The two most common types of folds are anticlines and synclines (Figure 1). Anticlines occur when compressional stresses squeeze sedimentary layers into arch-like folds. These upfolded structures are sometimes spectacularly displayed along highway roadcuts that pass through deformed strata. Typically found in association with anticlines are trough-like structures called synclines. Notice in Figure 1 that the right limb of the syncline is also the left limb of the adjacent anticline.
Depending on their orientation, these basic folds are described as symmetrical when the limbs are mirror images of each other and asymmetrical when they are not. An asymmetrical fold is said to be overturned if one or both limbs are tilted beyond the vertical. An overturned fold can also “lie on its side” so the axial plane is horizontal. These recumbent folds are common in highly deformed mountainous regions such as the Alps. Folds also can be tilted by tectonic forces that cause their hinge lines to slope rather than have a horizontal orientation (Figure 2A). Folds of this type are called plunging folds because the hinge lines of the fold dip downward (plunge) and penetrate Earth’s surface. Figure 2B shows the outcrop pattern produced when erosion removes the upper layers of a plunging anticline and exposes its interiors. Note that the outcrop pattern of an eroded anticline “points” in the direction it is plunging. The opposite is true for a syncline, which exhibits an outcrop pattern that points in the opposite direction of its plunge.
A good example of topography that results when erosional forces attack folded sedimentary strata is found in the Valley and Ridge Province of the Appalachians (Figure 3). It is important to recognize that ridges are not necessarily associated with anticlines, nor are valleys related to synclines. Rather, ridges and valleys result because of differential weathering and erosion. In the Valley and Ridge Province, for instance, resistant sandstone beds remain as imposing ridges separated by valleys cut into more easily eroded shale or limestone beds.
Authors: E.J. Tarbuck, F.K. Lutgensn illustrated byD. Tasa