Fig. 392.—Syncline, C. & O. canal, 3 miles west of Hancock, Md. The beds are shale and sandstone near base of the Silurian. (Walcott, U. S. Geol. Surv.)
When the beds incline in a single direction, they form a monocline. When beds are arched so as to incline away from one another, they form an up-fold or anticline (Figs. [388] to [391]). The anticline may depart from its simple form, as shown in Figs. [390] and [391]. When beds are curved downward so as to incline towards one another, they form a syncline ([Fig. 392]). When beds assume the position shown in [Fig. 393], the folds are said to be isoclinal. When they are arched so as to form a cone or dome, and incline in all directions from a central point, they are said to have a quaquaversal dip. When considerable tracts are bent so as to form great arches or great troughs with many minor undulations on the flanks of the larger, they are designated as geanticlines, or anticlinoria (Figs. [394] and [395]), and geosynclines or synclinoria (Figs. [396] and [397]). Folding is often accompanied by the development of slaty cleavage ([p. 440]).
Fig. 393.—Isocline. (Van Hise, U. S. Geol. Surv.)
Fig. 394.—Anticlinorium: diagrammatic. (Van Hise, U. S. Geol. Surv.)
Fig. 395.—Anticlinorium. General section in the central massif of the Alps. (Heim.)
Fig. 396.—Synclinorium: diagrammatic. (Van Hise, U. S. Geol. Surv.)