fault gouge - teaching hand specimens of clay from the San Andreas Fault
Take a continent and mash it up against one of the earth's oceanic crustal plates, grind them together a bit, and this is what you get - fault gouge - rock material ground to a fine clay. In this case, the collision is between the Pacific Plate and the North American Plate. In the Mecca Hills in California, the clay gouge has been squeezed up to form a linear ridge.
An offset of strata along with gouge on the line of displacement are marks of faulting. On a smaller scale, gouge is found in most fault zones, though not as a massive ridge. These specimens are clay, so students should handle with care. We have large specimens collected over 20 years ago that have not fallen apart.
These specimens dramatically show the squeezing that has taken place in this fault zone, where granitic rocks have been ground to a clay. They were selected because they have good slickensides - the striations formed on rocks that move against each other along the fault plane.
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