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in geology, bent or deformed arrangement of stratified rocks. These rocks may be of sedimentary or volcanic origin. Although stratified rocks are normally deposited on the earth's surface in horizontal layers (see stratificationstratification
(Lat.,=made in layers), layered structure formed by the deposition of sedimentary rocks. Changes between strata are interpreted as the result of fluctuations in the intensity and persistence of the depositional agent, e.g.
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), they are often found inclined or curved upward or downward. Arches, or upfolds, in stratified rock are called anticlines; depressions or downfolds, synclines. A third type of fold, the monocline, is a steplike structure sloping in one direction only. It is more correctly called a flexure and generally passes at depth into a fracture called a fault. An imaginary line drawn along the crest of an anticline or the trough of a syncline is its axis; the two sides curving away from the axis are the limbs. If both limbs, dipping in opposite directions, make the same angle with the horizontal, and if an imaginary axial plane passed through the axis and the center of the fold is vertical and divides the fold into two equal halves, the fold is symmetrical; if the limbs make unequal angles, and if the axial plane is inclined and does not bisect the fold, the fold is asymmetric. If one limb lies partly under the other, and the axial plane is inclined, the fold is overturned; if one limb lies almost completely under the other, and the axial plane is almost horizontal, the fold is recumbent. The axis of a fold cannot be indefinitely extended parallel to the horizontal, but plunges or emerges as the fold tapers off to a plane. Certain domes are very short anticlines with axes plunging at both ends, while some basins, similarly, are synclinal structures. Folds are commonly formed at some distance below the surface, but complete folds or portions of folds are exposed by erosion. Anticlines frequently have their crests eroded, till only the worn-down stumps of the two limbs remain. In a similar manner synclines may be eroded so that only the edges of the limbs project above the surface. The ridge crests of the Appalachian Mts. are eroded limbs of folds. The nature of the original fold can generally be determined from the arrangement of the outcrops, or exposed portions; thus, two outcrops dipping toward each other mark a syncline, and two outcrops dipping away from each other, an anticline. Folds on a grand scale, extending, for example, most of the length of a continent, are known as geosynclines and geanticlines. The immediate cause of folding is generally conceded to be the horizontal compression of the earth's surface, anticlines being squeezed up by this compression and synclines formed between anticlines. The problem of the ultimate cause of fold formation is similar to that of fault formation, both being earth movements involved in mountainmountain,
high land mass projecting conspicuously above its surroundings and usually of limited width at its summit. Although isolated mountains are not unusual, mountains commonly form ranges, comprising either a single complex ridge or a series of related ridges.
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 building and plate tectonicsplate tectonics,
theory that unifies many of the features and characteristics of continental drift and seafloor spreading into a coherent model and has revolutionized geologists' understanding of continents, ocean basins, mountains, and earth history.
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. Porous and permeable rocks of anticlines often contain oil and gas reservoirs. Organic remains of late Paleozoic tree fern swamps were converted to anthracite coal during the folding of the Appalachian Mts.



(also flexure), a tectonic structure in which a stratum or bed is strongly bent in two opposite directions. A fold consists of five elements, namely, two bends and three limbs. Two of the limbs are outside the bends; the third, or middle, limb connects the bends. Each element is described by its own bedding parameters, and the relationships between the parameters determine the many varieties of folds.

According to the bedding of the limbs, we distinguish among simple, plunging, and cross folds; according to the inclination of the hinges of the bends, folds are classified as vertical, inclined, and nonplunging (Figure 1). The size of a fold ranges from fractions of a meter to many kilometers. The inclination of the limbs ranges from barely discernible to vertical.

Figure 1.

Note on Figure 1. Location of the strata in the major types of folds:, (a) simple fold, (b) plunging fold, (c) cross fold, (d) nonplunging fold, (e) inclined fold, (f) vertical fold; the symbol Fold indicates that the strata are horizontal, the symbol Fold shows the direction in which the strata dip

Folds are found in cratons and folded regions. Especially large folds are found along the margins of cratons and the edges of syneclises. Some folds affect the accumulation of sediment and determine the distribution of the facies and thicknesses of sedimentary strata. Folds are sometimes associated with petroleum deposits.


A plication or doubling, as of various parts of the body such as membranes and other flat surfaces.
A bend in rock strata or other planar structure, usually produced by deformation; folds are recognized where layered rocks have been distorted into wavelike form.


1. a hollow in undulating terrain
2. Geology a bend in stratified rocks that results from movements within the earth's crust and produces such structures as anticlines and synclines
3. Anatomy another word for plica


a church or the members of it


An invisible line on a Web page that is at the bottom of the first full page on screen. See above the fold.
References in classic literature ?
So seem'd Farr off the flying Fiend: at last appeer Hell bounds high reaching to the horrid Roof, And thrice threefold the Gates; three folds were Brass Three Iron, three of Adamantine Rock, Impenitrable, impal'd with circling fire, Yet unconsum'd.
The goatherd gave the goat which he held by the horns a couple of slaps on the back, saying, "Lie down here beside me, Spotty, for we have time enough to return to our fold.
Tahiti is a lofty green island, with deep folds of a darker green, in which you divine silent valleys; there is mystery in their sombre depths, down which murmur and plash cool streams, and you feel that in those umbrageous places life from immemorial times has been led according to immemorial ways.
At nightfall he drove his flock back to the fold, saw that the gate was secure and retired to his cave for refreshment and for dreams.
On the other hand, I have continued to fold my clothes, and to keep my little diary.
He placed it on the table, opened the last fold only, and kept his hand on the rest.
Try to get all the sleep you can, for we must start early to avoid the heat of the day," and there was the sound of the guide leaving and arranging the folds of the mosquito net behind him to keep out the night- flying insects.
Unnumbered waves Have broidered with green moss the marble folds About her feet.
This precious commodity they brought to the house, enveloped in multitudinous folds of leaves; and as a special mark of the esteem in which they held me, would spread an immense leaf on the ground, and dropping one by one a few minute particles of the salt upon it, invite me to taste them.
The visitor's daughter was already smoothing down her dress with an inquiring look at her mother, when suddenly from the next room were heard the footsteps of boys and girls running to the door and the noise of a chair falling over, and a girl of thirteen, hiding something in the folds of her short muslin frock, darted in and stopped short in the middle of the room.
Swiftly it enveloped her in its embrace, covering her completely in its thick folds, and then it swayed back upon its stem.
In the first place, sir,' said Mr Tappertit, producing a small pocket-handkerchief and shaking it out of the folds, 'as I have not a card about me (for the envy of masters debases us below that level) allow me to offer the best substitute that circumstances will admit of.