fold¹ / fōld/ • v. [tr.] 1. bend (something flexible and relatively flat) over on itself so that one part of it covers another: she folded all her clothes and packed all her bags. ∎  (fold something in/into) mix an ingredient gently with (another ingredient), esp. by lifting a mixture with a spoon so as to enclose it without stirring or beating: fold the egg whites into the chocolate mixture. ∎  [intr.] (of a piece of furniture or equipment) be able to be bent or rearranged into a flatter or more compact shape, typically in order to make it easier to store or carry: the deck chair folds flat | [as adj.] (folding) a folding chair. ∎  bend or rearrange (a piece of furniture or equipment) in such a way: he folded up his tripod. ∎  [intr.] (fold out) be able to be opened out; unfold: the sofa folds out. ∎  (of a bird) collapse (its wings) and lay them flat against its body: the crow folded its wings to a sharp angle and dive-bombed the vulture. ∎  (often be folded) Geol. cause (rock strata) to undergo bending or curvature: [as n.] (folding) a more active period of igneous activity caused intense folding. 2. cover or wrap something in (a soft or flexible material): a plastic bag was folded around the book. ∎  hold or clasp (someone) closely in one's arms with passion or deep affection: Bob folded her in his arms and kissed her.3. [intr.] inf. (of an enterprise or organization) cease operating as a result of financial problems or a lack of support: the club folded earlier this year. ∎  (esp. of a sports player or team) suddenly stop performing well or effectively: he folded in the second round. ∎  (of a poker player) drop out of a hand: an unerring knack for knowing when to fold and when to stay in.• n. 1. (usu. folds) a form or shape produced by the gentle draping of a loose, full garment or piece of cloth: the fabric fell in soft folds. ∎  an area of skin that sags or hangs loosely. ∎  chiefly Brit. an undulation or gentle curve of the ground; a slight hill or hollow: the house lay in a fold of the hills. ∎  Geol. a bend or curvature of strata.2. a line or crease produced in paper or cloth as the result of folding it. ∎  a piece of paper or cloth that has been folded: a fold of paper slipped out of the diary.PHRASES: fold one's arms bring one's arms together and cross them over one's chest.fold one's hands bring or hold one's hands together.DERIVATIVES: fold·a·ble adj.fold² • n. a pen or enclosure in a field where livestock, esp. sheep, can be kept. ∎  (the fold) a group or community, esp. when perceived as the locus of a particular set of aims and values: he's performing a ritual to be accepted into the fold.• v. [tr.] shut (livestock) in a fold.

fold
1. A bend in rock strata or in any planar feature. The feature (e.g. bedding, cleavage, or layering) is deflected sideways and the amount and direction of dip is altered. Four principal regimes are responsible for folding: layer-parallel or lateral compression; differential vertical subsidence; differential shearing; and thrusting. In a simple anticline—syncline fold pair, an individual fold consists of a curved hinge zone and two planar limbs. An imaginary fold axis lies parallel to the hinge zone (line) and marks the intersection of the axial plane (or surface) with this zone. This basic geometric form gives rise to many fold profiles, including parallel, similar, concentric, open, and isoclinal fold types. To define the attitude of a fold accurately, the orientation of both the hinge line and the axial plane have to be measured. Varying orientations of the hinge line and axial plane may give rise to widely differing fold attitudes, thus vertical, upright and inclined (horizontal and plunging, see PLUNGE), and reclined and recumbent forms may be described.

2. In seismic reflection sampling, the number of offset distances which sample one common depth point. For example, if one CDP is sampled at 24 offset distances it is referred to as ‘24-fold’ coverage. The signals recorded for the CDP on each separate trace are then summed by stacking to improve the signal-to-noise ratio.

Fold

A fold is a bend in a body of rock or sediment that forms due to a change in pressure. Wavelike folds are composed of layers of 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 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 Alps,

stratified rock layers that originally covered an area about 300 miles (482 km) wide were squeezed together until they had a width of less than 120 miles (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, which are arched, upfolded strata that generally appear convex upward, and synclines, which are 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 them. At the core of an anticline lie stratigraphically older layers, with younger layers comprising the outermost layers. The opposite is true in a syncline, where the youngest layers form the core, with the oldest beds situated at the outside.

A line drawn along points of maximum curvature is called the axis. The inclined rock that lies on either side 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 steplike ridges rising from flat or gently sloping terrain.

As folding intensity increases, the 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, because they 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.

See also Fault; Tectonics; Unconformity.

Fold

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 .

See also Fault; Tectonics; Unconformity.

fold In geology, a bend in a layer of rock. An upfold is an anticline; a downfold a syncline. The line around which the rock is folded is termed the fold axis. The fold system may be symmetrical, asymmetrical, overturned or recumbent (with the axis of the fold horizontal). A single folding is a monocline.