Fig. 18.—Anticlinal and synclinal folds.
The imaginary line passing longitudinally through a fold, about which the strata appear to be bent, is the axis; and the plane lying midway between the two sides of a fold and including the axis is the axial plane. The two principal kinds of folds are the anticline ([Fig. 18], A), where the strata dip away from the axis; and the syncline ([Fig. 18], B), where they dip toward the axis. They are commonly, but not always, correlative, like hill and valley.
Rock-folds are of all sizes, from almost microscopic wrinkles to great arches miles in length and breadth, and thousands of feet in height. The smaller folds, or such as may be seen in hand specimens and even in considerable blocks of stone, are commonly called contortions, and it is interesting to observe that they are, in nearly everything except size, precisely like the large folds, so that they answer admirably as geological models. Large folds, however, are almost necessarily curves, but contortions are frequently angular ([Fig. 19]). With folds, as with waves, the small undulations are borne upon the large ones; but the contortions are not uniformly distributed. An inspection of [Fig. 18] shows that when the rocks are folded they must be in a state of tension on the anticlines (A), and in a state of compression in the synclines (B), and the latter is evidently the normal position of the puckerings or contortions of the strata, as shown in [Fig. 20]. Contortions are also most commonly found in thin-bedded, flexible rocks, such as shales and schists. And when we find them in hard, rigid rocks, like gneiss and limestone, it must mean either that the structure was developed with extreme slowness, or that the rock was more flexible then and possibly plastic.
Fig. 19.—Contorted strata.
Fig. 20.—Contorted syncline.
Fig. 21.—Section of anticlinal mountains.