Mazon Creek Fossils are world famous. Every museum has some. Mazon Creek fossil beds were discovered in 1857. Former Grundy County Judge George Bedford collected these and displayed them in his office window for passersby to see.
World famous because they are so abundant and diverse—400 different plant specimens, 320 animals—jellyfish, shrimps, clams, many soft-bodied critters that are hard to fossilize.
Found in ironstone concretions of Francis Shale—unique to Mazon Creek. Formed during Mid-Pennsylvania epoch of the Carboniferous Period. At that time Mazon Creek was a tropical, swampy place, full of growing trees, vines and shrubs.
As this stuff died and buried in the swampy sediment, they decayed, giving off carbon dioxide in the process, which combined with iron in the water. This chemical process formed a protective nodule of ironstone around the fern or worm or whatever and turned it into a Mazon Creek Fossil.
Strip mining in the southern part of Grundy County has also turned up a lot of fossils.