Ironstone concretions containing Mazon Creek fossils are found
in various types of natural and man-made outcrops of a rock called
the Francis Creek Shale.
Most of the fossils come from the Mazon Creek area of Grundy,
Will, Kankakee, and Livingston counties. In this area fossils are
recovered from natural exposures on Mazon Creek, from active and
abandoned strip mines, from shaft mines, and from mine spoil
Fossils like those from Mazon Creek are recovered from a few
other areas in Illinois. The Francis Creek Shale is exposed along
streams and in strip mines in LaSalle county between the Vermillion
River and Marseilles.
Mazon Creek nodules and fossils are also found in museums
worldwide. This is because the fossils from the area are abundant
and exceptionally well-preserved. Most major natural history
museums in the world probably have at least a few nodules.