Folding
(c) 2001 Andrew Alden, licensed to About.com, Inc. (fair use policy)
This exposure of chocolate chert at San Francisco's Bernal Heights shows good examples of folding in stratified rock. This chert formed on the seafloor, probably as silica-rich fluids partly replaced clay minerals. At that time its layers were smooth and level. It was later disturbed during the complex series of plate motions that pushed present-day San Francisco above the sea. Despite the variety of motions that the rock was subjected to, it is only a little wrinkled at the scale of this outcrop. This tough, competent rock underlies much of San Francisco along with serpentinite, making up a colorful patchwork of stone.
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