Besides the anticlinal and synclinal folds already explained, there are folds that slope in only one direction, one-sided or _monoclinal_ folds (Fig. 23). Anticlinal and synclinal folds are _symmetrical_ when the dip or slope of the strata is the same on both sides and the axial plane is vertical. The great majority of folds, however, are _unsymmetrical_, the opposite slopes being unequal, and the axial planes inclined to the vertical (Fig. 24, _A_). This means that the compressing or plicating force has been greater from one side than from the other, as indicated by the arrows. It acted with the greatest intensity on the side of the gentler slope, the tendency evidently having been to crowd or tip the fold over in the direction of the steep slope. When the steep slope approaches the vertical, this tendency is almost unresisted, and when it passes the vertical, gravitation assists in overturning the fold (Fig. 24, _B_). Such highly unsymmetrical folds, including all cases where the two sides of the fold slope in the same direction, are described as _overturned_ or _inverted_, although the latter term is not strictly applicable to the entire fold, but only to the strata composing the under or lee side of it. Fig. 24, _B_, shows that these beds are completely inverted, the older, as the figures indicate, lying conformably upon the newer. This inversion is one of the most important features of folded strata, and it has led to many mistakes in determining their order of succession. In the great mountain-chains, especially, it is exhibited on the grandest scale, great groups of strata being folded over and over each other as we might fold carpets. An inverted stratum is like a flattened S or Z, and may be pierced by a vertical shaft three times, as has actually happened in some coal mines. Folds are _open_ when the sides are not parallel, and _closed_ when they are parallel, the former being represented by a half-open, and the latter by a closed, book. Closed folds are usually inverted, and when the tops have been removed by erosion (Fig. 25), the repetition of the strata may escape detection, and the thickness of the section be, in consequence, greatly overestimated. Thus, a geologist traversing the section in Fig. 25 would see thirty-two strata, all inclined to the left at the same angle, those on the right apparently passing below those on the left, and all forming part of one great fold. The repetition of the strata in reverse order, as indicated by the numbers, and the structure below the surface, show, however, that the section really consists of only four beds involved in a series of four closed folds, the true thickness of the beds in this section being only one-eighth as great as the apparent thickness. [Illustration: Fig. 25.—Series of closed folds.]