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Bible Encyclopedias
Fold
1911 Encyclopedia Britannica
or 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, &c. The word is common to Teutonic languages, cf. Ger. falten, Dutch vouwen (for vouden), &c., and the ultimate Indo-European root is found in Gr. 7 Micci', Lat. plicare, plectere, to plait, pleat, weave, and in the suffixes of such words as borXauws, duplex, double, simplex, &c. 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 faked, and cognate forms are found in Dutch vaalt, &c. It apparently meant a planked or boarded enclosure, cf. Dan. fjael, Swed. fjol, 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.
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 mono- FIG. I. - 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.
dines, 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. I. 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.
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 FIG. 2. - Plan of Anticlinal and Synclinal Folds.
(I-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. Section on line C D. FIG. 3. - Section of Anticlinal and Synclinal Folds on the line CD (fig. 2).
anticlinal axis (A) where the section crosses, bed No. 4 forms the crown of the arch, Nos. I, 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.
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 the71orth, 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.
