7. Folds

Hinge Line

An actual line you could draw on the hinge of a fold

Fold Axis

Also known as a generator line, it is an imaginary line that doesn't have a fixed location

Axial Trace

The line on a geological map where the axial surface of a fold intersects the Earth's surface

Axial Surface

An imaginary plane that bisects the limbs of a fold in geology, connecting the hinge lines of each folded layer

Inflection Point

A point of a curve at which a change in the direction of curvature occurs

Inflection Line

A line where the sense of curvature of a rock fold changes, such as from convex to concave or vice versa

Limb

One of the sides or arms of a rock fold, such as an anticline or syncline

Interlimb Angle

The angle between the two limbs of a fold, measured in the profile plane and used to classify the fold tightness

Hinge Zone

The area of maximum curvature in a geological fold, where the two limbs (sides) of the fold meet

Hinge Point

The location of maximum curvature along a fold in rock layers, typically found in the hinge zone where the fold is tightest

Amplitude

The magnitude or strength of the fold

Wavelength

The distance between two troughs or peaks

Enveloping Surface

A surface containing all the crests of a fold or set of folds

Cylindrical Fold

The fold axis has a constant orientation, parallel to every strike orientation of the folded surface

Non-Cylindrical Fold

Also known as a conical fold, the orientation of the hinge line is not the same everywhere

Monocline

A step-like fold in rock layers where a zone of steeply dipping rock is sandwiched between horizontal or gently dipping layers; is “m” shaped

Anticline

A type of fold that is an arch-like shape and has its oldest beds at its core

Syncline

A fold with younger layers closer to the center of the structure

Antiform

A topographic feature which is composed of sedimentary layers in a convex formation, but may not actually form a real anticline

Synform

A geological structure characterized by a downward curvature of sedimentary

Parallel Fold

A fold that maintains a constant thickness when measured perpendicular to the fold, but the thickness varies parallel to the axial surface

Similar Fold

A fold that maintains a constant thickness when measured parallel to the axial surface, but the thickness varies perpendicular to the fold, causing layers to thin in the limbs and thicken in the hinges

Decollement

A fault or a gliding plane where a mass of rock has detached and moved independently from the rocks below it

Chevron Fold

V-shaped geological folds characterized by straight, sharp-hinged limbs that form in regularly layered rock sequences, like alternating sandstones and shales

Flexural Fold

Folds that form when the mechanical influence of layering is strong and the mean ductility of the rock volume is moderate to low

Parasitic Fold

Also known as a drag fold, it is a smaller fold that forms on the limb of a larger fold, sharing a similar orientation

M Fold

A symmetric fold where the two limbs are mirror images of each other in a cross-section perpendicular to the axial trace

S Fold

A type of asymmetrical fold that forms when rock layers bend under pressure, appearing to have a counterclockwise rotation when viewed from above

Z Fold

An asymmetric fold in rock layers formed by the bending of rock under compressional forces

First-Order Fold

An initial geological fold, like an anticline or syncline, that is formed during the early stages of a rock deformation process, where rock layers are bent under pressure

Second-Order Fold

A smaller, secondary fold that develops on the limbs of a larger, first-order fold

Third-Order Fold

Most commonly refers to a parasitic fold in geology, and is a series of folds developing under a larger fold

Intersection Lineation

The intersection of two planar elements