Fig. 405.—Cross-section of [Fig. 404] along the line ab.
Fig. 406.—Complex folding. Section across the Alps from the neighborhood of Zürich toward Como: about 110 miles. (Heim and Prestwich.)
Fig. 407.—Generalized fan fold of the central massif of the Alps. (Heim.)
Fig. 408.—Intimate crumpling of beds near head of Sperry glacier, Mont. (Meyers.)
Fig. 409.—Intimate crumpling in detail, accompanied by faulting. Jasper Hill, Ishpeming, Mich. (Meyers.)
Much the larger portion of the earth’s surface is occupied by beds that retain nearly their original horizontal attitude; but in mountainous regions the beds have usually suffered bending, folding, crumpling, and crushing, in various degrees, in the course of the deformations that gave rise to the mountains. Distortion is on the whole most intense and characteristic in the most ancient rocks known, the Archean, in which a distorted condition is nearly universal, so far as observation goes. Distortion is assigned chiefly to lateral thrust arising from the shrinkage of the earth, as explained in the chapter on Earth Movements. The simpler, and some rather complex forms of deformation, are shown in the preceding figures, but the folding is sometimes much more complex ([Fig. 406]), the folds sometimes “fan” ([Fig. 407]), and the beds of which they are composed are sometimes intricately crumpled (Figs. [408] to [410]). Among these various phases of deformation there are all gradations and combinations. Overturned folds reverse the order of the strata in the under limb of the fold. After such folds have been greatly eroded, so that their outer form is lost and their relations have become obscure, the reversed beds are likely to be interpreted as though they lay in natural order. In such a case as that represented in [Fig. 411], a complex structure may be interpreted as a simple one. Thus the strata of [Fig. 411] may have the structure shown in [Fig. 412], [413], or [414], so far as dip and strike show.