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?
Answer & explanation
Answer: (c) Only one of the Statements II and III is correct and that explains Statement I
Porosity is how much empty space a rock holds; permeability is how easily water passes through it. Chalk is a soft limestone, and limestone is porous and gives up its pore water freely, which explains Statement I. Clay is actually highly porous (USGS puts it at about 50%), but its pores are so tiny that water barely moves, so Statement III ('not at all porous') is wrong.
- ✓ Statement I Chalk is a variety of limestone (Britannica), and USGS shows limestone yielding most of its pore water freely, while groundwater moves very slowly through relatively impermeable materials such as clay and shale.
- ✓ Statement II Chalk, as a limestone, is porous: USGS lists limestone at 20% porosity with 18% specific yield, so it takes up and releases water readily. That is why chalk is permeable, so II explains I.
- ✗ Statement III Clay is not free of pores: USGS lists its porosity at 50%, but only 2% of that drains freely because the pores are tiny. Britannica notes that permeability falls as the average pore size becomes smaller.
Remember · Porosity = pore space; permeability = ease of water flow. Chalk (limestone): porous and permeable. Clay: highly porous but nearly impermeable because its pores are minute.
📘 Read it in NCERT: Class 11 Fundamentals of Physical Geography, Ch 5 (practise this chapter)
Sources
- U.S. Geological Survey, Water Science School: Aquifers and Groundwater (copy of the USGS page) ↗ “Groundwater moves very slowly through relatively impermeable materials such as clay and shale.”
- Encyclopaedia Britannica: Permeability (geology) ↗ · reference work “The porosity of a soil horizon increases as its texture becomes finer, whereas the permeability decreases as the average pore size becomes smaller.”
- NCERT Class 11 · Fundamentals of Physical Geography, Chapter 5 “If the rocks are permeable and soluble and water is available only then karst topography develops.”
- U.S. Geological Survey, Water-Supply Paper 2220: Basic Ground-Water Hydrology (Heath, 1983): porosity, specific yield and specific retention ↗ “Clay ----------------------- 50 2 48 Sand ---------------------- 25 22 3 Gravel --------------------- 20 19 1 Limestone ------------------ 20 18 2”
- Encyclopaedia Britannica: Chalk ↗ · reference work “chalk , soft, fine-grained, easily pulverized, white-to-grayish variety of limestone .”
Question and answer: UPSC's official GS Paper I (2025, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). ·