Kink Folds
(c) Kimberley Motz, reproduced by permission. (fair use policy)
Stratified rocks are like pages in a great book, and like paper pages they can be folded in intricate ways. Creases like these are classified as kink folds, which have sharp hinges and flat sides. Because their sides have unequal lengths, these are specifically called zigzag folds.
This Canadian roadcut exposes a set of thin shaly layers sandwiched between two thick sandstone layers, which cannot flex as tightly as the thin layers. (See penny for scale.) The kinks arose as a result of shear between the thick layers, the right-hand one moving up relative to the left-hand one. Structural geologists can untangle quite intricate folding sequences, helping decipher the geologic history of an area.
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