coquina - display specimen of gray oyster shell coquina from the Imperial Formation, Imperial County, California

Coquina is a sedimentary rock composed almost entirely of shells, marine or non-marine. The southern Imperial Valley was submerged under the Imperial Sea, a shallow arm of the proto-Gulf of California, roughly 3 to 6 million years ago, in the Late Pliocene and Early Miocene. The warm and shallow subtropical waters were home to numerous oyster reefs. As the sea grew shallower, currents and wave action broke up oyster beds and packed the shells tightly into channels. Calcium carbonate has cemented them into dense coquina beds that cap small ridges in the Imperial Formation south of the Coyote Mountains near Ocotillo in Imperial County, California. 

The coquina that most students see in the classroom is white, easily collected from extensive beds in Florida. It was widely used as a building stone in 16th to 19th century Spanish colonial structures. In forts, when struck by cannonballs, the porous structure absorbed the blow instead of shattering like hard stone or brick. This gray coquina should make students think.

Select a specimen: When more than one specimen is shown, you can select a particular specimen by telling us what is in the photo with it, a black and silver pen, a black mechanical pencil (it has an eraser), or one of those plus some number of coins, or you can let us make the selection.

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