The Baker Beach Rocks
17 - Blueschist metamorphic blocks

Near the boundary of the Serpentinite we find these unique rocks which contain minerals that can only be created by incredible squeezing but not be heated that significantly.  They don't look all that impressive, but geologists who have cut thin slices of these rocks and observed them under strong microscopes assure us that they are truly unique rocks.  They have a somewhat mysterious origin but it is surely related to that enormous collision between the old ocean floor and the continent.  All of the rocks we have been observing during this field trip are dramatic evidence of what occurred to create the Coast Range of California.

Close up of the high pressure - low temperature rocks
(quarter for scale)

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If you haven't visited the Marin Headlands portion make sure that you do!