Class 11 Geography Chapter 2 Structure and Physiography Notes in English

1Geological Structure of India

The earth is about 4,600 million years old. In all that time, two kinds of forces have kept reshaping its surface: forces acting from inside the earth (endogenic forces) and forces acting from outside, on the surface (exogenic forces). You already know that the Indian Plate was once south of the equator, broke away from the Australian Plate, and has been drifting north ever since. It is this long push north, combined with endogenic and exogenic forces working together, that has given India its present geological structure.

Based on differences in geological structure and rock groups, India is divided into three geological divisions:

1.1 The Peninsular Block

• Its northern boundary is an irregular line running from Kachchh, along the western side of the Aravali Range near Delhi, then roughly parallel to the Yamuna and the Ganga up to the Rajmahal Hills and the Ganga delta.
• The Karbi Anglong and Meghalaya Plateau (northeast) and Rajasthan (west) are also extensions of this block. In West Bengal, the Malda fault separates the northeastern part from the Chotanagpur plateau.
• In Rajasthan, the desert and desert-like features simply overlie this old block.
• It is made mainly of very ancient gneisses and granites, and has stood as a rigid block since the Cambrian period, with only its western coast submerged and a few parts changed by tectonic activity.
• As part of the Indo-Australian Plate, it has still seen vertical movements and block faulting. The rift valleys of the Narmada, the Tapi and the Mahanadi, and the Satpura block mountains, are examples of this.
• It mainly carries relict and residual mountains (worn down over a very long time): the Aravali hills, the Nallamala hills, the Javadi hills, the Veliconda hills, the Palkonda range and the Mahendragiri hills.
• Its river valleys are shallow with low gradients. Most east-flowing rivers form deltas before entering the Bay of Bengal, the Mahanadi, the Krishna, the Kaveri and the Godavari being the well-known examples.

1.2 The Himalayas and Other Peninsular Mountains

• Unlike the rigid Peninsular Block, these mountains are young, weak and flexible in structure.
• They are still being shaped by endogenic and exogenic forces, which keep producing new faults, folds and thrust planes.
• They are tectonic in origin and are cut through by fast-flowing rivers that are still in their youthful stage, so the landforms here, gorges, V-shaped valleys, rapids and waterfalls, are all signs of a young, actively-eroding landscape.

1.3 Indo-Ganga-Brahmaputra Plain

• This third division is the plain formed by the Indus, the Ganga and the Brahmaputra rivers.
• It began as a geosynclinal depression that reached its largest size during the third phase of Himalayan mountain building, about 64 million years ago.
• Since then it has been steadily filled up by sediment brought down by both the Himalayan and the Peninsular rivers.
• The average depth of alluvial deposits here is 1,000-2,000 m.

2Physiography and the Six Divisions

India’s land shows huge variety. The north has a wide belt of rugged land with mountain ranges, sharp peaks, beautiful valleys and deep gorges. The south is a stable tableland with highly dissected plateaus, worn-down rocks and rows of scarps. In between the two lies the vast Northern Plain.

Based on these broad differences, India is divided into six physiographic divisions:

3The Northern and North-Eastern Mountains

This division has two parts: the Himalayas, and the Northeastern hills. The Himalayas are not a single ridge, they are a series of parallel ranges.

Himalayan range	Also called	Notes
Greater Himalaya	Himadri, the central axial range	The highest, continuous range; about 2,500 km long (east-west), 160-400 km wide (north-south)
Trans-Himalaya	Karakoram, Zaskar, Ladakh ranges lie in this belt	North of the Greater Himalaya, in the rain shadow, holds India’s cold desert
Middle Himalaya	Himachal	Lower, more broken ranges between the Greater Himalaya and the Shiwalik
Shiwalik	Outer Himalaya	The youngest, southernmost and lowest range, facing the plains

• Direction changes along the length of the Himalayas: northwest to southeast in the northwestern part of India, east-west in the Darjiling and Sikkim region, southwest to northwest in Arunachal Pradesh, and north-south in Nagaland, Manipur and Mizoram.
• Standing almost like a long, strong wall, the Himalayas separate the Indian subcontinent from Central and East Asia. They are not just a physical barrier, they are also a climatic, drainage and cultural divide.

4The Northern Plains

• Formed by alluvium (silt) brought down by the Indus, the Ganga and the Brahmaputra.
• Stretches about 3,200 km from east to west, with a width of 150-300 km. Alluvium depth reaches 1,000-2,000 m at its deepest.
• Haryana and Delhi form the water divide between the Indus and the Ganga river systems.
• The Brahmaputra flows northeast to southwest before turning almost 90° south at Dhubri, just before entering Bangladesh.
• The soil is fertile alluvium, so this belt grows wheat, rice, sugarcane and jute, and supports a very large population.

Zone (north to south)	Width	What it is like
Bhabar	8-10 km	Narrow belt right at the Shiwalik foothills; streams drop their pebbles and boulders here and often sink underground
Tarai	10-20 km	South of the Bhabar; the streams re-emerge here with no fixed channel, making it marshy and swampy, with thick natural vegetation and varied wildlife
Bhangar	–	Old alluvium south of the Tarai; the higher, older part of the alluvial plain
Khadar	–	New alluvium, laid down closer to present-day rivers; renewed almost every year by floods

• The mature stage of the rivers here has created landforms like sand bars, meanders, ox-bow lakes and braided channels. The Brahmaputra plains are known for their river islands and sand bars, and are subject to periodic floods.
• These mighty rivers also build some of the world’s largest deltas at their mouths, the Sunderbans delta being the best-known example. Away from the delta, this is otherwise a nearly featureless plain, at a general elevation of 50-150 m above mean sea level.

5The Peninsular Plateau

• Rises from about 150 m (near the river plains) up to 600-900 m, in an irregular triangle.
• Its outer boundary runs from the Delhi ridge (northwest, an extension of the Aravalis) to the Rajmahal hills (east), the Gir range (west) and the Cardamom hills (south), with a further extension northeast into the Shillong and Karbi-Anglong plateau.
• It is made of a series of flat-topped (“patland”) plateaus: Hazaribagh, Palamu, Ranchi, Malwa, Coimbatore and Karnataka plateaus among them.
• It is one of the oldest and most stable landmasses of India. Elevation generally falls from west to east, which also matches the direction most of its rivers flow.
• Its landforms include tors, block mountains, rift valleys, spurs, bare rocky surfaces, hummocky hills and wall-like quartzite dykes that act as natural sites for storing water. Black soil dominates its western and northwestern part.
• It has gone through repeated phases of upliftment and submergence with crustal faulting, the Bhima fault being worth remembering for its repeated seismic activity. The northwestern part has a complex relief of ravines and gorges, the Chambal, Bhind and Morena ravines being the well-known examples.

Based on relief, the Peninsular Plateau is grouped into three parts:

5.1 The Deccan Plateau

• Bounded by the Western Ghats (west), the Eastern Ghats (east) and the Satpura, Maikal range and Mahadeo hills (north).
• The Western Ghats have different local names: Sahyadri in Maharashtra, Nilgiri hills in Karnataka and Tamil Nadu, and Anaimalai and Cardamom hills in Kerala.
• Western Ghats are higher and more continuous than the Eastern Ghats, at an average elevation of about 1,500 m, rising from north to south. Most Peninsular rivers rise here.
• Eastern Ghats are discontinuous and low, and have been heavily eroded by rivers like the Mahanadi, the Godavari, the Krishna and the Kaveri. Their ranges include the Javadi hills, the Palconda range, the Nallamala hills and the Mahendragiri hills.
• The Eastern and Western Ghats meet at the Nilgiri hills.

Anaimudi, on the Anaimalai hills, is the highest peak of the entire Peninsular Plateau. Dodabetta, on the Nilgiri hills, is the second highest.

5.2 The Central Highlands

• Bounded on the west by the Aravali range. The Satpura range forms a series of scarped plateaus, 600-900 m high, and marks the Deccan Plateau’s northern boundary.
• A classic example of relict mountains: highly denuded and broken into discontinuous ranges. The extension reaches Jaisalmer in the west, covered there by longitudinal sand ridges and crescent-shaped dunes (barchans). Metamorphic rocks like marble, slate and gneiss confirm its long geological history.
• General elevation is 700-1,000 m, sloping towards the north and northeast. Most tributaries of the Yamuna rise in the Vindhyan and Kaimur ranges; the Banas, rising in the Aravalli, is the only significant tributary of the Chambal from the west.
• Its eastern extension is the Rajmahal hills, south of which lies the mineral-rich Chotanagpur plateau.

5.3 The Northeastern Plateau

• An extension of the main Peninsular plateau. A huge fault is believed to have formed between the Rajmahal hills and the Meghalaya plateau when the Indian Plate moved northeast during the Himalayan mountain-building, and the resulting depression was later filled by rivers.
• Today the Meghalaya and Karbi Anglong plateau stand detached from the main Peninsular Block. Meghalaya plateau is further divided into the Garo, Khasi and Jaintia Hills, named after the tribal groups who live there. There is a further extension into the Karbi Anglong hills of Assam.
• Like Chotanagpur, this plateau is rich in coal, iron ore, sillimanite, limestone and uranium. It receives the maximum rainfall from the southwest monsoon, which has left it highly eroded, Cherrapunji shows a bare rocky surface with almost no permanent vegetation.

6The Indian Desert, Coastal Plains and Islands

6.1 The Indian Desert

• Lies northwest of the Aravali hills, also called Marusthali.
• Undulating land dotted with longitudinal dunes and barchans, and gets less than 150 mm of rainfall a year, giving it an arid climate and thin vegetation.
• During the Mesozoic era this region was under the sea, evidence for this comes from the wood-fossil park at Aakal (fossils about 180 million years old) and marine deposits near Brahmsar, close to Jaisalmer.
• Although the rock underneath is an extension of the Peninsular Plateau, extreme aridity has carved its own surface features here: mushroom rocks, shifting dunes and oases (mostly in the south).
• By slope, it is split into a northern part sloping towards Sindh and a southern part sloping towards the Rann of Kachchh.
• Most rivers here are ephemeral (short-lived, seasonal); the Luni river in the south is the significant exception. Low rainfall and high evaporation make it a water-deficit region. Some streams simply disappear into a lake or a playa (an example of inland drainage), and these playa lakes hold brackish water that is used to make salt.

6.2 The Coastal Plains

India’s long coastline is split into two, on the basis of location and how active the coastal processes are.

6.3 The Islands

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

	Bay of Bengal (Andaman & Nicobar)	Arabian Sea (Lakshadweep & Minicoy)
Number	About 572 islands and islets	About 36 islands, 11 of them inhabited
Location	6°N-14°N, 92°E-94°E	8°N-12°N, 71°E-74°E; 220-440 km off the Kerala coast
Origin	Believed to be elevated tops of submarine mountains; a few smaller ones are volcanic	Entirely built of coral deposits
Internal split	Andaman (north) and Nicobar (south), separated by the Ten Degree Channel	Amini Island (north) and Cannanore Island (south), separated by the Nine Degree Channel
Special feature	Barren Island: India’s only active volcano	Minicoy: the largest island, 453 sq km
Climate	Convectional rainfall, equatorial vegetation, coral deposits and beaches	Storm beaches of pebbles, shingles, cobbles and boulders on the eastern side

• Andaman and Nicobar’s two main sub-groups are the Ritchie’s archipelago and the Labrynth island.
• Important peaks: Saddle Peak (North Andaman, 738 m), Mount Diavolo (Middle Andaman, 515 m), Mount Koyob (South Andaman, 460 m) and Mount Thuiller (Great Nicobar, 642 m).