Friday Rocks #38: Folded Chert

I had to dig this one deep out of my archives, so sorry for the poor image quality.

[caption id="attachment_579" align="aligncenter" width="720"]Folded chert in the Marin Headlands Folded chert in the Marin Headlands[/caption]

This is a roadcut in the Marin Headlands, North of San Francisco California. Chert is made of microcrystalline quartz from radiolaria, microscopic protozoa with silicate skeletons. When radiolaria die their skeletons sink to the bottom of the ocean floor. After compaction and diagenesis, chert is made.

The chert in the Marin Headlands is part of an accretionary wedge block and is folded into chevron folds.

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Wahrhaftig, Clyde. "Structure of the Marin Headlands block, California: A progress report." (1984): 31-50.

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