=The arches and troughs of the folded strata.=—Every series of folds is made up of alternating arches and troughs. The arches of the strata the geologist calls _anticlines_ or _anticlinal folds_, and the troughs he calls _synclines_ or _synclinal folds_ (Fig. 21). When a stratum is merely dropped in a bend to a lower level without producing a complete arch or a complete trough, this half fold is termed a _monocline_.
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.