Permeability of chalk and clay — UPSC Prelims PYQ 2025

Consider the following statements:

1. 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.
2. Statement II: Chalk is porous and hence can absorb water.
3. Statement III: Clay is not at all porous.

Which one of the following is correct in respect of the above statements?

1. Both Statement II and Statement III are correct and both of them explain Statement I
2. Both Statement II and Statement III are correct but only one of them explains Statement I
3. Only one of the Statements II and III is correct and that explains Statement I
4. Neither Statement II nor Statement III is correct

Explanation

Only one of the Statements II and III is correct and that explains Statement I

• Chalk is very permeable due to its high porosity and interconnected pores, often forming aquifers.
• Clay is very low in permeability due to tightly packed fine grains, though it may still be porous. Thus, the statement I is geologically sound.
• Statement II is correct & Explains Statement I:  Chalk is porous, enabling it to absorb and transmit water, contributing to its permeability. Hence, this explains the chalk part of Statement I.
• Statement III is incorrect: Clay is porous but not permeable due to poor pore connectivity. The claim that clay is “not at all porous” is factually wrong.

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