When a body of rock is squeezed from the sides by tectonic forces, it is likely to fracture if it is cold and brittle. However, if it is warm enough and/or under enough pressure, it will behave in a ductile manner instead.
The terms that describe different parts of a fold are illustrated in Figure 13.6. The axial plane is an imaginary surface that slices through a fold along its hinge, where the folding is tightest. The sloping beds on either side of the axial plane are the limbs of the fold. The general region around the hinge is the hinge zone, and the line that is formed by the axial plane cutting into the hinge zone is called the hinge line.
Folds are classified according to which way the hinge points. If it points up the fold is called an anticline. If it points down the fold is called a syncline. Some sequences of rocks are folded into a single anticline or syncline, but in other areas it is possible to find a series of anticlines and synclines in a fold train (as in Figure 13.7). An anticline or syncline is described as symmetrical if the angles between each of limb and the axial plane are generally similar, and asymmetrical if they are not. If the axial plane is sufficiently tilted that the beds on one side have been tilted past vertical, the fold is known as an overturned anticline or syncline.
A very tight fold, in which the limbs are parallel or nearly parallel to one another is called an isoclinal fold (Figure 13.8). Isoclinal folds that have been overturned to the extent that their limbs are nearly horizontal are called recumbent folds.
Folds in the Landscape
Folds can be of any size, and it’s very common to have smaller folds within larger folds (Figure 13.9). Large folds can have wavelengths of tens of kilometres, and very small ones might be visible only under a microscope.
Anticlines are not necessarily, or even typically, expressed as ridges in the terrain, nor synclines as valleys. Folded rocks get eroded just like all other rocks. The topography that results depends in part on how resistant individual layers are to erosion. In Figure 13.10, the topography is low where there are anticlines, and higher where there are synclines. This has happened because the uppermost yellow layer is relatively easy to erode compared to the uppermost grey layer, and the green layer beneath it.
Exercise 13.1 Folding Style
This photograph shows folding in the same area of the Rocky Mountains as Figure 13.1. Describe the types of folds using the appropriate terms from above (symmetrical, asymmetrical, isoclinal, overturned, recumbent, etc.). You might find it useful to sketch in the axial planes first.