Compositional or metamorphic layers of rocks may bend during ductile deformation to produce folds. Folds commonly form during regional horizontal shortening in orogenic (mountain building) belts at microscopic to regional scales in all rock types (given suitable deformation conditions). Even rocks that at Earth's surface may be brittle and shatter when rapidly deformed, may fold during the application of regional, tectonic stresses over a long period of time at depth. Such a change in rock rheology is due to elevated temperature and confining pressure and the presence of fluids at deeper levels of the crust.

Upright layers (where young beds overlie older beds) that are arched upward are called anticlines. If the direction of younging (facing) is not known, such folds are called antiforms. Layers that are bent downward are called synclines (where beds are upright) or synforms where facing is not known. Cylindrical folds show the same...