Blue sandstone is unusual. This blue sandstone marks the base of the Pliocene-age Etchegoin Formation of the western San Joaquin Valley in California. The unusual color is caused by a montmorillonite group clay that coats the dark-colored sand grains. This authigenic (formed in place) coating developed as the sediment was being deposited. Sandstones with a striking bluish color are found in a number of formations in California. These coated sandstones all contain a considerable amount of andesitic clasts and are highly permeable.
This sandstone is not at all cemented - a specimen dropped instantly becomes sand, showing that more than compression is required for lithification. The addition of a cement, often calcium carbonate or silica, would complete lithification and result in a coherent sandstone.
These specimens are fragile. Expect some crumbling with student handling. If not dropped, and if handled carefully, they can survive student examination though they will lose sand grains. Useful for a discussion of the processes that form a sedimentary rock: deposition of the sediment, compression and then cementing of the sediment clasts (fragments, from the Greek root klastos, for broken) by silica, calcite or iron oxide.
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