"The Mazon Creek fossil beds are a conservation lagerst\u00E4tte found near Morris, in Grundy County, Illinois. The fossils are preserved in ironstone concretions, formed approximately 300 million years ago in the mid-Pennsylvanian epoch of the Carboniferous period. These concretions frequently preserve both hard and soft tissues of animal and plant materials, as well as many soft-bodied organisms that do not normally fossilize."@en . . . "97001272" . . "Mazon Creek Fossil Beds"@en . "1997-09-25Z"^^ .