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Photograph of mylonitic and cataclastic rock in the southeastern San Bernardino Mountains, southern California; pencil is about 6 inches (15 cm) long. These rocks occur in the upper plate of the Vincent Thrust, a zone of crustal movement and dislocation that underlies much of the Transverse Ranges Province. The original rock type probably was plutonic tonalite or quartz diorite that has been subjected to high-strain squeezing, stretching, and grinding of the mineral crystals to yield a rock whose original grain-to-grain relationships have been completely modified; such a high-strain rock is known as cataclasite or mylonite, depending on the brittle or ductile nature of the deformation style. Photo by J.C. Matti, USGS, July, 1979.

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