An abundance of siderite (iron-rich) nodules, many of which contain fossils, makes Mazon Creek a unique place for the collection of marine Carboniferous fossils from around 300 million years ago.
Nestled within many of the nodules are complete and perfectly preserved shells and many of the nodules found here include the impressions of soft-bodied jellyfish and worms. With no hard (calcareous) body parts it is extremely rare for soft-bodied animals to be preserved as fossils. Specimens include the molluscs Pterochiton and Aviculopecten, the worms Archisymplectes, Hystriciola, Dryptoscolex, Astreptoscolex, Escinites, Fossundecima and Coprinoscolex, and the crustaceans Cyclus and Kallidecthes.