📘 IAS Prelims 2025 — GS-I Q.28 | Classroom Explanation

📌 Question

Q.28) 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

✅ Correct Answer: (c)

🧑‍🏫 Classroom Explanation

This question tests understanding of the difference between porosity and permeability, a classic UPSC trap area in geomorphology and groundwater studies.

🔍 Key Concept: Porosity vs Permeability

• Porosity refers to the presence of pore spaces in a rock
• Permeability refers to the ability of water to flow through interconnected pores

➡️ A rock can be:

• Porous but impermeable
• Porous and permeable
• Non-porous and impermeable

UPSC often checks whether aspirants confuse these two terms.

🔍 Statement-wise Analysis

✅ Statement I: Correct

• Chalk:

• Has numerous pore spaces
• Pores are well-connected
• Hence, water can pass through easily
• Therefore, chalk is highly permeable

• Clay:

• Composed of extremely fine particles
• Pores are very small and poorly connected
• Water movement is highly restricted
• Hence, clay is least permeable / impermeable

➡️ Statement I correctly describes permeability characteristics.

✅ Statement II: Correct

• Chalk is composed of microscopic calcareous shells
• This creates a porous structure
• Pores are sufficiently interconnected
• Hence, chalk:

• Absorbs water
• Allows water to move through it

➡️ Statement II is correct and explains Statement I

❌ Statement III: Incorrect

• Clay is porous, but:

• Pores are extremely fine
• Pore spaces are not well connected

• Thus:

• Clay has high porosity
• But very low permeability

➡️ Saying clay is “not at all porous” is incorrect.

🧮 Logical Elimination (UPSC Style)

• Statement II → True
• Statement III → False
• Only Statement II explains Statement I

➡️ Correct option: (c)

🎯 Final Answer

✅ Correct Answer: (c)

📦 Info Box — Extra Points (Prelims Value Add)

• Chalk:

• Porous + Permeable
• Important aquifer rock in limestone regions

• Clay:

• Porous but impermeable
• Acts as aquiclude
• Causes waterlogging above clay layers

• UPSC frequently tests:

• Porosity ≠ Permeability
• Aquifer vs Aquiclude vs Aquitard

🔗 Bridge Points (Future Questions)

• Karst topography and groundwater movement
• Limestone vs sandstone vs shale
• Springs, seepage, and water table behavior

📍 GS Mapping

GS Paper I
Subject: Geography
Theme: Geomorphology — Groundwater & Rocks
Nature: Conceptual + Reasoning