The bending or warping of rocks is called folding. Monoclines are folds in which both sides are horizontal, which often form over deeper faults. Anticlines are upward-pointing arches that have the oldest rocks in the center, and synclines are downward-pointing arches, with the oldest rocks on the outside edges of the structure. Though many other geometric varieties of folds exist, most are variations of these basic types. The fold hinge is the region of maximum curvature on the fold, whereas the limbs are the regions between the fold hinges. Folds may be further classified according to how tight the hinges are, which can be measured by the angle between individual fold limbs. Gentle folds have interlimb angles between 180° and 120°, open folds have interlimb angles between 120° and 70°, close folds between 70° and 30°, tight folds have interlimb angles of fewer than 30°, and isoclinal folds have interlimb angles of 0°. Folds may be symmetrical, with similar lengths of both fold limbs, or asymmetrical, in which one limb is shorter than the other limb. Fold geometry may also be described by using the orientation of an imaginary surface (the axial surface), that divides the fold limbs into two symmetric parts, and the orientation of the fold hinge. Folds with vertical axial surfaces
Folded rock strata in Austrian Alps (Bernhard Edmaier/Photo Researchers, Inc.)
and subhorizontal hinges are known as upright gently plunging folds, whereas folds with horizontal hinges and axial surfaces are said to be recumbent.
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