The impressions of fossilized leaves are commonly found in coal. The most common fossils are of ferns, mosses and horsetails, all of which still grow on the Earth today. Coal was formed in shallow bogs, where plants died and were gradually buried by more material, until pressure and time converted them into coal. The oldest coal was formed 350 million years ago, and the process still continues in swamps and bogs. Peat is the earliest stage in the formation of coal. It has not yet been subjected to the pressure that will eventually turn it into a hard material.
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