concretion - cannonball concretion from the lower sandstone member of the Monterey Formation - hand/display specimen
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Concretions are a typical structural feature of some sedimentary rocks, particularly sandstones. Early in the burial history of a rock, before it becomes solidified, a cement will precipitate around a nucleus, often a fragment of calcium carbonate, filling the pore spaces as the sediment becomes consolidated into rock. The sediment grains then become cemented into a rough sphere around that fragment. Occasionally neighboring concretions grow against each other and are cemented together.
At Piru Dam on Piru Creek, a roadside slope failed in the 1970s, depositing hundreds of these concretions on the road. These were simply bulldozed into the lake to clear the roadway. The large one was photographed in 1900. These are rarely found today.
Monterey Formation, lower sandstone member - Middle to late Miocene Piru Creek, Ventura County, Calif.
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