KAJ R. SVENSSON / SCIENCE PHOTO LIBRARY KAJ R. SVENSSON / SCIENCE PHOTO LIBRARY
Fossil clam. Clams are a type of bivalve (two-shelled) marine mollusc. In a marine environment, a fossil is created when an animal is buried in sand or mud after death, and the normal decay processes do not occur. The minerals of the hard parts of the animal, in this case the shell, are gradually replaced with rock. This fossil was found in sandstone on the shoreline of the US Pacific coast in Oregon. Here, sandstone layers bearing various fossils are continually uplifted by the Pacific Plate as it strikes the North American Plate. As the sandstone layers reach the intertidal zone, the fossils are exposed by waves, wind and rain action.
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