UPSC Prelims 2025 Q28: Geography – Physical Geography & Geomorphology

UPSC Prelims 2025, General Studies Paper I · Geography

Consider the following statements: Statement I: In the context of effect of water on rocks, chalk is known as a very permeable rock whereas clay is known as quite an impermeable or least permeable rock. Statement II: Chalk is porous and hence can absorb water. Statement III: Clay is not at all porous. Which one of the following is correct in respect of the above statements?

(a) Both Statement II and Statement III are correct and both of them explain Statement I
(b) Both Statement II and Statement III are correct but only one of them explains Statement I
(c) Only one of the Statements II and III is correct and that explains Statement I
(d) Neither Statement II nor Statement III is correct

Answer: (c) Only one of the Statements II and III is correct and that explains Statement I

Explanation

Rock Types - Porosity and Permeability:

Rock	Porosity	Permeability	Water Behaviour
Chalk	HIGH - many pores	HIGH - pores are interconnected, water flows freely	Water passes through easily
Clay	HIGH - many pores	LOW (impermeable) - pores are tiny and disconnected, capillary forces prevent flow	Water does NOT flow through

Statement I: CORRECT. Chalk is permeable; clay is impermeable (or least permeable).

Statement II: CORRECT and explains Statement I. Chalk's pores are large and interconnected — water flows freely.

Statement III: INCORRECT. Clay does have pores (it is porous) — but they are so tiny and disconnected that water cannot flow through. Clay is porous but impermeable.

Only Statement II explains Statement I — not Statement III (which is factually wrong).

Answer: (c) Only one of the Statements II and III is correct and that explains Statement I.

Why each option is right or wrong

(a) Incorrect. Both II and III cannot explain Statement I — Statement III is factually wrong about clay having no pores.
(b) Incorrect. Both statements are not correct — Statement III is wrong (clay does have pores, just very tiny and disconnected).
(c) CORRECT. Only Statement II is correct and explains Statement I. Statement III is wrong — clay IS porous (has pores) but is impermeable because pores are tiny and disconnected.
(d) Incorrect. Statement II is definitively correct — chalk's interconnected pore structure explains its permeability.

Key point

Chalk = porous AND permeable (interconnected pores). Clay = porous but IMPERMEABLE (tiny, disconnected pores block flow). Porosity ≠ Permeability.

Remember

Chalk = porous AND permeable. Clay = porous but IMPERMEABLE (tiny disconnected pores block water flow).

Common mistakes

Assuming clay has no pores. Clay is actually highly porous — it has tiny pores. But those pores are so small and disconnected that water cannot flow through — hence impermeable.
Confusing porosity (has pores = yes/no) with permeability (water can FLOW through = yes/no). These are separate properties. Clay = porous but impermeable.
Not knowing chalk's pore structure — chalk (calcium carbonate) has large, interconnected pores that allow free water flow, unlike clay's disconnected micropores.

Exam tip

Porosity vs Permeability: Porous = has pores. Permeable = water FLOWS through. Chalk = both. Clay = porous only (water cannot flow despite having pores). This is a classic Earth science distinction UPSC tests.

Sources

NCERT Class 11 Geography - Fundamentals of Physical Geography (Rock weathering)
Geomorphology - Rock properties

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