2.6 What is Physiography?

Physiography^? of an area is the outcome of three things working together: structure (the rocks and their arrangement), process (the forces acting on those rocks), and the stage of development (how far the landscape has progressed in its evolution). India's land is one of the most diverse on the planet — a rugged northern wall of mountain ranges with high peaks, deep gorges and beautiful valleys; a vast plain in the middle; and a stable southern table-land of dissected plateaus, denuded rocks and series of scarps.

2.7 The North & North-Eastern Mountains

This division includes the Himalayas and the North-eastern hills. The Himalayas are not a single ridge but a series of parallel ranges. Among the most important is the Greater Himalayan range — which itself includes the Great Himalayas and the Shiwalik foothills.

2.7.1 Orientation: A Range That Bends

The Himalayas change direction as one travels along their length:

The Great Himalayan range — also called the central axial range — is approximately 2,500 km long from east to west, with a width that varies between 160 and 400 km from north to south. On a map of Asia, the chain stands almost like a strong, long wall between the Indian subcontinent and the countries of Central and East Asia.

2.8 The Northern Plain

South of the mountains stretches the Northern Plain, formed by alluvial deposits of the Indus, Ganga and Brahmaputra river systems. The plain runs approximately 3,200 km from east to west, with an average width of 150–300 km. The maximum depth of alluvium ranges between 1,000 and 2,000 m.

2.8.1 Three Belts from North to South

From the Shiwalik foothills southwards, the plain divides into three major zones, with the alluvial belt itself further split into Bhangar and Khadar:

The plain shows the characteristic features of a mature stage of fluvial landform development — sand bars, meanders, oxbow lakes and braided channels. The Brahmaputra plains in particular are known for their riverine islands and sand bars, and most of the area suffers periodic floods and shifting river courses that produce braided streams.

The plain is also where two great river systems part ways. The states of Haryana and Delhi form a low water-divide between the Indus and the Ganga systems. The Brahmaputra, meanwhile, flows from the north-east to the south-west until it takes an almost 90° southward turn at Dhubri before entering Bangladesh. The Plain's deep alluvial cover supports wheat, rice, sugarcane and jute — and feeds a substantial share of India's population.

2.9 The Peninsular Plateau

South of the plain rises India's largest physiographic unit — the Peninsular Plateau, an irregular triangle that climbs from about 150 m above the river plains to elevations of 600–900 m. Its outer corners are marked by the Delhi ridge in the north-west (an extension of the Aravalis), the Rajmahal hills in the east, the Gir range in the west and the Cardamom hills in the south. The Shillong and Karbi Anglong plateaus form a detached north-eastern extension.

The Peninsula is a mosaic of patland plateaus — Hazaribagh, Palamu, Ranchi, Malwa, Coimbatore, Karnataka and others. This is one of the oldest and most stable landmasses in India. The general slope is from west to east, which is why so many of its great rivers flow eastward to the Bay of Bengal.

Recurrent phases of upliftment and submergence, with crustal faulting and fractures, have given the plateau a varied relief. The Bhima fault deserves a special mention because of its recurring seismic activity. The north-western plateau has a notably complex relief of ravines and gorges — those of Chambal, Bhind and Morena are well-known examples.

2.9.1 Three Sub-divisions of the Peninsular Plateau

(i) The Deccan Plateau

The Deccan is bordered by the Western Ghats in the west, the Eastern Ghats in the east, and the Satpura, Maikal range and Mahadeo hills in the north. The Western Ghats are higher and far more continuous than the Eastern Ghats and go by different local names along their length:

The average elevation of the Western Ghats is about 1,500 m, increasing from north to south. Two notable peaks dominate the southern stretch:

Most Peninsular rivers originate in the Western Ghats. The Eastern Ghats, by contrast, are discontinuous and low; they have been eroded heavily by the Mahanadi, Godavari, Krishna and Kaveri. Important Eastern Ghat ranges include the Javadi hills, Palconda range, Nallamala hills and Mahendragiri hills. The two ghats finally meet at the Nilgiri hills.

(ii) The Central Highlands

The Central Highlands are bounded to the west by the Aravali range. The Satpura range forms a series of scarped plateaus to the south, generally at elevations of 600–900 m, marking the northernmost boundary of the Deccan plateau. The Satpura is a classic example of relict mountains — highly denuded, discontinuous remnants of much older ranges.

The plateau extends as far west as Jaisalmer, where it has been buried by longitudinal sand ridges and crescent-shaped sand dunes called barchans^?. The whole region has undergone heavy metamorphic processing — its rocks include marble, slate and gneiss. Average elevation of the Central Highlands is 700–1,000 m above mean sea level, and the region slopes towards the north and north-east. Most tributaries of the Yamuna originate in the Vindhyan and Kaimur ranges; the Banas is the only significant tributary of the Chambal that rises from the Aravalli on the west. To the east, the Central Highlands extend into the Rajmahal hills, south of which lies the mineral-rich Chotanagpur plateau.

(iii) The Northeastern Plateau

This is, in fact, an extension of the main Peninsular plateau. As the Indian plate pushed northward during the Himalayan orogeny, a huge fault opened between the Rajmahal hills and the Meghalaya plateau. Sediment later filled that depression, leaving the Meghalaya and Karbi Anglong plateaus standing detached from the main Peninsular Block.

Like the Chotanagpur plateau, the Meghalaya plateau is rich in mineral resources — coal, iron ore, sillimanite, limestone and uranium. It receives the maximum rainfall from the south-west monsoon, which is why Cherrapunji shows a bare rocky surface stripped of any permanent vegetation cover.

2.10 The Indian Desert

To the north-west of the Aravali hills lies the Great Indian Desert — a land of undulating topography dotted with longitudinal dunes and barchans. Annual rainfall here falls below 150 mm; the climate is arid and vegetation cover is thin. Because of these features, the desert is also called Marusthali^?.

Although the underlying rock structure is an extension of the Peninsular plateau, extreme arid conditions have given the desert its own distinctive surface — sculpted by physical weathering and wind action. Pronounced features include mushroom rocks, shifting dunes and oases (mostly in the southern part).

On the basis of orientation, the desert can be divided into two parts: the northern part slopes towards Sindh, and the southern part slopes towards the Rann of Kachchh. Most rivers here are ephemeral. The Luni, in the southern desert, is the only river of any significance. With low precipitation and very high evaporation, Marusthali is a water-deficit region. Some streams disappear into the sand after running for a short distance, joining a lake or playa — a textbook example of inland drainage. The brackish water of these lakes and playas is the main source of salt for the surrounding region.

2.11 The Coastal Plains

India's long coastline is divided on the basis of location and active geomorphological processes into the Western and Eastern coastal plains.

2.11.1 The Western Coastal Plains — Submerged

The western coast is a textbook example of a submerged coastal plain. Tradition has it that the city of Dwaraka, once part of the Indian mainland, is now submerged under the sea — the kind of evidence used to argue that this coast was drowned. Submergence has made it narrow, but also gives it deep water close to shore — natural conditions for ports and harbours. Kandla, Mazagaon, JLN port Navha Sheva, Marmagao, Mangalore and Cochin are leading natural ports on the west coast.

From north to south, the western coast can be divided into:

The plain is narrow in the middle and broader at both ends. Rivers crossing the western coastal plain do not form deltas — slope and submergence both work against delta-building. The Malabar coast has its own distinguishing feature: kayals^? (backwaters), used for fishing, inland navigation and tourism. The famous Nehru Trophy Vallamkali (boat race) is held every year in Punnamada Kayal, Kerala.

2.11.2 The Eastern Coastal Plains — Emergent

By contrast, the eastern coast is broader and is an example of an emergent coast. Well-developed deltas of the Mahanadi, Godavari, Krishna and Kaveri punctuate it. Because of its emergent nature it has fewer natural ports and harbours; the continental shelf extends up to 500 km into the sea, which is too gentle a slope for deep-water harbours. Notable east-coast ports include Visakhapatnam, Paradip, Chennai, Tuticorin and Ennore.

Figure 2.4: Comparing the West and East coastal plains — submerged vs emergent character produces sharply different port geographies and delta patterns.

2.12 The Islands

India has two major island groups — one in the Bay of Bengal and one in the Arabian Sea.

2.12.1 The Bay of Bengal Islands — Andaman & Nicobar

The Bay of Bengal group consists of about 572 islands and islets, situated roughly between 6°N–14°N and 92°E–94°E. The two principal sub-groups of islets are Ritchie's archipelago and the Labrynth island. The whole chain is divided into the Andaman in the north and the Nicobar in the south, separated by the Ten Degree Channel.

Most of these islands are believed to be elevated portions of submarine mountains. Some smaller islands are volcanic in origin — Barren Island, India's only active volcano, sits in this group. Important peaks include:

The coastlines of these islands hold coral deposits and beautiful beaches. The islands receive convectional rainfall and support an equatorial type of vegetation.

2.12.2 The Arabian Sea Islands — Lakshadweep & Minicoy

The Arabian Sea islands include Lakshadweep and Minicoy, scattered between 8°N–12°N and 71°E–74°E, at distances of 220 km–440 km off the Kerala coast. The entire group is built of coral deposits. There are about 36 islands, of which 11 are inhabited. Minicoy is the largest, with an area of 453 sq. km. The whole archipelago is split by the Nine Degree Channel — Amini Island lies north of it, the Canannore Island lies south. The islands typically have storm beaches consisting of unconsolidated pebbles, shingles, cobbles and boulders along their eastern seaboard.