fault gouge - hand/display specimen of clay from the San Andreas Fault Zone

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.

This gouge dramatically shows the squeezing that has taken place in this fault zone, where granitic rocks have been ground to a clay. The field photo shows what happens in zones of plate collision. These tilted sedimentary rocks are less than a mile from the San Andreas in the Mecca Hills.

Select a specimen: You can select a specimen by telling us what is in the photo with it, a black and silver pen, a black mechanical pencil or one of those plus some number of coins, or you can let us make the selection.

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