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A petrified forest is the fossil remains of a forest where the organic material of the wood is replaced by minerals, usually silicates such as quartz.
A petrified forest more than 300 million years old has been found in the remains of mine workings. The forest is so large that it underlies parts of Illinois, Indiana and Kentucky and extends over thousands of hectares. The most famous petrified forest in the US, Petrified Forest National Park, is located in Arizona and contains fossilized trees that lived in the Late Triassic period about 225 million years ago.