Curvatures in layers of rocks, generally found in rock formations that were originally horizontal and produced by stresses in the crust of the earth.

The angle of deformation of folds ranges from very small to extremely contorted; their size ranges from a few inches to many miles.

When folded rocks erode, the result is a series of parallel ridges and valleys; such as, those that can be observed in the Appalachian Mountains, the mountain range in the eastern United States extending from Quebec, Canada, to the Gulf of Mexico.