---
title: "Define geological planes with figures"
subject: "Engineering Geology I"
chapter: 5
key: "semester_3/engineering_geology_i:define-geological-planes-with-figures"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/define-geological-planes-with-figures
site: BCT Engineering
---

# Define geological planes with figures

Engineering Geology I · Chapter 5: Structural Geology

- Asked in: 2081 Kartik (new course), 2 marks
- Video: [What is Strike and Dip and How is it Indicated on a Map?](https://www.youtube.com/watch?v=-yPcPlIHDl0) — Ruppert Rocks Geology
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/define-geological-planes-with-figures
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5

## Question

Define the various geological planes with suitable figures.

## Short answer

Geological planes are flat surfaces in rock masses, including bedding planes (separate sedimentary beds), joint planes (fractures with no movement), fault planes (fractures with displacement), foliation/cleavage planes (mineral alignment in metamorphic rocks), axial planes (divides a fold symmetrically), and unconformity planes (erosional surface). The orientation of a plane is given by its strike and dip.

## Answer

A **geological plane** is a more or less flat surface in a rock mass that separates rocks of different properties or along which the rock can break or move. The common geological planes are:

1. **Bedding plane** - the surface that separates two successive beds of sedimentary rock.
2. **Joint plane** - the surface of a fracture along which there has been no movement.
3. **Fault plane** - the surface of a fracture along which the rock has moved.
4. **Foliation / cleavage plane** - planes of parallel alignment of minerals in metamorphic rocks.
5. **Axial plane** - the imaginary plane that divides a fold into two nearly equal halves.
6. **Unconformity plane** - an erosion surface separating younger and older rocks.

```text
  ------------------  bedding plane
  ------------------
  ====/============   fault plane (with movement)
     /   |  |  |      joint plane (no movement)
```

The orientation of every plane is expressed by its attitude: **strike** (bearing of the horizontal line on the plane) and **dip** (maximum inclination, measured perpendicular to the strike).

```text
        horizontal plane
        ______________
       /   strike    /|
      /____________ /_|  dip angle
     inclined plane
```

## Check yourself

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

1. A bedding plane is
   A. a fracture with movement
   B. the surface separating two successive beds of sedimentary rock
   C. a plane dividing a fold into halves
   D. a plane of mineral alignment
   Answer: B. Bedding planes separate successive sedimentary beds. (Key point: Bedding plane)

2. What distinguishes a joint plane from a fault plane?
   A. No movement along a joint plane, movement along a fault plane
   B. Joint plane has movement, fault plane has none
   C. Both always have equal movement
   D. Joint planes occur only in metamorphic rock
   Answer: A. Displacement is the distinguishing feature. (Key point: Joint vs fault plane)

3. The axial plane of a fold
   A. is the erosion surface under younger beds
   B. is the horizontal line on a bed
   C. is the plane of mineral alignment
   D. divides the fold into two nearly equal halves
   Answer: D. It is an imaginary plane bisecting the fold. (Key point: Axial plane)

4. How is the attitude of a plane expressed?
   A. By strike only
   B. By dip measured along the strike
   C. By strike and dip, with dip measured perpendicular to the strike
   D. By thickness and colour
   Answer: C. Dip is the maximum inclination, perpendicular to strike. (Key point: Strike and dip)

## Related in Engineering Geology I

- [Primary and secondary structures; strike and dip](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aprimary-and-secondary-structures-strike) — Engineering Geology I, chapter 5, asked 2×