1. Folds are the wavy undulations as a result of plastic deformation developed in rocks by compressive forces acting at or near the surface of the earth crust.
A simple fold consists of two parts. One convex part which represents the crest of the fold and the other concave part which represents the trough.

2. Folds are best displayed by stratified formations such as sedimentary or volcanic rocks or their metamorphosed equivalents. But any layered or foliated rock, such as gabbro or granite gneiss, may show folds.

3. Some folds are a few miles across
Some folds are a few miles across. The width of other is to be measured in feet or inches or even fractions of an inch. Folds of continental proportions are hundreds of miles wide. Folds may be observed directly or may be inferred from various kinds of data.

4. If the cross-section of a single folded surface is examined, it will be seen that there are certain points at which the curve surface generally reaches its highest position above a horizontal datum plane, and other points where it reaches its lowest position. c h i h t
c – crest point
t – trough point
h – hinge point
i – inflection point

5. Crest Point: The highest point on the fold is called crest point.
Crest line: The line connecting the highest points on the same surface in an infinite number of cross sections.
Crestal Plane: A plane which contain all the crest line of successive layers is called crestal plane.

6. Trough Point: The lowest point in the fold is known as trough point.
Trough Line: The line connecting the lowest points on the same surface is called trough line.
Trough Plane: A plane contains all the trough line of successive layers is called trough plane.
Crest line and Trough line may be either rectilinear or curved in space and they may or may not be parallel to one another.

7. Culmination Point: The point where a crest line or trough line reaches a maximum height above the horizontal datum plane is known as culmination point.
Depression Point: The point where the crest or trough line has a minimum height is known as the depression point.

8. Hinge Line: It is the line on the folded surface represents the maximum curvature. The azimuth or direction of a hinge line is often termed axial trend of the fold.
Hinge lines, like crest and trough lines, may be rectilinear or curved and the adjacent hinges may be parallel or convergent. Hinge has got a specific position on the fold.
Fold Axis: Fold axis is the line parallel to the fold hinge. It is that straight line moving parallel to itself that generates the fold. It has no position on the fold.
Hinge Zone: The surface region of the fold around the fold hinge is known as hinge zone. Hinge zone is considered to be that portion of the curved surface adjacent to the hinge line where the curvature of the fold exceeds that of the circular arc.

9. Hinge Surface / Axial Surface: The surface containing all the hinge lines of successive layers. It may be a simple flat plane or curved surface.
Fold Profile: The cross-section of the folded surface perpendicular to the hinge line at any point in the fold is known as the fold profile.

10. Inflection Point: A point where the curve changes from convex to concave, i.e., where the rate of change of the slope is zero, is called inflection point.
Inflection Line: The line joining the points of inflections on the same surface in an infinite number of cross-section is known as inflection line.

11. Fold Limb / Flank: is the region of the folded surface between the two adjacent hinges on the same surface. Every limb is mutually shared by two adjacent folds.
Fold Domain: The points where inflections occur are the limits of individual folds in the cross section and is called fold domain.

12. Median Plane: The surface which contains all the lines of inflections in the same folded layer is known as median plane.
Enveloping Surfaces: The folded layer may form either a periodic or irregular wave form that oscillates between two limiting surfaces called the enveloping surfaces.

13. Core: The central part of the fold.
Interlimb Angle / Dihedral Angle: The minimum angle between the fold limbs as measured in the fold profile.
Dihedral angle
Interlimb angle
Core: The central part of the fold.

14. Wavelength : The wavelength of a fold is the distance between the two inflection lines defining the fold domain, and this distance is the half wavelength of the fold (W/2). A
W/2
Amplitude (A): The perpendicular distance between the median plane and a line drawn at the vertex / extremity of the fold parallel to the median plane.

15. i.e., (I) axial plane must be perpendicular to median plane.
Fold Symmetry: Symmetric fold is one in which the hinge / axial surface bisects the fold in such a way that one half is the mirror image of the another.
i.e., (I) axial plane must be perpendicular to median plane.
(II) the length of the fold limb must be equal.
horizontal datum
horizontal datum
Asymmetric Fold: A fold with unequal limb and axial plane not perpendicular to median plane, i. e., axial plane does not bisect the fold into two identical halves.

16. Fold Geometry
Plunge: The angle of inclination of the fold axis with horizontal measured in the vertical plane.
Plunge
Horizontal line
Line – linear structure
Vertical Plane
Inclined Plane
Line – linear structure
Horizontal line
Pitch / Rake
Inclined Plane
Rake / Pitch: The angle measured in some specified inclined plane, between a line and a horizontal.

17. Axial Plane Trace: The line of intersection of hinge / axial plane with the ground surface.