When a body of rock, especially sedimentary rock, is squeezed from the sides by tectonic forces, it is likely to fracture and/or become faulted if it is cold and brittle, or become folded if it is warm enough to behave in a plastic manner.
The nomenclature and geometry of folds are summarized on Figure 12.2.1. An upward fold is called an anticline, while a downward fold is called a syncline. In many areas it’s common to find a series of anticlines and synclines (as in Figure 12.2.1), although some sequences of rocks are folded into a single anticline or syncline. A plane drawn through the crest of a fold in a series of beds is called the axial plane of the fold. The sloping beds on either side of an axial plane are limbs. 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.
Exercise 12.1 Folding Style
This photograph shows folding in the same area of the Rocky Mountains as Figure 12.0.1. Describe the types of folds using the appropriate terms from above (symmetrical, asymmetrical, isoclinal, overturned, recumbent etc.). You might find it useful to first sketch in the axial planes.