---
title: "Classification of sedimentary rocks"
subject: "Engineering Geology I"
chapter: 4
key: "semester_3/engineering_geology_i:classification-of-sedimentary-rocks"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/classification-of-sedimentary-rocks
site: BCT Engineering
---

# Classification of sedimentary rocks

Engineering Geology I · Chapter 4: Petrology

- Asked in: 2081 Baisakh (new course), 2 marks; 2081 Bhadra, 3 marks; 2076 Chaitra, 3 marks; 2073 Shrawan, 3 marks (asked 4 times)
- Video: [Sedimentary Rock Classification](https://www.youtube.com/watch?v=zVbuA3ZevIw) — Inside the Ram Skull
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/classification-of-sedimentary-rocks
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4

## Question

Classify sedimentary rocks.

## Short answer

Sedimentary rocks are classified as clastic (conglomerate, sandstone, shale) from fragments, chemical (rock salt, gypsum) from precipitation, and organic (coal, coral limestone) from remains. Clastic types are subdivided by grain size into rudaceous, arenaceous, and argillaceous.

## Answer

Sedimentary rocks are classified mainly by their mode of origin into three groups, and also by grain size and composition.

#### 1. Clastic (mechanical, detrital) rocks
Formed from broken fragments of older rocks, transported and deposited, then cemented. Classified by grain size:
- **Rudaceous** (> 2 mm): conglomerate (rounded gravel), breccia (angular fragments).
- **Arenaceous** (1/16-2 mm): sandstone, arkose, greywacke.
- **Argillaceous** (< 1/16 mm): shale, mudstone, siltstone, clay.

#### 2. Chemical rocks
Formed by precipitation from solution or evaporation.
- Limestone (chemical), dolomite, rock salt, gypsum, chert, travertine.

#### 3. Organic (biogenic) rocks
Formed from remains of plants and animals.
- Calcareous: fossiliferous limestone, coral limestone, chalk.
- Carbonaceous: peat, lignite, coal.
- Siliceous: diatomite.

```text
 Sedimentary
   |-- Clastic    : conglomerate, sandstone, shale
   |-- Chemical   : rock salt, gypsum, chert
   `-- Organic    : coal, coral limestone
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Sedimentary rocks are classified by mode of origin into
   A. clastic, chemical and organic groups
   B. plutonic, hypabyssal and volcanic groups
   C. acidic, intermediate and basic groups
   D. foliated and non-foliated groups
   Answer: A. Origin gives clastic, chemical and organic sedimentary rocks. (Key point: Main groups)

2. Which clastic group has grains larger than 2 mm?
   A. Arenaceous (sandstone)
   B. Rudaceous (conglomerate, breccia)
   C. Argillaceous (shale)
   D. Siliceous (diatomite)
   Answer: B. Rudaceous rocks have gravel-sized grains over 2 mm. (Key point: Clastic grain size)

3. What distinguishes breccia from conglomerate?
   A. Breccia has rounded gravel; conglomerate has angular fragments
   B. Breccia is chemical; conglomerate is organic
   C. Breccia has angular fragments; conglomerate has rounded gravel
   D. Breccia is finer than 1/16 mm
   Answer: C. Angular fragments mean breccia; rounded gravel means conglomerate. (Key point: Conglomerate vs breccia)

## Related in Engineering Geology I

- [Texture of sedimentary rocks](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Atexture-of-sedimentary-rocks) — Engineering Geology I, chapter 4, asked 2×
- [Identify sedimentary rocks in the field](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aidentify-sedimentary-rocks-in-the) — Engineering Geology I, chapter 4
- [Recognising sedimentary, metamorphic, igneous rocks](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arecognising-sedimentary-metamorphic-igneous-rocks) — Engineering Geology I, chapter 4
- [Formation of sedimentary and metamorphic rocks](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aformation-of-sedimentary-and-metamorphic) — Engineering Geology I, chapter 4
- [Texture of sedimentary rocks and rock cleavage](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Atexture-of-sedimentary-rocks-and) — Engineering Geology I, chapter 4
- [Petrography; structure of sedimentary rocks](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Apetrography-structure-of-sedimentary-rocks) — Engineering Geology I, chapter 4

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