as you go away from the mountains.

By region, the plain is divided into three parts. The Punjab Plains in the west (Punjab, Haryana and the adjoining areas) are made by the Indus and its five tributaries; the land between two rivers is a doab (do = two, ab = water). The Ganga Plains extend from the Ghaggar river to the Teesta and cover Uttar Pradesh, Bihar, Jharkhand's north and West Bengal. The Brahmaputra Plains (Assam Valley) lie in the east. Within the Ganga plains, local landforms include bhur (sand ridges built by wind in the western Ganga-Yamuna doab), reh or kallar (salt crust in dry areas), and the ravines (badlands) of the Chambal.

The Peninsular Plateau is the oldest and most stable part of India, a great triangular tableland of hard crystalline rocks (granite, gneiss, schist) that once belonged to the southern supercontinent Gondwana. Its sides are marked by hills: the Aravalli and Vindhya in the north-west and north, the Western Ghats in the west and the Eastern Ghats in the east. Because the whole block tilts gently towards the east, most big peninsular rivers (Mahanadi, Godavari, Krishna, Kaveri) flow east into the Bay of Bengal. The two exceptions, the Narmada and Tapi, flow west because they run in long rift valleys (down-faulted troughs).

The plateau has two broad parts, separated by the Narmada. North of it is the Central Highlands (the Malwa Plateau between the Aravalli and the Vindhya, the Bundelkhand and Baghelkhand uplands, and in the east the Chotanagpur Plateau). South of the Narmada, the Deccan Plateau is a triangle of land between the Satpura in the north, the Western Ghats in the west and the Eastern Ghats in the east. In the north-east, the Meghalaya (Shillong) Plateau with the Garo, Khasi and Jaintia hills is a detached piece of the peninsula, cut off from the Chotanagpur by the Garo-Rajmahal gap, which was filled by Ganga-Brahmaputra deposits.

The Deccan Traps (the word 'trap' comes from a Swedish word for stairs, because the lava flows form steps) are one of the largest volcanic provinces in the world. They were formed by fissure eruptions of basaltic lava about 66 million years ago, near the end of the Cretaceous period. They cover much of Maharashtra and parts of Gujarat, Madhya Pradesh, Karnataka and Telangana. The weathering of this basalt gives the black cotton soil (regur), ideal for cotton.

Why do the Western Ghats get so much rain? The Southwest Monsoon winds blow from the Arabian Sea and meet the high wall of the ghats. The moist air is forced to rise, cools, and drops heavy rain (2,000 to 3,000 mm and more) on the windward western slopes. The air that crosses over is dry, so the land east of the ghats (the interior Deccan) gets little rain and is called the rain-shadow region. This is why the west coast has dense evergreen forests and the Deccan interior is drier. The Western Ghats have three major gaps: the Thal Ghat (Mumbai-Nashik), the Bhor Ghat (Mumbai-Pune) and the Pal Ghat (Palakkad Gap, linking Kerala and Tamil Nadu), through which roads and railways cross.

West of the Aravalli lies the Indian (Thar) Desert. It covers most of western Rajasthan and extends into parts of Gujarat, Haryana and Punjab, continuing into Pakistan. The Aravalli range lies parallel to the Southwest Monsoon winds and does not block them, but its low height lets the winds pass without giving rain. So the Thar gets less than about 150 mm of rain a year. Its landscape is rolling sand dunes (including crescent-shaped barchans), rocky outcrops, and dry lake beds called playas. The only river of any size is the Luni, which rises in the Aravalli, flows south-west and disappears in the marshes of the Rann of Kutch; the Ghaggar is a seasonal stream that dries up here.

The Coastal Plains run along both sides of the peninsula. Both are the narrow strips between the Ghats and the sea, but they are very different. The western coastal plain is narrow (about 10 to 25 km in most places), because the Western Ghats come close to the sea. It is a coast of submergence: the sea has drowned the river mouths, creating estuaries, lagoons and backwaters. The eastern coastal plain is wider (about 100 km or more) and is a coast of emergence, with big deltas built by the Mahanadi, Godavari, Krishna and Kaveri.

One more feature belongs here. The Rann of Kutch in Gujarat is a vast seasonal salt marsh, flooded in the monsoon and a white salt crust in the dry season. It is divided into the Great Rann and Little Rann; the Little Rann is the home of the Indian wild ass (Khur). Although it lies in the desert belt, geographically it is a former sea inlet that silted up. The Sundarbans at the mouth of the Ganga-Brahmaputra is a mangrove delta shared by India and Bangladesh and is a UNESCO World Heritage Site.

India's islands fall into two very different families. The Andaman and Nicobar Islands in the Bay of Bengal are the tops of a submerged mountain chain, an extension of the Arakan Yoma range of Myanmar, so they are hilly and forested. Lakshadweep in the Arabian Sea is made of coral; its low islands are atolls built on the underwater Laccadive-Chagos ridge. One family is mountain-born, the other coral-born, and that single idea answers most questions about their landscapes.

The islands are also home to some of India's most isolated peoples. In the Andamans live the Great Andamanese, Onge, Jarawa and Sentinelese; in the Nicobars live the Nicobarese and Shompen. The Sentinelese of North Sentinel Island and the Shompen of Great Nicobar live largely cut off from the outside world. These groups are protected as Particularly Vulnerable Tribal Groups, and entry into their reserves is restricted by law.