San Francisco Bay Region Geology and Geologic Hazards

Stories > East of Napa Valley - Mesozoic (dinosaur-age) marine fossil (Buchia)

East of Napa Valley - Mesozoic (dinosaur-age) marine fossil (Buchia)

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Buchia fossils from the Jurassic sedimentary rocks of the Great Valley complex, found along Highway 128 east of Napa Valley.
Jurassic and Cretaceous sedimentary rocks of both the Great Valley and Franciscan basement complexes contain fossils of the ancient mollusk, Buchia. Modern mollusks include clams, snails, and squid.


Fossils form when the remains of plants and animals, or traces of their lives such as footprints, are buried in sediments and preserved when the sediments are converted to sedimentary rocks.  Sometimes the original animal or plant material is preserved, other times only an imprint or mold is preserved, and sometimes the original material is replaced by new material in the same shape as the original.  The upper Buchia in the photo is only a mold, but the lower fossil retains some of the original "mother of pearl".

Learn more about fossils at the USGS or by visiting the Paleontology Portal External link - link policy applies Web site.



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