Coquina
(c) 2006 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Coquina ("co-KEEN-a") is a sedimentary rock composed chiefly of shell fragments. Because shells are made of calcium carbonate, coquina is formally a limestone. It's not common, but when you see it you want to have the name handy. Coquina is the Spanish word for cockleshells or shellfish. Coquina forms near shore, where wave action is vigorous and sorts the sediments well. Notice that the pieces are all broken and rounded by the abrasion of the waves.
Most limestones have some fossils in them, but coquina is the extreme version. This specimen is in the American Museum of Natural History, encased in glass because it isn't a strong stone. A well-cemented, strong version of coquina is called coquinite. A similar rock, composed chiefly of shelly fossils that lived where they sit, unbroken and unabraded, is called a coquinoid limestone. That kind of rock is called autochthonous (aw-TOCK-thenus), meaning "arising from here." Coquina is made of fragments that arose elsewhere, so it is allochthonous (al-LOCK-thenus). Those are handy words in geology.
See more photos in the Coquina Gallery.
See more sedimentary rocks in the Sedimentary Rocks Gallery.
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