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Petrified wood specimen
Petrified wood specimen. Petrifaction describes the fossilisation of wood. It occurs when pieces of wood fall into sediments and are rapidly covered, preventing normal decay. Over long periods of time the wood decays. As it decays, the wood is replaced by minerals that are deposited from solution in ground water. This forms an exact replica of the original, preserving the texture of the bark and even the growth rings. This specimen is from Petrified Forest National Park, Arizona, USA. Fossils from this park date from the Late Triassic, about 225 million years ago., by DIRK WIERSMA/SCIENCE PHOTO LIBRARY
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