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Petrified logs. Petrifaction is a term used to describe the fossilisation of wood. It occurs when pieces of wood, such as these logs, fall into sediments and are rapidly covered. Over a long period of time, the wood decays. As it decays, the wood is replaced by minerals which are deposited from solution in ground water. This forms a perfect replica of the original, even down to its growth 'rings'. Photographed in Arizona, USA. (KEYSTONE/SCIENCE PHOTO LIBRARY/DAVID PARKER)