FOLD, a pleat or bend in a flexible material, or a curve in any surface, whence its particular application in geology with which this article deals. The verb "to fold" (O. Eng. fealdan) meant originally to double back a piece of cloth or other material so as to form a pleat, whence has evolved its various senses of to roll up, to enclose, enfold or embrace as with the arms, to clasp the hands or arms together, etc. The word is common to Teutonic languages, cf. Ger. falten, Dutch vouwen (for vouden), etc., and the ultimate Indo-European root is found in Gr., Lat. plicare, plectere, to plait, pleat, weave, and in the suffixes of such words as , duplex, double, simplex, etc. Similarly the termination "-fold" is added to numbers implying "so many," e.g. twofold, hundredfold, cf. "manifold." The similar word for an enclosure or pen for animals, especially for sheep, and hence applied in a spiritual sense to a community of worshippers, or to the whole body of Christians regarded as Christ's flock, must be distinguished. In O. Eng. it is falæd, and cognate forms are found in Dutch vaalt, etc. It apparently meant a planked or boarded enclosure, cf. Dan. fjael, Swed. fjöl, plank.
In geology, a fold is a bend or curvature in the stratified rocks of the earth's crust, whereby they have been made to take up less horizontal space. The French equivalents are pli, plissement, ridement; in Germany, Falte, Faltung, Sattelung are the terms usually employed. It is comparatively rarely that bedded rocks are observed in the position in which they were first deposited, a certain amount of buckling up or sagging down of the crust being continually in progress in one region or another. In every instance therefore where, in walking over the surface, we traverse a series of strata which gradually, and without dislocations, increase or diminish in inclination, we cross part of a great curvature in the strata of the earth's crust.
Such foldings, however, can often be distinctly seen, either on some cliff or coast-line, or in the traverse of a piece of hilly or mountainous ground. The observer cannot long continue his researches in the field without discovering that the rocks of the earth's crust have been almost everywhere thrown into curves, usually so broad and gentle as to escape observation except when specially looked for. The outcrop of beds at the surface is commonly the truncation of these curves. The strata must once have risen above the present surface, and in many cases may be found descending to the surface again with a contrary dip, the intervening portion of the undulation having been worn away.
Fig. 1. - Section of the Isle of Wight - a Monoclinal Curve, a, Chalk; b, Woolwich and Reading beds; c, London clay; d, Bagshot series; e, Headon series; f, g, Osborne and Bembridge series.
The curvature occasionally shows itself among horizontal or gently inclined strata in the form of an abrupt inclination, and then an immediate resumption of the previous flat or sloping character. The strata are thus bent up and continue on the other side of the tilt at a higher level. Such bends are called monoclines, monoclinal folds or flexures, because they present only one fold, or one half of a fold, instead of the two which we see in an arch or trough. The most notable instance of this structure in Britain is that of the Isle of Wight, of which a section is given in fig. 1. The Cretaceous rocks on the south side of the island rapidly rise in inclination till they become nearly vertical. The Lower Tertiary strata follow with a similar steep dip, but rapidly flatten down towards the north coast. Some remarkable cases of the same structure have been brought to light by J.W. Powell in his survey of the Colorado region.
Fig. 2. - Plan of Anticlinal and Synclinal Folds.
It much more frequently happens that the strata have been bent into arches and troughs, so that they can be seen dipping under the surface on one side of the axis of a fold, and rising up again on the other side. Where they dip away from the axis of movement the structure is termed an anticline or anticlinal fold; where they dip towards the axis, it is a syncline or synclinal fold. The diagram in fig. 2 may be taken to represent a series of strata (1-17) thrown into an anticline (AA') and syncline (BB'). A section drawn across these folds in the line CD would show the structure given in fig. 3. Here we see that, at the part of the anticlinal axis (A) where the section crosses, bed No. 4 forms the crown of the arch, Nos. 1, 2 and 3 being concealed beneath it. On the east side of the axis the strata follow each other in regular succession as far as No. 13, which, instead of passing here under the next in order, turns up with a contrary dip and forms the centre of a trough or syncline (B). From underneath No. 13 on the east side the same beds rise to the surface which passed beneath it on the west side. The particular bed marked EF has been entirely removed by denudation from the top of the anticline, and is buried deep beneath the centre of the syncline.
Fig. 3. - Section of Anticlinal and Synclinal Folds on the line CD (fig. 2).
Such foldings of strata must always die out unless they are abruptly terminated by dislocations. In the cases given in fig. 2, both the arch and trough are represented as diminishing, the former towards the north, the latter towards the south. The observer in passing northwards along the axis of that anticline finds himself getting into progressively higher strata as the fold sinks down. On the other hand, in advancing southwards along the synclinal axis, he loses stratum after stratum and gets into lower portions of the series. When a fold diminishes in this way it is said to "nose out." In fig. 2 there is obviously a general inclination of the beds towards the north, besides the outward dip from the anticline and the inward dip from the syncline. Hence the anticline noses out to the north and the syncline to the south.
Simple Folds. - In describing rock-folds special terms have been assigned to certain portions of the fold; thus, the sloping sides of an anticline or syncline are known as the "limbs," "slopes," "flanks" or "members" of the fold; in an anticline, the part X, fig. 3, the angle of the bend, is the "crest" or "crown" (Ger. Gewölbebiegung, Fr. charnière anticlinale), the corresponding part of a syncline being the "trough-core" or "base," Y, fig. 3 (Ger. Muldenbiegung, Fr. charnière synclinale). The portion of an anticline which has been removed by denudation is the "aerial arch," dotted in fig. 3. The innermost strata in a fold constitute the "core," arch-core A, fig. 3, or trough-core B, in the same figure. In the majority of folds the bending of the strata has taken place about an "axial plane" (often called the "axis"), which in the examples illustrated in fig. 3 would pass through the points A and B, perpendicularly to the horizontal line CD. In powerfully folded regions the axial planes of the folds are no longer upright; they may be moderately inclined, producing an "inversion," "inverted fold" or "overfold." When the inclination of the axial plane is great a "recumbent overfold" is produced (Fr. pli couché, Ger, liegende Falte). In a fold of this kind (fig. 4) we have an "arch limb" (a), a middle limb (b) and a floor or "trough limb" (c). X and Y are the upper and lower bends respectively. One of the important functions of a fold is its direction; this of course depends upon the orientation of the axial plane. The crest-line of an anticline or trough-line of a syncline is rarely horizontal for any great distance; its departure from horizontality is designated the "pitch," and the fold is said to pitch (or dip) towards the north, etc. Most simple folds - with the exception of very shallow curvatures of wide area, - when considered in their entirety, are seen to be somewhat canoe-shaped in form. There are three variations of the simple fold dependent upon the position of the limbs, (1) the limbs may tend to diverge as they recede from the crest (fig. 3), sometimes styled an "open anticline"; (2) the limbs may be parallel in "closed" folds (commonly known as isoclinal folding); (3) the limbs may make an open angle or widen out towards the crest (fig. 4). This is known as a fan-shaped fold (Fr. pli en éventail, Ger. Fächerfalte); another variant of the same form is the mushroom fold (Fr. pli en champignon). The axial plane is not always extended: it may be so abbreviated that the folding appears to have taken place about a point; anticlines of this type are variously designated "short-anticlines," "brachyanticlinaux" or "domes"; similarly, there are "short-synclines," "brachysynclinaux" or "cuvettes." The dip in cases of this kind has been described as "qua-qua versal" or "periclinal."
Complex Folding. - Sometimes a simple fold has been itself subjected to further folding repeated more than once, it is then termed a "refolded fold" (Fr. pli replié); fine examples may be observed in the Alps and in other mountain chains. A great regional major fold containing within itself a number of minor "special" or subsidiary folds is described as a "geanticline" (Fr. structure en éventail composé), or as a "geosyncline" (Fr. structure en éventail renversé). Even folds of lesser magnitude may be highly complex in regions of extreme crustal movement, and may contain smaller folds of the first, second, third or higher order (Fr. couches gaufrées [fig. 5]). In its smaller manifestation, this class of folding passes into "crumpling" or "puckering," where quite a large number of folds may be crowded into a single hand specimen. In "frilling" or "frilled structure" the folds have still smaller amplitude, and in many highly corrugated rocks minute folds are observable with the microscope that do not appear to the unaided eye. When a series of adjacent isoclinal overfolds has passed into a series of thrusts (see Fault), the so-called "imbricated" structure (Fr. structure imbriquée, Ger. Schuppenstruktur) is generated. Occasionally crust-blocks resembling "graben" and "horsts" are circumscribed by folds instead of faults; when this is so they have been called respectively "infolded graben" or "overfolded horsts."
The heterogeneous character of great masses of strata has always had a marked influence on the nature of the folding; some beds have yielded much more readily than others, certain beds will be found to be faulted, while those above and below have folded without fracture. In many examples of apparent plasticity it can be shown that this effect has been produced by an infinite number of minute slippings within the rock substance.
Fig. 5. - Curved and Contorted Rocks, near Old Head of Kinsale. (Du Noyer.)
The larger rock folds have produced important economic results. For example, in many coal regions the deposits have been conserved in some districts in the synclines or "basins," while they have been removed by denudation from the uplifted anticlines in others. Near the crest of anticlines is commonly an enriched portion of the ground in mineralized districts; and, in the case of water supply, the tilt of the strata determines the direction of the underground flowage. Again, the most convenient site for oil wells is the crest of an anticline or "dome," where an impervious stratum imprisons the gas and oil in a subjacent saturated layer under pressure.
For a discussion of the question of the distribution and arrangement of the great folded regions of the earth's crust, see E. Suess, Das Antlitz der Erde, English translation. The Face of the Earth, vols. i., ii., iii., iv. (Oxford). See also E. de Margerie and A. Heim, Les Dislocations de l'écorce terrestre (Zurich, 1888); A. Rothpletz, Geotektonische Probleme (Stuttgart, 1894).
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)