FOLD, a pleat or bend in a flexible material. The termination "-fold" added to numbers, as hundredfold, is of the same origin (O.Eng. fealdan), but fold in the sense of an enclosure for ani mals and hence applied to a community of worshippers has another root, apparently meaning a planked or boarded enclosure (O.Eng. faloed).
With but a few exceptions all sedimentary rocks have been formed from the deposition of material, carried down from land masses, in horizontal or very nearly horizontal sheets. Since the earth's crust has always been in a state of movement in one region or another, the stresses set up are relieved in various ways such as fracture (see FAULT), sagging down, buckling up and crumpling. Except then, in comparatively recent strata, one rarely finds the stratified rocks lying in the position in which they were laid down. The sheets are folded and fractured to any degree, from a condition when they show a hardly perceptible inclination to the horizontal to one when they have been con torted and folded over upon themselves so as to give rise to extremely complicated attitudes. Such curved and folded strata are rarely seen except on a small scale in cliff sections from any one particular view-point : pari passu with folding on a large scale denudation has been at work to plane down the land masses formed by such arching of the strata, so that what one observes are more or less highly inclined beds outcropping at the surface in different parts of the folded region, these beds being the truncated limbs of the folds.
In describing folds certain terms are used. Fig. i shows dia grammatically a simple system of folding; strata curved upwards into an arch, giving rise to a simple anticlinal fold or anticline, and curved downwards into a trough, a simple synclinal fold or syncline. The axis of folding is an imaginary line along the crest or the trough and defines the direction of folding. The axial plane divides the fold symmetrically along its axis; in a simple sym metrical fold it will be vertical, but in an asymmetrical fold it will be inclined and may cease to be a plane, becoming a curved surface. On either side of the fold the inclined strata form the flanks, limbs or slopes. The innermost strata or rock mass form the core. No fold can have an axis of indefinite length, so that it is found that folds die away longitudinally somewhat after the manner of the rumpling which can be produced with the hand in a small area of a table cloth; though the stresses which give rise to folding in stratified rocks may find relief in some other fashion than folding beyond the margin of these rocks, and hence a fold may terminate differently. Most folds are therefore somewhat like an inverted feeding trough with tapering ends, and the axis, following the crest, will plunge downwards. The anticline is here said to pitch or dip in a certain direction, and the angle of inclina tion to the horizon of the axis at any point is called the pitch.
Folds may assume many forms, simple and complex, sym metrical and asymmetrical. There is the simple anticline (and syncline) where the axial plane is vertical; the asymmetrical form where the axial "plane" is inclined or a curved surface. A dome is a special case of the symmetrical anticline : here there is no axis and folding appears to have taken place round a point. The dip of the strata around this point is said to be quaquaversal or periclinal. The converse of this structure, basin shaped, is called a centro-clinal fold. Both these forms are somewhat rare. A monocline is a fold with only one limb : it passes generally into a fault. The axial plane may be inclined at any angle, from the case where the asymmetry is hardly perceptible to the case where it curves into a horizontal position : if the axial plane is inclined at such an angle that the strata of one limb are reversed, the structure is called an over f old, and when one limb is horizontal, a recumbent fold. In regions of severe folding, a number of axial planes inclined in the same direction give rise to isoclinal folding. On a large scale, the earth's crust may be arched up into a broad anticlinal fold, a geanticline, and if numerous subordinate flexures are present this is called an anticlinorium. The converse, resulting from depression, is a geosyncline and a synclinorium. A fan structure is a great anticline flanked by one or more asymmetrical folds whose axial planes incline inwards. Over wide areas, but on a smaller individual scale, strata are frequently found to be folded in such a complex and irregular manner that they are said to be contorted. Great compression has caused the beds to behave as if they were plastic, since they thin out in the limbs and swell out in the arches. Not only is this seen in large cliff sections but the same occurs in hand specimens and even in rock slices under the microscope. Constituents of rocks, like pebbles in conglomer ates are drawn into lenticular shapes and fossils are distorted.