The exertion of compressional stress on rocks over a long period of time causes them to deform. If the deformation occurs in a pliable manner, there can be folding in the rocks, according to Tulane University.
When the stress is so intense that the fold closes, leaving each section parallel to the other, this is called an isoclinal fold. Folds are symmetrical if each section slopes at the same angle and asymmetrical if the angles are different. Overturned folds have one section that is almost bent upside down. Chevron folds have straight sections and no curve at the center. As well as classifying folds by their appearance, the nature of the bending of the rock determines whether the fold is anticline, monocline or syncline.
If the horizontal layers of rock slope away from the center of the bend, it creates an anticline fold. This occurs when the layers fold upward.
For a monocline fold, both sections of the fold remain horizontal. This type of fold also occurs when horizontal layers of rock bend upward.
Syncline folds have two sections that slope down toward the bend of the fold. They generally occur with anticlines.