This document defines key terms related to rock folding and describes the different types of folds. It begins by explaining that folds form in rocks due to stresses that cause undulations or curvatures over time. Different parts of folds are then defined, including the hinge, limb, axis, and more. Causes of folding like tectonic movement and mechanisms like buckling are outlined. Folds are also classified based on features such as closure, symmetry, plunge, and hinge shape. Measurement criteria for folds including height, width, and tightness are provided. The presentation aims to deliver a lecture on rock folding structures and was compiled using various academic sources.

1. Fold
Mithun Ray
Department of Geography
Malda College (University of Gour Banga)
E-mail: mithun.ray147@gmail.com

2. Fold
Undulations or bends or curvatures developed in the rocks of the crust as a
result of stresses to which these rocks have been subjected from time to time
in the past history of the Earth.
The word fold was used by Hall (1815) for the first time to explain the rock structures

3. Causes of Folding
 Tectonic Causes
Earth Movement
 Gravity Sliding
 Differential
Compression
 Non Tectonic Causes
 Rock Fall
Intrusions

4. Different Parts of a Fold

5. Hinge point: point located at the maximum curvature
Hinge line: a line joining hinge point
Hinge zone or hinge area: region on the folded surface near hinge line

6. Enveloping surface: a surface joining successive hinge lines, located on the same folded later
OR
The enveloping surface is the surface tangent to individual hinges along a folded layer.

7. Crest point: point located at maximum height of the folded layer with reference to a horizontal
reference plane.
Crest line: a line joining successive crest point.
Crestal plane/surface: plane/surface joining all the crest lines in a fold.
Culmination point: point where crest line reaches its maximum elevation

8. Difference between Hinge Line and Crest Line in Folding

9. Trough: a point located at minimum height with reference to a horizontal reference plane.
Trough line: a line joining point of minimum height.
Trough plane/surface: plane/surface joining all the trough line in a fold.
Depression point: point where the trough line reaches its minimum elevation.

10. Inflection point: point of minimum or zero curvature in a single layer fold.
Inflection line: a line joining points of minimum or zero curvature in a single fold on the same
surface. This line may also be curved.
Median surface: a surface passing through successive inflection lines.

12. Fold limb: portion of the folded surface between hinge and inflection line.

13. Fold axis: an imaginary line in space along which the fold is generated. It does not have a fixed
position on the folded surface unlike the hinge. The hinge line is commonly found to be curved, but
where it appears as a straight line it is called the fold axis.
Axial plane or axial surface: an imaginary plane which divide the fold almost at two equal half. Or
it can be defined as an imaginary surface or plane joining multiple hinge lines in a single fold
affecting various layers.

15. Dip and Strike
DIP is the acute angle that a rock surface makes with a horizontal plane. STRIKE is the direction
of the line formed by the intersection of a rock surface with a horizontal plane

17. The size of a single fold is measured by its
Height > distance between crest and trough
Width > distance between the inflection points bounding a fold
Amplitude > distance from crest to width, measured parallel to the axial
plane
Wavelength > distance between two consecutive crests or troughs
Aspect ratio > the ratio of its amplitude to its width
Robert Twiss of University of California at Davis (Journal of Structural Geology, 1988) has suggested the
following terms to describe a fold’s aspect ratio:
wide (0.1 to <0.25); broad (0.25 to <0.63);
equant (0.5 to 2.0); short (1.50 to <4);
and tall (4 to <10).

18. https://www.geoexpro.com/articles/2015/01/folds-and-folding-part-ii

19. Tightness of a fold is measured by its interlimb angle (the angle between the
two limbs of a fold).
gentle (interlimb angle <180° to 170°),
broad (170° to 120°),
open (120 to 70°),
closed (70° or 30),
tight (<30° to 10°), and
isoclinal (<10° and the limbs have the same dip).
Michael Fleuty , Imperial College in London (1964)

20. Mechanism of Folding
 Buckling
Bending
Passive folding

21. Buckling occurs when the deforming force is applied parallel to rock layers. This is usually
caused by horizontal compressional tectonic forces and results in layer-parallel shortening of
rocks and thickening (relief) of the rock body perpendicular to stress direction.

22. Bending of rocks occurs when the deforming force is applied across (at high angle to) rock
layers.
For example, basement uplift along a fault, magma intrusion or salt diapirs all produce bends
(folds) in the overlying sedimentary rocks.

23. Passive folding
Passive folding occurs where the layering exerts no mechanical influence on the folding. In this
case the layering only serves as a visual expression of strain with no mechanical or competence
contrast to neighbouring layers

24. CLASSIFICATION OF FOLD:
Classification of fold based on fold closure
1. Antiform: fold domains having upward closure or negative curvature.
2. Synform: fold domains having downward closure or positive curvature.
3. Neutral fold: fold closing sideways.
4. Vertical fold: all surfaces in a neutral fold vertically dipping.

25. Classification of fold based on symmetry
1. Symmetrical fold: a fold in which the axial plane is a plane of symmetry and the two limbs dip
at the same angle but in opposite direction.
2. Asymmetrical fold: a fold in which the axial plane is not a plane of symmetry and the two
limbs dip at unequal angles in opposite direction.

26. Classification of fold based on plunge of fold axis:
1. Horizontal fold: a fold whose axis is horizontal.
2. Plunging fold: a fold whose axis is inclined.
3. Vertical fold: a fold whose axis is vertical

27. Classification of fold based on orientation of axial plane
1. Upright fold (Willis & Willis 1929): with vertical or nearly vertical axial plane.
2. Recumbent fold: with axial plane dipping at an angle of 10° or less.
3. Inclined fold (Willis & Willis 1929): With inclined axial plane.
4. Reclined fold (Sutton 1960): inclined fold in which the pitch of the fold axis on the axial plane
is between 80 and 100o

28. Classification of fold based on direction of younging relative to fold closure:
1. Anticline: a fold in which direction of younging is away from the fold core.
2. Syncline: a fold in which direction of younging is towards the fold core.
3. Anticlinorium: a large anticline with many smaller folds on its back.
4. Synclinorium: a large syncline with many smaller folds on its back

29. Classification of fold based on shape of hinge
1. Round-hinged or broad hinged fold: a fold with a broad hinge zone compared to the limb.
2. Chevron fold: a fold with straight limbs and with a sharp hinge.
3. Arrow-head fold: a fold with a sharp hinge and with distinctly curved limbs.
4. Cuspate fold: a train of fold with sharp hinge on one set of closures and with rounded hinges
on the oppositely directed closures.

30. This Power Point Presentation (PPT) has been
prepared only to deliver the lecture. The
materials (Maps, Diagrams and Images) used in
this presentation have been collected and
compiled by the presenter from various
academic blogs, research papers, books etc.

31. THANK YOU