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University of Illinois Extension

Mazon Creek - A Treasure of the Past

In northern Illinois, there is an area that contains a large amount of fossils.  This area is known as Mazon Creek.

a polychaete annelid, Fossundecima

Polychaete annelid, Fossundecima Credit: UC Museum of Paleontology

Tullimonster (Tullimonstrum)

Tullimonster (Tullimonstrum) Credit: UC Museum of Paleontology

Three hundred million years ago, what is now northern Illinois was part of a swampy marsh that included a river delta. Located near the equator, it had a tropical climate.  As animals died in this area, they were covered in mud and sediment.  This mud and sediment eventually formed shale.  Within the shale, a particular mineral formed around many of the buried animals, called siderite.  This helped preserve the hard and soft tissue of these animals, creating fascinating fossils that are present today.