---
title: "Fold classification, strike, true and apparent dip"
subject: "Engineering Geology I"
chapter: 5
key: "semester_3/engineering_geology_i:fold-classification-strike-true-and"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/fold-classification-strike-true-and
site: BCT Engineering
---

# Fold classification, strike, true and apparent dip

Engineering Geology I · Chapter 5: Structural Geology

- Asked in: 2081 Baisakh, 4 marks
- Video: [Strike and Dip 1 Definition](https://www.youtube.com/watch?v=Kg5326kqetU) — Steven Kidder
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/fold-classification-strike-true-and
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5

## Question

Describe the classification of fold. Define strike. What is the relation between apparent dip and true dip?

## Short answer

Folds are classified by convexity (anticline, syncline), axial plane attitude (symmetrical to recumbent), plunge, and shape. Strike is the bearing of a horizontal line on an inclined plane, perpendicular to dip direction. Apparent dip is always smaller than true dip, given by tan(apparent) = tan(true) × sin(angle from strike).

## Answer

#### Classification of folds
1. **On the basis of convexity**: anticline (upfold, older beds in core), syncline (downfold, younger beds in core), monocline.
2. **On the basis of axial plane**: symmetrical (upright, vertical axial plane), asymmetrical (inclined), overturned (both limbs dip the same way), recumbent (horizontal axial plane), isoclinal (parallel limbs).
3. **On the basis of plunge**: non-plunging (horizontal axis) and plunging (inclined axis) folds.
4. **On the basis of shape**: open, closed, tight, chevron, box, dome and basin.

#### Strike
The **strike** is the bearing of the horizontal line on an inclined plane (line of intersection of the plane with a horizontal plane). It is perpendicular to the dip direction.

#### Relation between apparent dip and true dip
The apparent dip is the dip measured in a direction oblique to the strike. It is always smaller than the true dip:

$$
\tan\delta_a = \tan\delta \times \sin\alpha
$$

where $\alpha$ is the angle between the strike and the direction of apparent dip. When $\alpha = 90°$, $\delta_a = \delta$; when $\alpha = 0°$, $\delta_a = 0°$.

## Check yourself

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

1. Which pair is right?
   A. Anticline has younger beds in the core
   B. Syncline has older beds in the core
   C. Anticline has older beds in the core; syncline has younger beds in the core
   D. Both have the same age in the core
   Answer: C. Upfold exposes older rocks in the core; downfold keeps younger in the core. (Key point: Fold by convexity)

2. Which fold has a horizontal axial plane?
   A. Recumbent
   B. Symmetrical
   C. Isoclinal
   D. Overturned
   Answer: A. Recumbent folds lie on their side. (Key point: Fold by axial plane)

3. A plunging fold is one with
   A. a horizontal axis
   B. an inclined axis
   C. a vertical axial plane
   D. parallel limbs
   Answer: B. Non-plunging folds have a horizontal axis. (Key point: Plunge)

4. What is the relation between apparent dip and true dip?
   A. Apparent dip is always larger
   B. They are always equal
   C. apparent = true / sin(alpha)
   D. tan(apparent) = tan(true) x sin(alpha); apparent is smaller
   Answer: D. alpha is the angle between the strike and the apparent dip direction. (Key point: Apparent and true dip)

## Related in Engineering Geology I

- [Strike-dip relationship and apparent dip](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Astrikedip-relationship-and-apparent-dip) — Engineering Geology I, chapter 5, asked 2×
- [Dip amount, strike; fold identification criteria](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Adip-amount-strike-fold-identification) — Engineering Geology I, chapter 5
- [True dip versus apparent dip](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Atrue-dip-versus-apparent-dip) — Engineering Geology I, chapter 5, asked 2×
- [Calculating apparent dip from true dip](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Acalculating-apparent-dip-from-true) — Engineering Geology I, chapter 5, asked 2×
- [Apparent dip in cross-section drawing](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aapparent-dip-in-crosssection-drawing) — Engineering Geology I, chapter 5
- [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×

## Related in other subjects

- [True dip from two apparent dips](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atrue-dip-from-apparent-dips) — Engineering Geology II, chapter 6
- [Reserve: limestone dip 30, apparent 20 m](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Areserve-limestone-dip-30) — Engineering Geology II, chapter 7
- [True thickness from apparent thickness 210 m](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Abh-quartzite-thickness-210-attitude) — Engineering Geology II, chapter 6