A layered rock that exhibits bends is said to be
folded. The layered rock was at one time uniformly straight but was stressed to develop a series of arches and troughs. A compressive stress compacts horizontal rock layers and forces them to bend vertically, forming fold patterns.
Anticlines and synclines. An anticline is a fold that is arched upward to form a ridge; a syncline is a fold that arches downward to form a trough (Figure ). Anticlines and synclines are usually made up of many rock units that are folded in the same pattern. The tip of a fold is called the nose. The center axis of a fold is called the hinge line and lies in the axial plane that separates the rocks on one side of the fold from the rocks on the other side that dip in the opposite direction. Extensive folding is represented by a repeated pattern of anticlines and synclines. Two anticlines are always separated by a syncline, and two synclines are always separated by an anticline. One side of the fold is called the limb; a side‐by‐side syncline and anticline share a limb. Frequently, an anticline or syncline can be identified only from the systematic change in the dips of the sloping rock units from one direction to the other, identifying the hinge line of the fold (Figure 1).
Figure 1
An Anticline and a Syncline
Plunging folds. Plunging folds have been tipped by tectonic forces and have a hinge line not horizontal in the axial plane. The angle between the horizontal and the hinge line is called the plunge and, like dip, varies from less than 1 degree to 90 degrees. Plunging folds characteristically show a series of V patterns on a bedrock surface (Figure 2).
Figure 2
A Plunging Anticline
Structural domes and basins. A structural dome, a variety of anticline, is a feature in which the central area has been warped and uplifted and all the surrounding rock units dip away from the center. Similarly, a structural basin is a variation of syncline in which all the beds dip inward toward the center of the basin. Basins and domes can be as large as 100 kilometers across.
Open, isoclinal, overturned, and recumbent folds. A variety of kinds of folds generally reflects increasing amounts of tectonic stress (Figure ). An open fold is a broad feature in which the limbs dip at a gentle angle away from the crest of the fold. Isoclinal folds have undergone greater stress that has compressed the limbs of the folds tightly together. The limbs of overturned folds dip in the same direction, indicating that the upper part of the fold has overridden the lower part. Depending on where the exposure is in an overturned fold, the oldest strata might actually be on top of the sequence and be misinterpreted as the youngest rock unit. Recumbent folds, found in areas of the greatest tectonic stress, are folds that are so overturned that the limbs are essentially horizontal and parallel.
Figure 3
Folds