The shapes of rock folds.—By the axial plane each fold is divided into two parts which are called its limbs, which may have either the same or different average inclinations. To describe now the shapes of rock folds and not the degree of compression of the district, some additional terms are necessary. Anticlines or synclines whose limbs have about the same inclinations are known as upright or symmetrical folds. The axial plane of the symmetrical fold is vertical ([Fig. 24]). If this plane is inclined to the vertical, the folds are unsymmetrical. So soon as the steeper of the two limbs has passed the vertical position and inclines in the same direction as the flatter limb, the fold is said to be overturned. The departure from symmetry may go so far that the axial plane of the fold lies at a very flat angle, and the fold is then said to be recumbent. The observant traveler by train along any of the routes which enter the Alps may from his car window find illustrations of most of these types of rock folds, as he may also, though generally less easily, in passing through the Appalachian Mountains.
Fig. 25.—Secondary and tertiary flexures superimposed upon the primary ones.
In regions which have been closely folded the larger flexures of the strata may be found with folds of a smaller order of magnitude superimposed upon them, and these in turn may show crumplings of still lower orders. It has been found that the folds of the smaller orders of magnitude possess the shapes of the larger flexures, and much is therefore to be learned from their careful study ([Fig. 25]). It is also quite generally discovered that parallel planes of ready parting, which are described as rock cleavage, take their course parallel to the axial plane within each minor fold. As was long ago shown by the pioneer British geologists, these planes of cleavage are essentially parallel and follow the fold axes throughout large areas.
Plate 2.
A. Layers compressed in experiments and showing the effect of a competent layer in the process of folding (after Willis).
B. Experimental production of a series of parallel thrusts within closely folded strata (after Willis).
