CAPF AC 2023 Q36: Geography – Physical Geography & Geomorphology
- (a) Granite
- (b) Limestone
- (c) Quartzite
- (d) Sill
Answer: (b) Limestone
Explanation
Which rock lets water through
How to think: permeable means water can seep through the rock.
Granite, quartzite and sill are hard, tightly locked rocks with almost no gaps.
Limestone is cracked by joints and dissolved by acid rain water, so water moves through it.
| Rock | Type | Permeability |
|---|---|---|
| Granite | Igneous (intrusive) | Impermeable |
| Limestone | Sedimentary | Permeable through joints and dissolved channels |
| Quartzite | Metamorphic | Impermeable |
| Sill | Igneous (a sheet intruded along bedding planes) | Impermeable |
Result in landscape: water sinks into limestone, dissolves it and forms caves, sinkholes and underground rivers, known as karst scenery.
Answer: (b) Limestone.
Why each option is right or wrong
- (a) Incorrect. Granite is crystalline and dense; water cannot pass through.
- (b) CORRECT. Limestone is jointed and soluble, so water passes through it.
- (c) Incorrect. Quartzite is a very hard metamorphic rock with no pores.
- (d) Incorrect. A sill is a cooled igneous sheet; it does not let water pass.
Key point
Permeable: limestone (joints and solution), sandstone, gravel, sand. Impermeable: granite, basalt, slate, quartzite, clay. Limestone region: karst topography with sinkholes, caves, stalactites and stalagmites.
Remember
Limestone is permeable; crystalline rocks are not.
Common mistakes
- Treating all rocks as impermeable, or all sedimentary rocks as permeable; shale is sedimentary and impermeable.
- Mixing a sill (horizontal igneous intrusion) with a sedimentary layer.
- Picking granite because it is familiar, without asking whether it has pores.
Exam tip
Educated guess: three options are igneous or metamorphic crystalline rocks. The odd one out, the only sedimentary rock, is limestone, and caves and sinkholes are only found there.
Sources
- NCERT Class XI Geography — Fundamentals of Physical Geography, Chapter 5 (Minerals and Rocks)
- NCERT Class XI Geography — Fundamentals of Physical Geography, Chapter 7 (Landforms and their Evolution: karst topography)