The Monterey Formation is famous for its siliceous composition. This Scanning Electron Microscope (SEM) image shows that the sediment (diatomite) is almost completely composed of the whole and broken pieces of diatoms. These microscopic plankton make their tests (shells) of opaline silica. Note the tip of a sponge spicule near the bottom right. Photo © Rick Behl (1997).
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