Ironstone concretions from the old strip-mine spoil around Braidwood and Mazonia, Illinois, and one of the very few places on Earth preserving soft-bodied animals in fine detail. Crack one and you may get a fern, a jellyfish, a worm, a shrimp, or the Tully Monster, Illinois’s state fossil, an animal so strange that palaeontologists still argue about what phylum it belongs to. The classic method is to collect nodules and freeze-thaw them at home until they split naturally. Access varies by parcel; Mazonia-Braidwood state area permits collecting, some ground is private.
Soft-bodied animals, ferns, the Tully Monster · Pennsylvanian, ~309 mya
Open to the public for collecting, and what you find is yours to keep. Check the posted rules before you start: they can change with the season and the landowner.
Other places on the map within a couple of hours.
