| Central California Ranges | |
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The Coast Range Thrust has pushed a thick sheet of mostly sedimentary rocks called the Great Valley sequence over the metamorphic rocks of the Franciscan complex. The New Idria region is a place where the thrust sheet has a hole in it where the Coast Range Ophiolite, at the base of the Great Valley sequence, peeks through. This body of rock was once oceanic lithosphere, altered to serpentinite early in its history. Serpentinite is poor in plant nutrients and contains many unusual minerals. One of them is serpentine asbestos; another is benitoite, California's state gem.
Given all that, I don't know what this crag of yellow rock is. There are exposures of it here and there in the Clear Creek valley.
All photos (c) 2003 Andrew Alden, licensed to About.com, Inc.
A crag of yellow rock rises above Clear Creek in the New Idria serpentinite zone.
