Folds Folds In general, folds are waves and bends in rocks. On the other hand, folds are a group of deformation in rocks which change the conditions of rocks and does not make fracture in rocks. Although folds occur in sedimentary rocks, layer-form igneous rocks and schistose metamorphic rocks, but they can also occur in massive rocks. Folds properties (the geometry of folded surfaces in three dimensions) Hinge line: It is a hypothetical line, which connects the points of a layer having maximum curving.   Axial plane: It is a hypothetical plane passing through all folds' layers, called axial plane. It can be horizontal, vertical or oblique.   Fold axis: It passes through the hinge line or successive folded surfaces.   Fold limb: Sides of a fold is called limb. Axial trace: The connection surface of axial plane with a horizontal or vertical plane is called axial trace. Horizontal surface is usually assumed the ground surface.   Crest line: It is a hypothetical line, which connects the highest points of a fold.   Trough line: It is a hypothetical line, which connects the lowest points of a fold.   Apex: The highest point of a fold is called apex.   Plunge: Situation of a fold is determined by its plunge. The angle of fold hinge is called plunge.                                       Anticline and syncline                                    Anticline: A fold, generally convex upward, whose core contains the stratigraphically older rocks. The dip of both sides of anticline is opposite to each other.   Syncline: A fold of which the core contains the stratigraphically younger rocks; it is generally concave upward.