A fold is a bend in a body of rock or sediment that forms due to a change in pressure. Wave-like folds are composed of layers of the earth's crust that bend and buckle under enormous pressure as the crust hardens, compresses, and shortens. Folds form much the same way as a hump arises in a sheet of paper pushed together from both ends.
Folds may be softly rolling or severe and steep, depending on the intensity of the forces involved in the deformation and the nature of the rocks involved. The scale of folding may be massive, creating mile upon mile of mountains like the Appalachian chain, traversing eastern
North America from Alabama to the Gulf of St. Lawrence in eastern Canada. In general, folded mountain belts represent periods of compression or squeezing during which the crust may be shortened significantly. During the formation of the European Alps, stratified rock layers that originally covered an area about 300 mi (482 km) wide were squeezed together until they had a width of less than 120 mi (193 km). Folds may also be minute, seen simply as tiny ripples a few centimeters in size.
Horizontal pressure results in two basic fold forms: anticlines, arched, upfolded strata that generally appear convex upward, and synclines, downfolds or reverse arches that are typically concave upward. An important and definitive characteristic of these folds is the relative position of the oldest and youngest layers within the fold. At the core of an anticline lie stratigraphically older layers. The outer most layers that make up the fold are younger in age. The opposite is true in the case of a syncline. At the core of a syncline are the youngest layers, with the oldest beds situated at the outside of the fold.
A line drawn along the points of maximum curvature of the fold is called the axis. The inclined rock that lies on either side of the axis are called the fold limbs. One limb of a downfold is also the limb of the adjacent upfold. Limbs on either side of a symmetrical fold are at relatively equal angles. A fold that has only a single limb it is known as a monocline. These often form step-like ridges rising from flat or gently sloping terrain.
As the intensity of the folding increases, the resultant folds often become more asymmetrical, i.e., one limb of an anticline dips at a steeper angle. In overturned folds, the angle of this limb becomes so steep that the tilted limb lies almost beneath the upper limb. Recumbent folds literally lie on their sides, with the lower limb turned completely upside-down.
In many cases the axis of the fold in not horizontal. Such folds are known as plunging folds, and are said to plunge in the direction that the axis is tilted. Folds with a curved axis are called doubly plunging folds. Domes are broad warped areas in which the plunge of the anticline is approximately equal in all directions. The corresponding synclinal structure is known as a structural basin.