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GeographyMediumAsked in: NDA · NDA-II 2019, GAT

Which one of the following sedimentary rocks is organically formed?

  1. A.Shale
  2. B.Chert
  3. C.Halite
  4. D.Chalk

Correct answer

D. Chalk

Explanation

The correct answer is D, Chalk. Chalk is a soft, white limestone built up from the tiny calcium carbonate shells of marine micro-organisms, so it is an organically formed sedimentary rock.

Sedimentary rocks are grouped by how they form. Mechanically formed rocks, such as sandstone and shale, are made of fragments of older rocks. Chemically formed rocks, such as halite and gypsum, crystallise out of water. Organically formed rocks come from the remains of living things: chalk and most limestones from shells and skeletons, and coal from buried plant matter. The famous white cliffs of Dover in England are made of chalk.

A is wrong because shale is a mechanically formed rock made of compacted clay and silt. B is wrong because chert is usually classed as a chemically formed rock of fine silica; though some chert includes silica from organisms, it is not the standard example. C is wrong because halite, or rock salt, is a chemically formed rock left when salty water evaporates.

Exam tip: mechanical = sandstone, shale; chemical = halite, gypsum; organic = chalk, limestone, coal.

Practice Questions

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Q2.GeographyAsked in: CDS · CDS-II 2017, GKMedium

Which of the following statements is/are correct? 1. Himalayan rivers have their origin in the snow-covered areas, hence are dry in winter season. 2. Rivers of the Peninsular Plateau have reached maturity. 3. Himalayan rivers depict all the three stages of normal cycle of erosion. Select the correct answer using the code given below.

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

Himalayan rivers are perennial, so statement 1 is wrong, while statements 2 and 3 are correct. Himalayan rivers such as the Ganga, Indus and Brahmaputra rise from glaciers and snowfields and also get monsoon rain, so they flow all year round, including in winter.

Q3.GeographyAsked in: CDS · CDS-II 2017, GKMedium

Which of the following statements related to latitude are true? 1. Rainfall, temperature and vegetation vary with latitude. 2. The difference between the longest day and the shortest day increases with latitude. 3. Indira Point is located approximately at 6°45' N latitude. Select the correct answer using the code given below.

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

All three statements about latitude are true. Latitude decides how directly the Sun's rays strike a place, so temperature, rainfall and the type of vegetation change from the equator towards the poles, from hot, wet rainforests to cold tundra.

Q4.GeographyAsked in: CDS · CDS-II 2017, GKMedium

Which of the following lakes is/are situated in Ladakh? 1. Tso Kar 2. Pangong Tso 3. Tsomgo 4. Tso Moriri Select the correct answer using the code given below.

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

Tso Kar, Pangong Tso and Tso Moriri are high-altitude lakes of Ladakh, while Tsomgo lies in Sikkim. Pangong Tso, at about 4,350 metres, is a long, narrow lake that stretches from Ladakh into Tibet, and most of its length lies on the Chinese side.