Chapter 2: Geological Evolution and Structure
High in the Nepal Himalayas, the Kali Gandaki river has cut a gorge so deep that it runs between two peaks that both rise above eight thousand metres. Along its stony bed, pilgrims and villagers gather smooth black stones. Crack one open and a perfect spiral lies coiled inside the rock. These are shaligrams — the fossilised shells of ammonites, sea animals that vanished from the Earth around the time the last dinosaurs did.
There is no ocean within a thousand kilometres of this gorge. Yet the rock in a trekker’s hand, lying thousands of metres above the sea, was once the soft mud of an ocean floor. That ocean had a name: the Tethys. For an immense stretch of geological time it lay across the belt where the Himalayas now stand, quietly collecting sediment washed off the lands on either side.
Then that sea floor was squeezed, folded and heaved upward into the highest mountains on the planet. The fossils rode up with it.
A rock is a record. This one is saying, more plainly than any textbook could, that a mountain range can begin its life as a sea — and that the ground beneath our feet has a history millions of years long. India’s other great divisions, the worn southern plateau and the flat plains between, keep records of their own, written in far older and far younger stone.
Diagram
Fig. 2.0-1 Caption: The floor of a sea, folded and lifted, becomes a mountain, and the sea creatures ride up with it. Brief: A three-stage schematic cross-section read left to right, not a map, no political boundaries. Stage 1 — a shallow sea labelled ‘Tethys Sea’ with horizontal layers of sediment (small downward arrows showing sediment settling) on its floor and one small coiled shell labelled ‘ammonite’ resting in the mud, layers labelled ‘marine sediments’. Stage 2 — the same layers squeezed by two large horizontal arrows converging from left and right, buckling the beds into folds. Stage 3 — the folded beds raised high above a new sea-level line with one upward arrow marked ‘continued uplift’, and the ammonite now sitting near a peak. Emphasise that the fossil-bearing bed is the same one throughout.
2.1 The Scale of Geological Time
Every rock carries a date, even if it wears no label. To read the Earth’s past, geologists first needed a shared calendar — a way of naming the vast stretches of time in which rocks formed. This section sets out that calendar in both its worldwide and its Indian forms, so the rest of the chapter has a framework on which to hang events.
2.1.1 The Language of Geological Time
Geology is the science of the Earth — the materials it is made of, its structure, and the long history recorded in its rocks. Within it, historical geology studies how that structure developed through time and space: the order in which events happened, and where.
Geological time is far too long to count in ordinary years, so it is divided into named blocks. An era is one of the largest primary divisions of geological time. A period is a shorter division within an era. Both measure time, not rock.
The rock itself is named separately. A group is the whole body of rock formed during an era; a system is the body of rock formed during a period. So an era is a slice of time, and its group is the pile of stone laid down while that slice lasted.
2.1.2 The Standard Geological Eras
Geologists worldwide use four standard eras, from oldest to youngest: the Pre-Cambrian, the Palaeozoic, the Mesozoic and the Cenozoic.
The Pre-Cambrian is by far the longest. It runs from the Earth’s origin, about 4,600 million years ago — written ‘mya’ from here on — to roughly 540 mya. To feel that scale, remember that a single million years is longer than the whole of recorded human history. The Palaeozoic then runs from about 540 to 252 mya, the Mesozoic from about 252 to 66 mya, and the Cenozoic from about 66 mya to today. These boundary ages are approximate, and are revised as dating methods improve.
The names carry meaning. The Pre-Cambrian takes its name from Cambria, the Latin name for Wales in the United Kingdom, where rocks of this age were first described; several periods are likewise named after the places where their rocks were first studied. The other three eras are named for the kind of life their rocks contain: Palaeozoic means ‘ancient life’, Mesozoic ‘middle life’ and Cenozoic ‘recent life’.
That naming points to how the eras are told apart. A fossil is the preserved remains or trace of a once-living thing, sealed in rock. Pre-Cambrian rocks hold almost no fossils. Palaeozoic rocks carry the earliest abundant life; Mesozoic rocks the middle life — the age of dinosaurs and of the ammonites whose shells opened this chapter; and Cenozoic rocks the recent life, including mammals.
Notice how uneven these eras are. The Pre-Cambrian alone covers nearly nine-tenths of the Earth’s whole history, while the Cenozoic — the era we live in — is a thin sliver at the very end. The timeline below draws each era with equal spacing, which is convenient but flattens that imbalance and hides just how vast the early history really was.
2.1.3 The Indian Geological Eras
Indian geologists use a second set of names, fitted to India’s own rock record, and place them in the same order as the standard eras. The four Indian eras, oldest to youngest, are the Archaean, the Purana, the Dravidian and the Aryan.
The Archaean covers the early Pre-Cambrian — the first-formed rocks of the Earth, mostly gneisses and granites, carrying no fossils. The Purana covers the late Pre-Cambrian. Together, the Archaean and Purana correspond to the standard Pre-Cambrian.
The Dravidian corresponds broadly to the Palaeozoic, roughly 540 to 252 mya. The Aryan runs from the close of the Palaeozoic, through the Mesozoic and Cenozoic, to the present day. In short: Archaean and Purana ≈ Pre-Cambrian, Dravidian ≈ Palaeozoic, and Aryan ≈ Mesozoic plus Cenozoic.
The Geological Survey of India (GSI), the body that surveys and maps the country’s rocks, uses this four-fold order — Archaean, Purana, Dravidian, Aryan — to arrange Indian formations in time. Exactly which rock systems fill each of these eras is the subject of the last section of this chapter.
2.2 The Geological Evolution of India’s Three Divisions
India is not one landmass but three, welded together and formed at very different times. In the south lies the ancient Peninsular Plateau; in the north, the young Himalayas; and between them, the flat Great Plain. Their combined story is the geological evolution of the subcontinent, and it runs from some of the oldest rocks on Earth to sediment being laid down today.
2.2.1 The Peninsular Plateau
The southern part of India is an old tableland, the Peninsular Plateau, surrounded by sea on three sides. Geologists hold it to be the oldest landform of the subcontinent — a piece of the Earth’s ancient crust that forms part of the Indian Plate.
During the Pre-Cambrian, sediment gathered in a large depression, and a block of crystalline rock rose out of it and never sank beneath the sea again. Since then this tableland has behaved as a rigid, inflexible block. It is often compared to a horst — an uplifted block of crust bounded by faults along its sides.
Diagram
Fig. 2.2-1 Caption: The peninsula rises and sinks as one rigid block, like a horst, but does not fold. Brief: A simple block cross-section. Draw a raised rectangular block of ancient crystalline rock, bounded on left and right by two faults (marked with small arrows showing vertical movement). Add upward arrows on the block to show it was uplifted as a whole; label ‘ancient crystalline basement’, ‘fault’, and ‘horst (uplifted block)’. Emphasise that the internal layers stay flat and unfolded — the block moves up or down, it does not crumple.
Because it is so rigid, the plateau received later sideways thrusts and orogenic (mountain-building) forces without its original basement being folded. As part of the larger Indo-Australian plate, it has been raised, lowered and cracked by block faulting, but never crumpled into folds.
It therefore carries mostly residual mountains — the worn-down stumps of far older ranges — the best example being the Aravalis, which run north-eastward across Rajasthan and its neighbours. Its river valleys are shallow, with gentle gradients, because the land is old and long worn down.
2.2.2 The Himalayas
The Himalayas are far younger, and are called Young Fold Mountains because they were raised by rock being folded into ranges. Through much of the Mesozoic, the whole Himalayan belt lay beneath a sea.
That sea was the Tethys, occupying a geosyncline — a long, slowly sinking trough in the crust that gathers great thicknesses of sediment. To its north lay the landmass of Angaraland, part of the northern continents; to its south lay Gondwanaland, the southern supercontinent that then included peninsular India. Rivers off both lands poured sediment into the Tethys for a very long time.
The uplift is best explained by the Theory of Plate Tectonics, which has replaced an older ‘geosynclinal’ theory of mountain-building. In it, the Earth’s rigid outer shell is broken into large plates that slowly move; where two plates collide, stresses build in the rock and produce folding, faulting and igneous activity.
Peninsular India, riding the Indian plate, broke away from Gondwanaland and drifted steadily north. It collided with the Eurasian plate roughly 50 million years ago. As it drove on, the Indian plate was forced partly beneath Eurasia; the Tethys closed; and its thick pile of sediment was squeezed, folded and thrust upward into the Himalayas. The marine fossils on the opening page rode up with that sediment.
Flowchart
Fig. 2.2-2 Caption: A sea becomes a mountain range: the plate-tectonic story of the Himalayas. Nodes: Gondwanaland breaks apart → the Indian plate splits off and drifts north → the Tethys Sea lies in its path, its floor piled with sediment from Angaraland and Gondwanaland → the Indian plate collides with the Eurasian plate (about 50 mya) → the Indian plate is pushed partly beneath Eurasia and the Tethys closes → the trapped sediments are squeezed, folded and faulted upward → the Himalayas rise → the plate keeps moving north, so uplift continues today
The cross-section makes the mechanism plain: one plate driving beneath another, and the sediment between them buckling upward.
Diagram
Fig. 2.2-3 Caption: The Indian plate drives beneath Eurasia, and the Tethys sediments buckle up into the Himalayas. Brief: A cross-section looking along the mountain belt. On the right, the Indian plate moves left (large horizontal arrow marked ‘northward’) and bends down beneath the Eurasian plate on the left. Between them, show the thick Tethys sediment layers being squeezed and folded upward into peaks, with an upward arrow marked ‘uplift’. Label ‘Indian plate’, ‘Eurasian plate’, ‘folded Tethys sediments = Himalayas’. Show the arrow directions clearly: Indian plate moving north and downward, sediments moving up.
The collision has not stopped. Satellite positioning (GPS) shows the Indian plate still creeping northward by a few centimetres a year, and the Himalayas rising by a few millimetres a year. The frequent, sometimes severe earthquakes of the belt are the surface sign of that continuing push.
2.2.3 The Great Plain
Between the rising Himalayas and the stable plateau lay a deep hollow. It formed as the Himalayas were heaved out of the Tethys and the northern edge of the peninsular block sagged downward under the strain.
Rivers descending from the young mountains in the north and from the old plateau in the south carried enormous loads of sediment into this hollow. Over time they filled it with alluvium — the fine sand, silt and clay that rivers deposit. The result is a vast, almost level plain of soft sediment, in places thousands of metres thick.
Diagram
Fig. 2.2-4 Caption: A north–south section across India: a deep alluvial trough sits between the young mountains and the old plateau. Brief: A north-to-south cross-section, not a map. On the left (north) draw the folded Himalayas; on the right (south) draw the flat-topped Peninsular Plateau; between them a broad trough filled deep with sediment, labelled ‘Great Plain — thick alluvium’. Add arrows from both the mountains and the plateau pointing inward and down into the trough, marked ‘rivers carrying sediment’. Label ‘Himalayas’, ‘Great Plain’, ‘Peninsular Plateau’.
The process has not ended. Where the great rivers meet the sea, they are still building new land. The steady seaward growth of the Sundarban delta, at the head of the Bay of Bengal, shows that plain-making is going on to this day.
So the Great Plain is not a finished landform but a work still in progress. The forces that built it over millions of years — rivers wearing down highlands and dropping the load on lowlands — are the forces at work at the delta’s edge today. That growth is not simple gain, however: the delta also sinks under its own weight, the sea is slowly rising against it, and dams and embankments upstream change how much sediment actually reaches the coast. Whether the delta gains or loses land depends on the balance between these.
2.3 The Peninsular Plateau and the Himalayas Contrasted
The two great uplands of India — the Peninsular Plateau, whose largest part is the Deccan, and the Himalayas — sit at opposite ends of the subcontinent’s history. Both are ‘high ground’, yet they could hardly be more different. Geographers set them side by side along three lines. Stratigraphy is the study of rock beds: their sequence, thickness and age. Geological structure is the study of folds and faults, and the igneous activity that goes with them. Physiography is the study of the surface landforms themselves — how mountains, plateaus and plains take their shape.
A Difference of Age and Rock
In stratigraphy, the contrast is one of age. The plateau is an old landmass of hard rock that has stood above the sea since Cambrian times; only its coastal margins were ever briefly drowned. The Himalayan belt, by contrast, lay under the Tethys for most of its history, right up to the moment it was folded into mountains. The plateau’s rocks are therefore ancient, and the Himalayas’ comparatively new.
The rock types follow from this. The plateau is built mainly of igneous and metamorphic rock — the hard, crystalline rock formed from cooled magma or reshaped by heat and pressure. The Himalayas are built largely of sedimentary rock — the hardened layers of sediment that once settled on the floor of the Tethys.
A Difference of Structure
In structure, the difference is between a block and a fold. The plateau has behaved as a rigid block: it has been lifted, dropped and cracked by faulting, like a horst, but its rock beds were never crumpled. The Himalayas are the opposite — young, weak and flexible, thrown into great folds and sliced by faults where two plates met. The Himalayas are fold mountains; the plateau is a faulted block.
A Difference of Shape and Rivers
In physiography, the two look and behave differently. Seen on a map, the peninsula is broadly triangular, while the Himalayan front curves in a long arc across the north. Their rivers differ as well. On the worn plateau, valleys are shallow and gradients gentle, so the rivers flow quietly. In the young, still-rising Himalayas, rivers run torrential and cut deep, carving gorges and showing every mark of youth. And where the plateau carries only residual mountains, the worn stumps of far older ranges, the Himalayas stand sharp, high and still growing.
Beneath every one of these contrasts lies a single cause: age and origin. The plateau is an ancient, rigid block of the Earth’s crust; the Himalayas are young folded mountains still being pushed upward. That one difference in history — an old stable block against a young rising fold — shows up in their rocks, their rivers and their very outlines.
2.4 The Rock Systems of India
Section 2.1 named India’s eras; this section fills them with rock. The Geological Survey of India recognises a series of named rock systems, each a body of rock formed at a particular time. Set in their correct order, these systems are the written record of everything the earlier sections described.
2.4.1 Geological Formations and Their Grouping
The Geological Survey of India has divided the country’s rocks into four major groups, arranged in chronological order: the Archaean, the Purana, the Dravidian and the Aryan — the same four eras met in Section 2.1. Within these groups, particular rock bodies are named as ‘systems’ after the places where they were first studied.
The two great regions of the country — the old Peninsula and the young Himalayas — did not share the same history, so they carry different formations. The table sets them side by side.
2.4.2 The Pre-Cambrian Systems
The oldest four systems all belong to the Pre-Cambrian — that is, to the Archaean and Purana eras. Their distribution across the Peninsula is shown on the map.
Map
Map 2.1 Caption: The major geological systems of India, from the oldest Archaean rocks to the most recent alluvium. Brief: A map showing the full extent of India as officially defined by the Government of India, including Jammu and Kashmir and Ladakh, with official boundaries; neighbouring countries and seas outlined for orientation only. Shade and key the major systems — Archaean and Dharwar, Cuddapah, Vindhyan, Gondwana, the Rajmahal and Deccan lava flows, and the younger alluvium and Himalayan belts. Include a clear legend distinguishing each. Label the Arabian Sea, Bay of Bengal and the Himalayan belt for orientation. Do not mark any area as disputed.
The Archaean system contains the first-formed rocks of the Earth. They are mostly gneisses and granites, carry no fossils, and are found across the peninsular states — Tamil Nadu, Andhra Pradesh, Telangana, Karnataka, Odisha, Madhya Pradesh, Chhattisgarh, Jharkhand and Rajasthan — as well as in parts of the Himalayas.
The Dharwar system holds the earliest sedimentary rocks, now so altered by heat and pressure that they survive in metamorphic form. They carry no fossils, but they are the most valuable rocks of all economically, being rich in metallic minerals such as gold, iron ore, manganese and copper. They are named after Dharwar in Karnataka, and also occur in Madhya Pradesh, Jharkhand, Meghalaya and Rajasthan.
The Cuddapah system, named after Cuddapah (Kadapa) in Andhra Pradesh, carries rocks rich in iron and manganese ore along with slate and marble. The Vindhyan system covers large areas of Madhya Pradesh, Chhattisgarh, Uttar Pradesh and Rajasthan, and generally lies over the Cuddapah rocks. Its limestones, sandstones, shales and slates are prized building stones — much of India’s fine red sandstone masonry comes from these beds.
2.4.3 From Gondwana to the Present
The younger systems belong to the Aryan era — the long stretch from the end of the Palaeozoic to the present.
The Gondwana system formed when, during the Upper Carboniferous, crustal movements sank basin-shaped depressions into the Peninsula. Countless land plants and animals were buried in them and, over time, turned to coal. These beds hold most of India’s coal and lie chiefly in the Damodar, Mahanadi and Godavari valleys. Their rocks carry the marks of great climatic swings, from icy cold to tropical and desert conditions.
Towards the end of the Mesozoic, around 66 million years ago, huge outpourings of lava flooded much of the Deccan and formed the Deccan Traps. Thin fossil-bearing sedimentary layers lie between the lava flows, which shows the flooding was not continuous but came in bursts. This was a time of great tectonic change: India had by then broken away from Gondwanaland and was drifting north, and would later collide with Eurasia to raise the Himalayas.
The Tertiary rocks, of the Cenozoic, are of comparatively recent origin and are found mostly in the Himalayas, where the folded Tethys sediments now stand; in the Peninsula they occur along the coasts of Gujarat, Kerala and Tamil Nadu, yielding lignite (brown coal), rock salt, gypsum and limestone. The Quaternary formations are the most recent of all: the Ice Age deposits of Kashmir, the alluvial plains of north India, the Thar desert of Rajasthan, the Rann of Kachchh, the laterite crusts of the Peninsula — a hard, red, iron-rich layer left by intense weathering in a hot, wet climate — and the black regur soils.
2.5 Summary
Geological time is divided into eras and periods. The rock formed during an era is called a group, and the rock formed during a period is called a system.
The four standard eras, oldest to youngest, are the Pre-Cambrian, the Palaeozoic, the Mesozoic and the Cenozoic. Fossils, growing from simple to complex life, help tell one era from another.
India’s own eras — Archaean, Purana, Dravidian and Aryan — run parallel to the standard ones. Archaean and Purana match the Pre-Cambrian, Dravidian matches the Palaeozoic, and Aryan matches the Mesozoic and Cenozoic together.
India is made of three geological divisions formed at very different times: the ancient Peninsular Plateau in the south, the young Himalayas in the north, and the Great Plain between them.
The Peninsular Plateau is the oldest landform — a rigid block of hard crystalline rock that behaves like a horst, rising and sinking without folding, and carrying only worn residual mountains.
The Himalayas are Young Fold Mountains raised from the floor of the Tethys Sea. As the Indian plate drove north and collided with Eurasia about 50 mya, the trapped sediments were folded and lifted, and the range is still rising.
The Great Plain formed as rivers from the mountains and the plateau filled a deep trough with alluvium. The process continues, as the seaward growth of the Sundarban delta shows.
The plateau and the Himalayas differ in stratigraphy (old hard rock against young soft rock), in structure (a faulted block against folded mountains) and in physiography (a triangular tableland with quiet rivers against an arc-shaped range with torrential ones) — all because one is an ancient stable block and the other a young rising fold.
The Geological Survey of India groups the country’s rocks in time order as Archaean, Purana, Dravidian and Aryan, and names individual rock bodies as systems.
The Pre-Cambrian systems include the Archaean (gneiss and granite), the mineral-rich Dharwar (gold, iron and manganese), the Cuddapah, and the Vindhyan (fine building stones).
The Aryan era holds the Gondwana coal beds, the Deccan Traps (whose weathering gives the black regur soil), the Tertiary rocks of the Himalayas and coasts, and the most recent Quaternary formations — alluvium, desert, laterite and black soil.
What This Chapter Cannot Tell You
The era boundary ages are approximate and shift as dating methods improve; the chapter fixes the order of events far more reliably than their exact dates. It shows which rocks lie where and in what sequence, but not how those ages are actually measured — that rests on methods taken up elsewhere. And it treats each division as a single whole; at the scale of one hill or quarry, the real rock story is far more tangled than these broad groupings suggest.
2.6 Key Terms
alluvium — the fine sand, silt and clay laid down by rivers, which builds up flat plains.
Angaraland — the ancient northern landmass that lay to the north of the Tethys Sea.
Archaean — India’s earliest geological era, matching the early Pre-Cambrian; its rocks are the oldest and carry no fossils.
Aryan — India’s most recent geological era, running from the end of the Palaeozoic through the Mesozoic and Cenozoic to today.
Cenozoic — the most recent standard era, from about 66 million years ago to the present; the era of ‘recent life’.
Cuddapah system — a Pre-Cambrian (Purana) rock system, rich in iron and manganese ore and some building stone.
Deccan Traps — the vast, step-like sheets of basalt formed by lava flows that flooded the Deccan near the end of the Mesozoic.
Dharwar system — a Pre-Cambrian rock system of altered sedimentary rocks, the most valuable economically for its metallic minerals.
Dravidian — India’s geological era matching the Palaeozoic, from about 540 to 252 million years ago.
era — one of the largest primary divisions of geological time.
fossil — the preserved remains or trace of a once-living plant or animal, sealed in rock.
geological structure — the study of folds and faults and the igneous activity linked to them.
Geological Survey of India (GSI) — the national body that surveys and maps the country’s rocks.
geology — the science of the Earth: the materials it is made of, its structure, and the history recorded in its rocks.
geosyncline — a long, slowly sinking trough in the crust that gathers great thicknesses of sediment.
Gondwana system — a rock system of the Aryan era that holds most of India’s coal, formed from plants buried in peninsular basins.
Gondwanaland — the ancient southern supercontinent that once included peninsular India, lying south of the Tethys Sea.
group — the whole body of rock formed during an era.
historical geology — the study of how the Earth’s crust developed through time and space.
horst — an uplifted block of the Earth’s crust bounded by faults along its sides.
laterite — a hard, red, iron-rich crust left by intense weathering in a hot, wet climate.
Mesozoic — the standard era from about 252 to 66 million years ago; the ‘middle life’ era of dinosaurs and ammonites.
orogenic — relating to mountain-building, and to the forces that raise mountains.
Palaeozoic — the standard era from about 540 to 252 million years ago; the ‘ancient life’ era of the earliest abundant life.
period — a division of geological time shorter than an era.
physiography — the study of how surface landforms — mountains, plateaus and plains — take shape.
plate — one of the large, slowly moving rigid pieces into which the Earth’s outer shell is broken.
Pre-Cambrian — the longest and oldest standard era, from the Earth’s origin to about 540 million years ago; its rocks hold almost no fossils.
Purana — India’s geological era matching the late Pre-Cambrian.
residual mountains — the worn-down stumps left behind when far older mountains are eroded away.
stratigraphy — the study of rock beds: their sequence, thickness and age.
system — the whole body of rock formed during a period.
Tethys — the ancient sea that lay where the Himalayas now stand, whose folded sediments became the range.
Theory of Plate Tectonics — the explanation that the Earth’s shell is broken into moving plates whose collisions build mountains by folding and faulting.
Vindhyan system — a Pre-Cambrian (Purana) rock system of limestone, sandstone and shale, prized for building stone.
Young Fold Mountains — geologically young mountains, like the Himalayas, that were raised by the folding of rock.
2.7 Points to Ponder
The Peninsular Plateau is called old and the Himalayas young, yet it is the young range that is changing fastest and the old plateau that barely stirs. In what sense, then, is ‘age’ the right word for either of them?
The Great Plain is still being built at the delta, even as rivers wear down the very mountains that feed it. Is India’s landscape being created or destroyed — and can it honestly be both at once?
Fossils let geologists tell one era from another, yet the Pre-Cambrian — nearly nine-tenths of the Earth’s history — left almost none. How might we still read the story of a time that recorded no life?
A timeline draws every era the same width, though the Pre-Cambrian is many times longer than the Cenozoic. What does that convenient drawing teach you correctly, and what does it quietly mislead you about?
A volcanic eruption 66 million years ago helps decide which crop a farmer plants on the Deccan today. How many other everyday choices might be set, unseen, by events in the deep geological past?
2.8 Objective Questions
- The oldest landform of the Indian subcontinent is: [1 mark]
- the Himalayas
- the Great Plain
- the Peninsular Plateau
- the coastal plains
- The Himalayas were folded up from the sediments of the: [1 mark]
- Arabian Sea
- Tethys Sea
- Bay of Bengal
- Gulf of Khambhat
- The whole body of rock formed during a period is called a: [1 mark]
- group
- era
- system
- horst
- Most of India’s coal is found in the rocks of the: [1 mark]
- Dharwar system
- Vindhyan system
- Cuddapah system
- Gondwana system
- Read the assertion and reason, then choose the correct option. [1 mark] Assertion (A): The Peninsular Plateau is often compared to a horst. Reason (R): It has behaved as a rigid block, rising and sinking along faults without being folded.
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
- Match each rock system or formation in Column A with what it is best known for in Column B. [1 mark]
| Column A | Column B |
|---|---|
| a. Dharwar system | i. fine building sandstone |
| b. Vindhyan system | ii. coal deposits |
| c. Gondwana system | iii. metallic minerals such as gold and iron |
| d. Deccan Traps | iv. basalt lava flows and black soil |
Fill in the blank: The ancient southern supercontinent that once included peninsular India was called __________. [1 mark]
State whether the following is true or false: The Great Plain of north India is a finished landform and is no longer growing. [1 mark]
2.9 Check Your Concepts
Define a geosyncline, and name the geosyncline from whose sediments the Himalayas were formed. [2 marks]
State any two characteristic features of the Peninsular Plateau. [2 marks]
Differentiate between the Peninsular Plateau and the Himalayas with reference to their rock type and their geological structure. [3 marks]
Give reasons why the rivers of the Peninsular Plateau flow in shallow valleys with gentle gradients, while those of the Himalayas run torrential and cut deep. [3 marks]
Explain, with reference to the Theory of Plate Tectonics, how the Himalayas were raised from the floor of the Tethys Sea. [4 marks]
Describe how the Great Plain of north India was formed, and give one piece of evidence that shows it is still forming today. [4 marks]
Give a brief account of the geological evolution of India’s three physiographic divisions — the Peninsular Plateau, the Himalayas and the Great Plain — showing how each was formed at a different time. [6 marks]
2.10 Map, Diagram & Data Interpretation
Study the flowchart in Fig. 2.2-2. In your own words, describe the sequence of stages by which the Tethys Sea became the Himalayas. [4 marks]
Look at Table 2.1. [3 marks]
- Name the two regions compared in its last two columns.
- In which standard era did the Deccan Traps form?
- State one thing this table does not tell you about the formations it lists.
Study the cross-section in Fig. 2.2-4. [3 marks]
- Which division lies in the deep trough between the mountains and the plateau?
- From where does the sediment that fills this trough come?
- Why is the land formed there almost perfectly flat?
With reference to Map 2.1, show and label the following on an outline map of India: (i) the area of the Deccan Traps, (ii) a region of the Gondwana coal-bearing rocks, and (iii) the alluvium of the Ganga plain. [3 marks]
The chapter notes that thin, fossil-bearing sedimentary layers lie between the lava flows of the Deccan Traps. What does the presence of these in-between layers tell us about the way the lava was erupted? [2 marks]