Mazon Creek is a Late Carboniferous fossil deposit in northeastern Illinois, roughly three hundred and nine million years old, in which plants and animals from a coastal river delta were rapidly encased in ironstone concretions called nodules, preserving soft bodied organisms, including the enigmatic Tully Monster, in unusually fine detail. Historically collected from coal mine spoil piles, the deposit remains one of the most productive sources of Carboniferous soft bodied fossils in North America.
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