Mazon Creek in Illinois contains concretions which, when cracked open, frequently contain beautifully preserved fossils. Both terrestrial and marine fossils have been recovered from Mazon Creek, including the enigmatic "Tully Monster" which is also the state fossil of Illinois. The deposit is Pennsylvanian in age.
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- Mazon Creek Fossils This is Illinois State Museum's online exhibit of the famous Mazon Creek fossils. The Mazon Creek fossils include many soft-bodied organisms that are not usually preserved.