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fold in sedimentary rock

Photograph of severely deformed sedimentary rock layers exposed in a roadcut of the Antelope Valley Freeway (Interstate Highway 19) within the San Andreas Fault zone near Palmdale, California; view is about 100 m wide (note light stanchion for scale). The sedimentary rocks originally were deposited as horizontal layers of sand and mud in streams and ponds that occupied a late Miocene landscape (about 15 to 9 million years ago). Squeezing related to movements within the San Andreas Fault zone caused the horizontal layers to be contorted into the folds visible in the roadcut, and even created a small fault that has broken one of the folds. These relations show nicely how geologic materials deform (strain) in response to dynamic earth forces (stress). Photograph copyright J.K. Nakata, August, 1988 (used with permission).

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