The Terminology of a Fold

Folds are common geological structures that form when rock layers are bent or deformed due to tectonic forces. They can be found in various shapes and sizes, ranging from microscopic wrinkles to massive mountain ranges. Understanding the terminology associated with folds is crucial for comprehending their formation and significance in geological studies.

Amplitude Half the height of the structure measured from crest to trough

Arc length The distance between two hinges of the same orientation measured over the folded surface

Axial surface The surface containing the hinge lines from consecutive folded surfaces

Crest The topographically highest point of a fold, which need not coincide with the fold hinge

Cross section A vertical plane through a fold

Culmination High point of the hinge line in a noncylindrical fold

Cylindrical fold Fold in which a straight hinge line parallels the fold axis; in other words, the folded surface wraps partway around a cylinder

Depression Low point of the hinge line in a noncylindrical fold

Fold axis Fold generator in cylindrical folds

Hinge The region of greatest curvature in a fold

Hinge line The line of greatest curvature

Inflection point The position in a limb where the sense of curvature changes

Limb Less curved portion of a fold

Noncylindrical fold Fold with a curved hinge line

Profile plane The surface perpendicular to the hinge line

Trough The topographically lowest point of a fold, which need not coincide with the fold hinge

Wavelength The distance between two hinges of the same orientation