See also folding

The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of plastic (i.e. permanent) deformation. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales. Folds form under very varied conditions of stress, hydrostatic pressure, pore pressure , and temperature, as evidenced by their presence in soft sediments, sedimentary rocks, the full spectrum of metamorphic rocks, and even as primary flow structures in some igneous rocks. A set of folds distributed on a regional scale constitutes a fold belt , a common feature of orogenic zones.

Types of folds

• Anticline: linear, strata dip away from axial center, oldest strata in center.
• Syncline: linear, strata dip toward axial center, youngest strata in center.
• Dome: nonlinear, strata dip away from center in all directions, oldest strata in center.
• Basin: nonlinear, strata dip toward center in all directions, youngest strata in center.
• Monocline: linear, strata dip in one direction between horizontal layers on each side.
• Recumbent: linear, fold axis oriented at low angle resulting in overturned strata below the fold axis.
• Slump: typically monoclinal, result of differential compaction or dissolution during sedimentation and lithification.