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GeographyMediumAsked in: UPSC CAPF · CAPF 2025, Paper I

Which one of the following is the main method by which potholes are created in bedrocks by the flowing stream?

  1. A.Abrasion
  2. B.Attrition
  3. C.Corrosion
  4. D.Solifluction

Correct answer

A. Abrasion

Explanation

The correct answer is A, Abrasion. Potholes are drilled into a stream's rocky bed by pebbles and boulders that the swirling water grinds against the rock.

On the rocky beds of hill streams, eddies trap stones in small hollows and spin them round and round. The stones scour the bedrock like a drill, deepening the hollow into a roughly circular pit called a pothole, which can grow from a few centimetres to several metres deep. This grinding of the bed and banks by the load a river carries is called abrasion, or corrasion. When large potholes form at the foot of a waterfall, they are called plunge pools.

B is wrong: attrition is the wearing down of the load itself as rock fragments collide and become smaller and rounder; it does not cut the bed.

C is wrong: corrosion (solution) is the chemical dissolving of rock by water, important in limestone but not the main way potholes are cut.

D is wrong: solifluction is the slow flow of wet soil over frozen ground, a mass movement and not stream erosion at all.

Exam tip: Abrasion (corrasion) cuts potholes and plunge pools; attrition rounds the pebbles; corrosion dissolves rock; hydraulic action is the force of the water itself.

Practice Questions

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Q1.GeographyAsked in: CDS · CDS-I 2018, GKHard

Statement I : Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth.

Statement II : The Earth rotates from west to east.

  1. A.Both the statements are individually true and Statement II is the correct explanation of Statement I
  2. B.Both the statements are individually true but Statement II is not the correct explanation of Statement I
  3. C.Statement I is true but Statement II is false
  4. D.Statement I is false but Statement II is true
Show answer
Correct answer: B. Both the statements are individually true but Statement II is not the correct explanation of Statement I

Both statements are true, but the direction in which the Earth spins does not explain why tides occur. Tides are caused mainly by the Moon's pull, with the Sun adding a smaller pull; the Moon's tide-raising force is roughly twice the Sun's because it is so much closer.

Q3.GeographyAsked in: UPSC CAPF · CAPF 2023, Paper IMedium

Consider the following statements :

1. Many of the world's largest mountain chains exist beneath the sea.

2. Some mountain chains are revealed as island arcs.

3. The mid-oceanic ridges form the longest mountain chains.

4. The mid-Atlantic ridge rises thirty-three metres above the floor of the Atlantic.

Which of the statements given above are correct ?

  1. A.1 and 2 only
  2. B.3 and 4 only
  3. C.1, 2 and 4 only
  4. D.1, 2 and 3 only
Show answer
Correct answer: D. 1, 2 and 3 only

Statement 4 is wrong: the Mid-Atlantic Ridge rises thousands of metres above the ocean floor, not thirty-three metres. The ocean floor has relief as varied as the land.

Q4.GeographyAsked in: UPSC CAPF · CAPF 2023, Paper IMedium

Consider the following statements :

1. The Great Northern Plains are formed by basins of three distinct river systems - the Indus, the Ganga and the Brahmaputra.

2. They are one of the most densely populated areas on Earth.

3. Between the Yamuna at Delhi and the Bay of Bengal, nearly 1600 km away, there is a drop of only 200 metres in elevation.

Which of the statements given above are correct?

  1. A.1, 2 and 3
  2. B.2 and 3 only
  3. C.1 and 2 only
  4. D.1 and 3 only
Show answer
Correct answer: A. 1, 2 and 3

All three statements correctly describe the Northern Plains of India. The Northern Plains were built from alluvium laid down by the Indus, the Ganga and the Brahmaputra and their tributaries, which together drain the Himalayas.