Fig. 21.—Diagrams representing a, an anticline; b, a syncline; and c, a monocline.
Any flexuring of the strata implies a reduction of their surface area, or, considering a single section, a shortening. If the arches and troughs are low and broad, the deformation of the strata is slight, the shortening is comparatively small, and the folds are described as open ([Fig. 22 b]). If they be relatively both high and narrow, the deformation is considerable, a larger amount of crustal shortening has gone on, and the folds are described as close ([Fig. 22 c]). This closing up of the folds may continue until their sides have practically the same slope, in which case they are said to be isoclinal ([Fig. 22 d]).
Fig. 22.—A comparison of folds to express increasing degrees of crustal shortening or progressive deformation within the zone of flow: a, stratum before folding; b, open folds; c, close folds; d, isoclinal folds.
The elements of folds.—Folds must always be thought of as having extension in each of the three dimensions of space ([Fig. 23]), and not as properly included within a single plane like the cross sections which we so often use in illustration. A fold may be conceived of as divided into equal parts by a plane which passes along the middle of either the arch or the trough, and is called the axial plane. The line in which this plane intersects the arch or the trough is the axis, which may be called the crestline in an anticline, and the troughline in a syncline.
In the case of many open folds the axis is practically horizontal, but in more complexly folded regions this is seldom true. The departure of the axis from the horizontal is called the pitch, and folds of this type are described as pitching folds or plunging folds. The axis is in reality in these cases thrown into a series of undulations or “longitudinal folds”, and hence pitch will vary along the axis.
Fig. 23.—Anticlinal and synclinal folds in strata (after Willis).
Fig. 24.—Diagrams to illustrate the different shapes of rock folds.