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

# Petrography; structure of sedimentary rocks

Engineering Geology I · Chapter 4: Petrology

- Asked in: 2081 Baisakh, 4 marks
- Video: [Sedimentary Rocks  Textures   Structures](https://www.youtube.com/watch?v=bJH-sf_JYy8) — Pet Eng S. A. (Franco Sivila)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/petrography-structure-of-sedimentary-rocks
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4

## Question

Define petrography. Describe the structure of sedimentary rocks.

## Short answer

Petrography is the systematic description and classification of rocks based on mineral composition, texture, and structure. Sedimentary structures include stratification, cross-bedding, graded bedding, ripple marks, mud cracks, and fossils, indicating depositional conditions.

## Answer

**Petrography** is the branch of petrology that systematically describes and classifies rocks on the basis of their mineral composition, texture and structure, using hand specimens and thin sections under the microscope.

#### Structures of sedimentary rocks
Structures are the large-scale features, seen in outcrops, showing the conditions of deposition.
1. **Stratification (bedding)**: arrangement in parallel layers, separated by bedding planes. Beds may be thick or thin (laminae).
2. **Cross-bedding**: inclined layers within a bed, from wind or current action (dunes, deltas).
3. **Graded bedding**: grain size changes gradually from coarse at bottom to fine at top within a bed.
4. **Ripple marks**: small wave-like ridges from water or wind movement.
5. **Mud cracks (desiccation cracks)**: polygonal cracks in dried mud.
6. **Fossils**: preserved remains of organisms.
7. **Concretions and nodules**: rounded masses of cementing minerals (chert nodules).
8. **Rain prints, sole marks, and stylolites**.

```text
 ====== bedding ======
 //////// cross-bedding
 ~~~~ ripple marks ~~~~
```

## Check yourself

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

1. Petrography
   A. studies origin only
   B. studies fossils
   C. describes and classifies rocks by composition, texture and structure
   D. studies earthquakes
   Answer: C. Uses hand specimens and thin sections. (Key point: Petrography)

2. Graded bedding shows
   A. fine at bottom to coarse at top
   B. uniform grains
   C. inclined layers
   D. grain size changing from coarse at bottom to fine at top
   Answer: D. Within a single bed. (Key point: Graded bedding)

3. Cross-bedding consists of
   A. inclined layers within a bed from wind or current action
   B. polygonal cracks
   C. wave-like ridges
   D. rounded masses of cement
   Answer: A. Seen in dunes and deltas. (Key point: Cross-bedding)

4. Mud cracks are
   A. ripple ridges
   B. polygonal desiccation cracks in dried mud
   C. fossils
   D. nodules
   Answer: B. They form when mud dries. (Key point: Mud cracks)

## Related in Engineering Geology I

- [Classification of sedimentary rocks](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aclassification-of-sedimentary-rocks) — Engineering Geology I, chapter 4, asked 4×
- [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