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Originally appearing in Volume V10, Page 598 of the 1911 Encyclopedia Britannica.
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FOLD , a pleat or See also:

bend in a flexible material, or a See also:curve in any See also:surface, whence its particular application in See also:geology with which this See also:article deals. The verb " to fold " (O. Eng. fealdan) meant originally to See also:double back a piece of See also:cloth or other material so as to See also:form a pleat, whence has evolved its various senses of to See also:roll up, to enclose, enfold or embrace as with the arms, to clasp the hands or arms together, &c. The word is See also:common to See also:Teutonic See also:languages, cf. Ger. fallen, Dutch vouwen (for vouden) , &c., and the ultimate Indo-See also:European See also:root is found in Gr. 7rXEKEIV, See also:Lat. plicare, plectere, to See also:plait, pleat, weave, and in the suffixes of such words as SLTrXaatos, duplex, double, simplex, &c. Similarly the termination " -fold " is added to See also:numbers implying " so many," e.g. twofold, hundredfold, cf. " manifold." The similar word for an enclosure or See also:pen for animals, especially for See also:sheep, and hence applied in a spiritual sense to a community of worshippers, or to the whole See also:body of Christians regarded as See also:Christ's See also:flock, must be distinguished. In O. Eng. it is faked, and cognate forms are found in Dutch vault, &c. It apparently meant a planked or boarded enclosure, cf. See also:Dan. fjael, Swed. fjol, See also:plank.

In geology, a fold is a bend or curvature in the stratified rocks of the See also:

earth's crust, whereby they have been made to take up less See also:horizontal space. The See also:French equivalents are pli, plissement, ridement; in See also:Germany, Falte, Faltung, Saltelung 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 See also:traverse a See also:series of strata which gradually, and without dislocations, increase or diminish in inclination, we See also:cross See also:part of a See also:great curvature in the strata of the earth's crust. Such foldings, however, can often be distinctly seen, either on some cliff or See also:coast-See also:line, or in the traverse of a piece of hilly or mountainous ground. The observer cannot See also:long continue his researches in the See also:field without discovering that the rocks of the earth's crust have been almost everywhere thrown into curves, usually so broad and See also:gentle as to See also: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 See also:present surface, and in many cases may be found descending to the surface again with a contrary See also:dip, the intervening portion of the undulation having been worn away. 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 See also:flat or sloping See also:character. The strata are thus See also:bent up and continue on the other See also:side of the tilt at a higher level. Such bends are called mono- clines, monoclinal folds or flexures, because they present only one fold, or one See also:half of a fold, instead of the two which we see in an See also:arch or trough. The most notable instance of this structure in See also:Britain is that of the Isle of See also:Wight, of which a See also:section is given in fig.

I. The Cretaceous rocks on the See also:

south side of the See also:island rapidly rise in inclination till they become nearly See also:vertical. The See also:Lower See also:Tertiary strata follow with a similar steep dip, but rapidly flatten down towards the See also:north coast. Some remarkable cases of the same structure have been brought to See also:light by J. W. See also:Powell in his survey of the See also:Colorado region. It much more frequently happens that the strata have been bent into See also:arches and troughs, so that they can be seen dipping under the surface on one side of the See also:axis of a fold, and rising up again on the other side. Where they dip away from the axis of See also:movement the structure is termed an anticline or anticlinal fold; where they dip towards the axis, it is a syncline or synclinal fold. The See also: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 See also: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 Section on line C D. (fig. 2). anticlinal axis (A) where the section crosses, See also:

bed No. 4 forms the See also:crown of the arch, Nos. 1, 2 and 3 being concealed beneath it, On the See also:east side of the axis the strata follow each other in See also:regular See also:succession as far as No. 13, which, instead of passing here under the next in See also: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 See also:west side. The particular bed marked EF has been entirely removed by denudation from the See also:top of the See also:anti-cline, and is buried deep beneath the centre of the syncline. Such foldings of strata must always See also: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 See also:

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 " See also:nose out." In fig. 2 there is obviously a See also: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. See also:Simple Folds.—In describing See also:rock-folds See also: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 See also:angle of the bend, is the " See also:crest " or " crown " (Ger. Gewolbebiegung, Fr. charniere anticlinale), the corresponding part of a syncline being the " trough-core " or " See also:base," Y, fig. 3 (Ger. Muldenbiegung, Fr. charniere 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 See also:

majority of folds the bending of the strata has taken See also:place about an " axial See also: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 " See also:inversion," " inverted fold " or " overfold." When the inclination of the axial plane is great a " recumbent overfold " is produced (Fr. pli couche, Ger. liegende Falte). In a fold of this See also:kind (fig. 4) we have an " arch See also:limb " (a), a See also:middle limb (b) and a ,;:x See also: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 See also: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 " See also:pitch," and the fold is said to pitch (or dip) towards the north, &c. Most simple folds—with the exception of very shallow curvatures of wide See also:area,—when considered in their entirety, are seen to be somewhat See also:canoe-shaped in form.

There are three See also:

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 See also:fan-shaped fold (Fr. pli en eventail, Ger. Facherfalte); another variant of the same form is the See also:mushroom fold (Fr. pli en chantpignon). 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 " See also: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 replie); See also:fine examples may be observed in the See also:Alps and in other See also:mountain chains. A great regional See also:major fold containing within itself a number of See also:minor " special " or subsidiary folds is described as a " geanticline (Fr. structure en eventail compose), or as a " geosyncline " (Fr. structure en eventail renverse). 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 gaufrees [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 See also:amplitude, and in many highly corrugated rocks See also:minute folds are observable with the See also:microscope that do not appear to the unaided See also:eye. When a series of adjacent isoclinal overfolds has passed into a series of thrusts (see See also:FAULT), the so-called " imbricated " structure (Fr. structure imbriquee, 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 See also:

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 See also:infinite number of minute slippings within the rock substance. The larger rock folds have produced important economic results. For example, in many See also: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 See also:case of See also:water See also:supply, the tilt of the strata determines the direction of the underground flowage. Again, the most convenient site for oil See also:wells is the crest of an anticline or " See also:dome," where an impervious stratum imprisons the See also:gas and oil in a subjacent saturated layer under pressure. For a discussion of the question of the See also:distribution and arrangement of the great folded regions of the earth's crust, see E. See also:Suess, Das Antlitz der Erde, See also:English See also:translation, The See also:Face of the Earth, vols. i., ii., iii., iv. (See also:Oxford). See also E. de Margerie and A. See also:Heim, See also:Les Dislocations de l'ecorce terrestre (See also:Zurich, 1888) ; A.

Rothpletz, Geotektonische Probleme (See also:

Stuttgart, 1894).

End of Article: FOLD

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