Denudation: ICSE Class 9 Geography solution

Summary

Denudation wears away the Earth’s surface. It involves breaking rocks, moving them, and reshaping the land. Weathering breaks down rocks in place due to weather conditions. Erosion uses agents like water or wind to pick up and carry away rock pieces. Mass movement is when earth materials slide down a slope by gravity. Transportation is this carrying of materials, and deposition is when these materials are laid down in a new location.

Rivers are powerful in the work of denudation. A river’s journey often has three stages: young, mature, and old. The water’s speed, its volume, and the amount of material it carries affect how a river shapes the land.

In its young stage, usually in hilly areas, a river flows fast with a steep slope. Its main work is eroding downwards. This creates deep, narrow valleys that are often shaped like the letter ‘V’. Waterfalls also form in this stage, where water drops suddenly over a hard rock layer that lies above softer rock. Sometimes, smaller, step-like water flows called rapids appear.

As the river enters its middle, or mature, stage, the land becomes flatter, and the river slows down. It starts to erode sideways more than downwards, making the valley wider. The river forms large bends that look like a snake’s path; these are called meanders. If a meander loop gets cut off, it can form a crescent-shaped lake called an oxbow lake.

In its old stage, the river flows very slowly towards its mouth, which is often the sea. Here, the river deposits most of the sediment it has carried. This can build up a flat, triangular piece of land called a delta. For a delta to form, the sea needs to be calm, and the river must carry a lot of sediment.

Wind also shapes land, especially in dry, desert regions where there is little plant cover to hold the soil. Wind erosion can lower the land by blowing away loose sand and dust. This process can create large depressions called deflation hollows.

Wind also deposits material, mainly sand, to form hills called sand dunes. The shape and size of sand dunes depend on the wind’s direction and strength, the amount of sand, and any obstacles. Sand dunes can move with the wind, which is called migration. This movement can sometimes cover oases or villages. Planting long-rooted trees can help stop dunes from moving.

There are different types of sand dunes. Barchans are crescent-shaped, or moon-shaped, dunes with horns pointing in the direction the wind is blowing. They have a gentle slope on the side facing the wind and a steep slope on the other side. Seifs are long, narrow ridges of sand that lie parallel to the direction of the prevailing wind. Their name comes from an Arabic word for ‘sword’, which describes their shape.

Textbook solutions (Morning Star/Total)

Multiple-Choice Questions

1. The process of breaking and removing the rocks from the surface of the earth:

(a) Erosion
(b) Mass movement
(c) Denudation
(d) Weathering

Answer: (c) Denudation

2. Denudation is the process that wears away landmass and reshapes the surface of the earth. Which of the following process is NOT involved in it?

(a) Weathering
(b) Erosion
(c) Deposition
(d) Mass wasting

Answer: (c) Deposition

3. The disintegration of rocks by atmospheric agents at or near the surface of the earth:

(a) Erosion
(b) Weathering
(c) Mass movement
(d) Denudation

Answer: (b) Weathering

4. The displacement of rocks by agents like wind, water or ice:

(a) Deposition
(b) Transportation
(c) Erosion
(d) Mass wasting

Answer: (c) Erosion

5. The large-scale movement of loose materials (rock-waste) down the slope on account of gravity:

(a) Mass movement
(b) Deposition
(c) Denudation
(d) Weathering

Answer: (a) Mass movement

6. The laying down of material that has been weathered, eroded and transported by natural processes such as water, wind and ice.

(a) Physical Weathering
(b) Deposition
(c) Denudation
(d) Tributary

Answer: (b) Deposition

7. The streams or small rivers which join the main river are called:

(a) Transporteries
(b) Tributaries
(c) Distributaries
(d) Meanders

Answer: (b) Tributaries

8. When a river divides into channels or smaller rivers they are called its:

(a) Tributaries
(b) Distributaries
(c) Meanders
(d) Transporteries

Answer: (b) Distributaries

9. In the upper course the dominant activity of a river is:

(a) Valley
(b) Meander
(c) Erosion
(d) Mass wasting

Answer: (c) Erosion

10. In the middle course of a river the gradient is gentle so the flow of the river slows down such that it appears like a snake over its floodplains forming which of the following?

(a) Deltas
(b) Gorges
(c) Meanders
(d) Rapids

Answer: (c) Meanders

11. _________ are moon-shaped dunes.

(a) Dunes
(b) Sand Dunes
(c) Barchans
(d) Seifs

Answer: (c) Barchans

12. Stationary, long and narrow sand-ridges which grow parallel to the direction of the prevailing wind and remain in a fixed position.

(a) Dunes
(b) Sand Dunes
(c) Barchans
(d) Seifs

Answer: (d) Seifs

13. The lowering of land, rounding of exposed rock surfaces and levelling of the peaks:

(a) Denudation
(b) Erosion
(c) Mass wasting
(d) Deposition

Answer: (a) Denudation

14. Which amongst the following is not a factor affecting work of a river?

(a) Velocity of water
(b) Quality of water
(c) Volume of water
(d) Load of water

Answer: (b) Quality of water

15. How do migratory dunes damage towns and villages?

(a) They cover all surfaces with thick layers of sand.
(b) The large mass of sand shifts further.
(c) They can be checked by building walls.
(d) All of the above.

Answer: (a) They cover all surfaces with thick layers of sand.

16. Name the landform that is formed as a result of the action of wind wherein loose materials are lifted and blown off to be deposited outside the desert margins.

(a) Sand Dunes
(b) Barchans
(c) Deflation Hallows
(d) Seifs

Answer: (a) Sand Dunes

Short Answer Questions

1. What do you mean by denudation?

Answer: Denudation is the process of breaking and removing the rocks from the surface of the earth. It is wearing away of landmass by various processes like weathering, erosion, mass movement and transportation. It results in lowering the level of land, rounding exposed rock surfaces and levelling the peaks.

2. Name the factors which affect work of a river.

Answer: The factors which affect the work of a river, specifically its force of erosion, include the degree of inclination or gradient of its course, the volume of water it carries, the load of the sediments, velocity of water, and water discharge. More broadly, various factors affect erosion, transportation and deposition work of a river, including velocity of water, volume of water, and the load.

3. What is the main function of a river in its upper course? Name the landforms formed in the upper course of a river.

Answer: In the upper course, the dominant activity of a river is erosion. The significant landforms resulting from erosion in the upper course of a river include V-shaped valleys and waterfalls. Other landforms associated with the young stage include gorges and rapids.

4. How is a waterfall formed?

Answer: A waterfall is generally formed due to large differences in the rate of erosion. The water that falls down the edge of a hard resistant rock may have at the bottom a soft rock. It is this soft rock that gets eroded fast and creates a hollow basin called a plunge pool. Smaller differences in the rate of erosion lead to the formation of step-like features known as rapids.

5. State the two characteristics of a river in its middle course.

Answer: Two important characteristics of the middle course of a river are:

(i) the lateral cutting is more active than down cutting; and
(ii) the volume of water in the river increases due to many tributaries joining it.

6. How are meanders formed? Give one example of a meander.

Answer: River meanders are bends of longitudinal courses. S-shaped meanders, which are common to most rivers, result when a channel forms a curved path. These are the result of both erosion and deposition work of rivers. Meanders generally form under conditions of a gentle slope and sufficient water in rivers. The river flow is diverted by an obstruction allowing the river to do lateral erosion work.

The Ganga in India and Mississippi in the USA are famous for their meanders.

7. Name two landforms in the lower course of a river.

Answer: Two landforms found in the lower course of a river are deltas and flood plains. Ox-bow lakes are also formed in this stage.

8. Where is wind erosion most predominant?

Answer: The effects of wind erosion are more pronounced in the desert region, as wind is the most effective agent of erosion, transportation and deposition in arid regions where there is little vegetation or moisture to bind the loose surface materials.

9. What are known as Deflation Hollows?

Answer: Deflation results in the lowering of the land surface to create depressions called deflation hollows. This occurs as deflation involves the lifting as well as the blowing away of loose materials from the ground.

10.Name two chief landforms of wind deposition.

Answer: Two chief landforms of wind deposition are Barchans and Longitudinal Dunes or Seifs. Barchans are described as moon-shaped dunes. Longitudinal Dunes or Seifs are long and narrow sand-ridges. These are types of sand dunes, which are hills of sand formed by the movement of winds where sand is being continuously moved, reshaped, and redeposited.

11. Name two chief types of sand dunes.

Answer: The two chief types of sand dunes seen in tropical deserts are Barchan and Longitudinal Dunes or Seifs.

12. State any two characteristics of barchans.

Answer: Two characteristics of barchans are:

(i) They are moon-shaped dunes.
(ii) The windward side is convex and gently-sloping while the leeward side (being sheltered) is concave and steep.

Complete the following table

Complete the following table

Stage	Effect on the river	Work of the river	Effect on land
Young Stage (Upper Course)
Maturity Stage (Middle Course)
Old Stage (Lower Course)

Answer:

Stage	Effect on the river	Work of the river	Effect on land
Young Stage (Upper Course)	Formation of gorge, rapids and waterfalls.	Mainly erosion.	Formation of V-shaped valleys.
Maturity Stage (Middle Course)	Meanders, river cliffs.	Transportation (some deposition), lateral erosion.	Valley widening, open, wide gentle-sloping valley.
Old Stage (Lower Course)	Deltas, flood plains, ox-bow lakes.	Mainly deposition.	Alluvium deposition, very gently-sloping, smooth, wide valley.

Structured Questions

1. (a) State the difference between weathering and denudation.

Answer: Weathering refers to the disintegration of rock by atmospheric agents at or near the surface of the earth, happening due to changes in weather conditions like temperature, moisture, and precipitation, with no such movement of resulting particles occurring in weathering itself. Denudation, on the other hand, is the broader process of breaking and removing the rocks from the surface of the earth; it is the wearing away of landmass by various processes which include weathering, as well as erosion, mass movement, and transportation.

(b) Name the various processes involved in denudation. What are the results of denudation?

Answer: The various processes involved in denudation are weathering, erosion, mass movement, and transportation.
The results of denudation include the lowering the level of land, rounding exposed rock surfaces, and levelling the peaks.

(c) Give a geographical reason for each of the following:
(i) Erosion is the dominant activity of a river in its upper course.

Answer: In the upper course, the river is young, flows swiftly due to steep gradients, and generally lies in a hilly area, flowing a considerable distance over a steep slope. It erodes its bed and banks and carries large boulders. Under the impact of the rush of water and the load the river carries, erosion action is swift and effective. Therefore, in the upper course, the dominant activity of a river is erosion.

(ii) Landforms of deposition are more prominent in the middle course of a river.

Answer: In the middle course, the river’s deposition work is more important than its erosion work, and therefore, the landforms of deposition are more prominent than the landforms of erosion. This is because, at this stage, the gradient is reduced, and the river flows more slowly. The energy required to transport the materials is just enough to drag large particles, and the volume of water in the river often increases due to many tributaries joining it. As a result, the river carries its load in different ways, and as it widens, deposition becomes more significant.

(iii) All the rivers do not form deltas.

Answer: Not all rivers form deltas because there are certain conditions required for their formation. These conditions include a nearly calm sheltered sea, a large amount of sediment supply, and a reasonable size of the river. Small rivers, for instance, do not form deltas.

(d) Describe the work of wind erosion and state two important landforms of wind erosion.

Answer: The work of wind erosion involves the wind picking up loose debris like sand and pebbles, lifting and carrying sand grains away, and transporting as well as depositing them in another location. Deflation, a key process of wind erosion, involves the lifting as well as the blowing away of loose materials from the ground. The sands and pebbles are carried in the air or driven along the ground, and finer dust and sands may be deposited even outside desert margins.

Two important landforms resulting from the work of wind, which includes erosion, are:

Deflation Hollows: These are depressions created by deflation, which is the lowering of the land surface due to the lifting and blowing away of loose materials.
Sand Dunes: These are hills of sand formed by the movement of winds, where sand is continuously moved, reshaped, and redeposited.

2. (a) Briefly describe the factors which affect the work of a river.

Answer: Various factors affect the erosion, transportation, and deposition work of a river. Some important factors include:

(i) Velocity of water: Erosion and transportation are both maximum when velocity is high. Deposition occurs when the velocity of water is low.
(ii) Volume of water: The larger the volume of water, the greater is the power of erosion and transportation. Deposition takes place better when there is a reduced volume of water as well as when the load is greater.
(iii) Load: Load is the material transported by a river. The load of a river does all the work of erosion and deposition.

(b) Describe the three stages of a river.

Answer: From its source to its mouth, a river is divided into three main sections or stages:

Upper Course (Young Stage): In this initial stage, the river flows swiftly as gradients are steep. It generally lies in a hilly area, flowing over a steep slope. The dominant activity is erosion, as the river erodes its bed and banks and carries large boulders.
Middle Course (Maturity Stage): In this second stage, the gradient is reduced, and the river flows more slowly. It typically begins when the river leaves the mountains and enters the plains. The energy is just enough to drag large particles. Lateral cutting is more active than down cutting, and the volume of water increases due to tributaries joining it. The river’s deposition work becomes more important than its erosion work.
Lower Course (Old Stage): In this third stage, the river flows sluggishly. Ideal conditions for depositional landforms exist as the load-transporting capacity is drastically reduced due to sluggish flow and the division of the river into many distributaries.

(c) Give a geographical reason for each of the following:
(i) The effects of wind erosion are more pronounced in desert areas.

Answer: The effects of wind erosion are more pronounced in desert areas because, in arid regions, there is little vegetation or moisture to bind the loose surface materials, making them easily erodible by the wind.

(ii) Migratory sand dunes are uncertain and dangerous.

Answer: Migratory sand dunes are uncertain and dangerous because when they migrate with the wind, they damage towns and villages. For example, the migration of barchans can be a threat to desert life as they sometimes encroach on an oasis, burying palm trees or houses.

(iii) Long rooted trees and sand holding grasses are grown bordering fields near deserts.

Answer: Long-rooted trees and sand-holding grasses are planted or grown bordering fields near deserts to halt the advance of the dunes in order to prevent the fertile land from being devastated. The shifting of a sand dune can also be checked by the growth of vegetation on the wind-ward slope.

(d) State the factors on which the shape and size of dunes depend?

Answer: The shape and size of sand dunes depend upon factors such as:

(a) The direction and force of wind.
(b) The speed of wind, carrying the dust and sand particles.
(c) The nature and amount of sand brought by the wind.
(d) Water features e.g., lakes, streams etc., in the close-by area.
(e) The nature of vegetation on the land.
(f) The nature of the surface, where the formation of a sand dune takes place.

Thinking Skills

1 What is the relationship between denudation, weathering and erosion? Give examples to support your answer.

Answer: Denudation is the overall process of wearing away the Earth’s surface, which includes weathering, erosion, mass movement and transportation. Weathering is the breakdown of rocks at or near the Earth’s surface due to atmospheric factors like temperature and moisture. For example, rocks may crack and break apart due to repeated heating and cooling. Erosion, on the other hand, involves the movement of rock particles by natural agents such as rivers, wind or ice. A river, for instance, can erode its bed and banks, carrying sediment downstream. Therefore, weathering prepares the material, and erosion transports it — both are part of denudation, which shapes and lowers the land over time.

2. While travelling from Haridwar to Prayagraj, the course of which river did you follow? List the landforms associated with it along the course you followed.

Answer: While travelling from Haridwar to Prayagraj, the river followed is the Ganga. This river passes through different stages along its course:

In the upper course near Haridwar, the Ganga flows swiftly over steep slopes, leading to the formation of V-shaped valleys, gorges, and waterfalls due to intense erosion.
In the middle course, as it enters the plains, the gradient reduces and lateral erosion becomes prominent. Here, meanders and river cliffs are common, and the river carries more load due to added tributaries.
By the time it reaches Prayagraj, the Ganga is in its lower course. The flow becomes slow and deposition dominates. The river forms ox-bow lakes, flood plains, and eventually contributes to the Ganga-Brahmaputra delta, one of the largest in the world.

Project/Map Work

1. On the outline map of the world mark one of the following in each continent (i) Rivers which form deltas; (ii) Desert.

Answer: Do it yourself. Here are the instructions:

Take a blank world map (outline only).
Use coloured pencils or markers to clearly label.
In each continent, mark:
- One major river that forms a delta (e.g. Nile in Africa, Mississippi in North America, Amazon in South America, Ganga in Asia, Danube in Europe, Murray-Darling in Australia, Lena in Eurasia).
- One major desert (e.g. Sahara in Africa, Sonoran in North America, Atacama in South America, Thar in Asia, Kalahari in Africa, Simpson in Australia, Gobi in Asia).
Label the features clearly and neatly.
Use a legend or key to indicate which colour/mark is for rivers and which is for deserts.

2. Prepare a power point presentation on the effects of rising sea levels due to global warming in the world’s most populated delta – the Ganga Brahmaputra delta.

Answer: Do it yourself. Here are the instructions:

Slide 1: Title slide – “Effects of Rising Sea Levels in the Ganga-Brahmaputra Delta”
Slide 2: Introduction – Location of the delta and its population.
Slide 3: Causes of sea level rise (focus on global warming and melting glaciers).
Slide 4: Effects on the delta – e.g. flooding, salinisation of soil, loss of homes, migration.
Slide 5: Impact on agriculture and biodiversity.
Slide 6: Social and economic consequences for local communities.
Slide 7: Measures being taken to protect the delta.
Slide 8: What more can be done – suggestions and solutions.
Slide 9: Conclusion – summarise key points.
Slide 10: References and image credits (if used).

3. In this chapter you have studied the action of winds that results in desertification in arid areas. Make a list of activities that could prevent and reverse desertification.

Answer: Do it yourself. Here are the instructions:

Plant long-rooted trees and shrubs to hold the soil and stop wind erosion.
Grow sand-holding grasses on dunes to prevent them from shifting.
Create windbreaks using rows of trees or bushes to reduce wind speed.
Avoid overgrazing by controlling the number of animals on the land.
Practise crop rotation and sustainable farming to maintain soil health.
Use drip irrigation and water-saving methods to prevent the land from drying out.
Build check dams to stop water runoff and encourage groundwater recharge.
Restore degraded land by adding organic matter and mulching to improve soil quality.
Raise awareness among local communities about the importance of protecting land.
Fence off threatened areas to allow vegetation to regrow naturally.

Extras

Additional MCQs

1. What is denudation?

A. Breaking and removing rocks
B. Formation of mountains
C. Folding of land
D. Volcanic activity

Answer: A. Breaking and removing rocks

2. Which process is not involved in denudation?

A. Weathering
B. Erosion
C. Mass movement
D. Volcanic eruption

Answer: D. Volcanic eruption

3. What is weathering?

A. Disintegration of rock by atmospheric agents
B. Displacement of rock by water
C. Movement of loose material downslope
D. Laying down of sediments

Answer: A. Disintegration of rock by atmospheric agents

4. What is erosion?

A. Displacement of rock by agents like wind or water
B. Breaking of rock in place
C. Deposition of sediments
D. Movement of loose material downslope

Answer: A. Displacement of rock by agents like wind or water

5. What is mass movement?

A. Movement of loose material downslope due to gravity
B. Flow of water in a river
C. Wind-driven sand transport
D. Disintegration of rock

Answer: A. Movement of loose material downslope due to gravity

6. What is transportation?

A. Movement of material by water or wind
B. Breaking of rocks by temperature change
C. Laying down of sediments
D. Collapse of overhangs

Answer: A. Movement of material by water or wind

7. What is deposition?

A. Laying down of transported material
B. Displacement of rock particles
C. Splitting of rock by frost
D. Flow of river water

Answer: A. Laying down of transported material

8. Which of the following is an active agent of denudation?

A. Wind
B. Mass movement
C. Weathering
D. Transportation

Answer: A. Wind

9. Which of the following is not an active agent of denudation?

A. Water
B. Wind
C. Vegetation
D. Glacial ice

Answer: C. Vegetation

10. Which factor increases river erosion when it is high?

A. Load
B. Volume of water
C. Velocity of water
D. Deposition rate

Answer: C. Velocity of water

11. Which factor increases river power when it is larger?

A. Gradient
B. Load
C. Volume of water
D. Channel width

Answer: C. Volume of water

12. What does the load of a river consist of?

A. Water volume
B. Sediments
C. Channel slope
D. Bank vegetation

Answer: B. Sediments

13. When does river deposition occur?

A. When velocity is high
B. When velocity is low
C. When volume is high
D. When load is low

Answer: B. When velocity is low

14. A stream joining a main river is called?

A. Distributary
B. Tributary
C. Meander
D. Oxbow lake

Answer: B. Tributary

15. Channels formed when a river divides near its mouth are called?

A. Tributaries
B. Distributaries
C. Estuaries
D. Floodplains

Answer: B. Distributaries

16. The starting point of a river is called?

A. Mouth
B. Source
C. Estuary
D. Delta

Answer: B. Source

17. The end point where a river enters the sea is called?

A. Source
B. Valley
C. Mouth
D. Levee

Answer: C. Mouth

18. Which stage of a river’s course is characterised by steep gradients and rapid flow?

A. Upper course
B. Middle course
C. Lower course
D. Old stage

Answer: A. Upper course

19. In which stage does lateral erosion dominate?

A. Upper course
B. Middle course
C. Lower course
D. Youth stage

Answer: B. Middle course

20. In which stage does a river flow sluggishly and form many depositional landforms?

A. Upper course
B. Middle course
C. Lower course
D. Maturity stage

Answer: C. Lower course

21. What valley shape is formed by vertical erosion in the upper course?

A. U-shaped valley
B. V-shaped valley
C. Ox-bow valley
D. Floodplain

Answer: B. V-shaped valley

22. What is a waterfall?

A. A vertical fall of water from height
B. A bend in a river
C. A triangular deposit
D. A dry hollow

Answer: A. A vertical fall of water from height

23. What is a plunge pool?

A. Depression at a waterfall base
B. River mouth channel
C. Sand dune hollow
D. River cliff

Answer: A. Depression at a waterfall base

24. Jog Falls has a plunge of how many metres?

A. 60
B. 778
C. 986
D. 150

Answer: A. 60

25. What is a meander?

A. S-shaped bend in a river
B. Vertical water fall
C. Horn-shaped dune
D. Delta finger

Answer: A. S-shaped bend in a river

26. What is an oxbow lake?

A. Cut-off meander lake
B. Gently sloping valley
C. Triangular deposit
D. Sand dune

Answer: A. Cut-off meander lake

27. What is the typical shape of a river delta?

A. Circular
B. Square
C. Triangular
D. Linear

Answer: C. Triangular

28. Which delta is one of the largest and growing towards the sea?

A. Nile delta
B. Mississippi delta
C. Ganga-Brahmaputra delta
D. Amazon delta

Answer: C. Ganga-Brahmaputra delta

29. What is a deflation hollow?

A. Depression caused by wind erosion
B. Hill of sand
C. River terrace
D. Underground reservoir

Answer: A. Depression caused by wind erosion

30. What are sand dunes?

A. Hills of sand
B. Rocky outcrops
C. River islands
D. Glacial tills

Answer: A. Hills of sand

31. Which dune is moon-shaped with horns pointing downwind?

A. Barchan
B. Seif
C. Transverse
D. Parabolic

Answer: A. Barchan

32. Which dune grows parallel to the prevailing wind direction?

A. Barchan
B. Seif
C. Star
D. Parabolic

Answer: B. Seif

33. Which factor does not affect dune shape?

A. Wind speed
B. Amount of sand
C. Vegetation
D. Temperature

Answer: D. Temperature

34. What can prevent dune migration?

A. Rainfall
B. Vegetation
C. Mining
D. Fire

Answer: B. Vegetation

35. Which vegetation is used to stabilise sand dunes?

A. Moss
B. Long-rooted trees
C. Short grass
D. Shrubs

Answer: B. Long-rooted trees

36. What does the term ‘gradient’ refer to?

A. Valley width
B. Slope inclination
C. Water volume
D. Sediment load

Answer: B. Slope inclination

37. What does river load consist of?

A. Water only
B. Sediments
C. Rocks only
D. Organic matter

Answer: B. Sediments

38. Which landform along river banks during floods is formed by deposition?

A. Levee
B. Gorge
C. Waterfall
D. Delta

Answer: A. Levee

39. What is the tidal mouth of a river called?

A. Delta
B. Estuary
C. Floodplain
D. Meander

Answer: B. Estuary

40. Under a waterfall, which rock type erodes faster?

A. Hard rock
B. Soft rock
C. Igneous rock
D. Metamorphic rock

Answer: B. Soft rock

Additional Assertion and Reason

1. Assertion (A): Denudation is the process of breaking and removing the rocks from the surface of the earth.
Reason (R): Denudation involves various processes like weathering, erosion, mass movement, and transportation.