GEORGE RANALLI / SCIENCE PHOTO LIBRARY GEORGE RANALLI / SCIENCE PHOTO LIBRARY
Petrified log. Petrifaction is the process by which organic material is fossilized. Here, the log was rapidly covered by sediment so it only decayed slowly. As it decayed, the wood was replaced by silica deposited from ground water. In some cases a perfect replica of the original wood may be formed. Photographed in Petrified Forest National Park, Arizona, USA.
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