ABSTRACT
Folds are perhaps the most distinctive feature of deformed rocks. Folds occur in all kinds of rocks, including sedimentary, igneous, and metamorphic. Folds in sedimentary rocks are perhaps best known in mountain fold and thrust belts. In igneous rocks, the folded surfaces of pahoehoe lava flows are perhaps the best example, and folded schlieren (wispy dark layers) in many light-colored plutonic rocks are another. In metamorphic rocks folds can be defined by almost any curved surface, including foliation, compositional layering, cleavage, dikes, veins, or geologic unit contacts. Folds come in all sizes, ranging from delicate crenulations on a foliation surface (Figs. 15.15-15.17) to folds many kilometers across. They also come in a variety of shapes, from graceful symmetric arcs to confusing messes reminiscent of a failed origami project. Folds illustrated here are defined by curved layers, curved foliation surfaces, and even curved fold axial planes.
