Three hundred million years ago, what’s now Illinois was covered by an inland sea teeming with aquatic life. The sea is long gone, but the life is still there, roughly 60 miles southwest of Chicago. An entire Paleozoic ecosystem is preserved at a site called Mazon Creek (“muh-ZAHN”), which sprawls across more than 250 individual locations in an area of northern Illinois that stretches roughly from the city of Morris to the village of Essex. It’s one of the most important fossil deposits on the planet and in certain circles, also one of the most famous. Still, a lot of people who live within an hour’s drive of Mazon Creek have never heard of it. “I had no idea that something like this was so close to Chicago, a world-class fossil location,” said Jeff Allen. “It just sort of blew my mind.” In 2024, Allen had recently retired from a career as a computer scientist and was casting about for a new purpose. He found it at Mazon Creek, joining long-time fossil hunters from the Earth Science Club of Northern Illinois (ESCONI) on expeditions to the site. “The first time I went out was before I met anybody, and I collected probably 50 pounds of rocks,” Allen recalled. “Then I started meeting people from ESCONI and started to tag along on some of the trips. I started realizing, ‘Oh my god, that first collection was mostly junk.’” Some facts: Most Mazon Creek specimens are found in waste piles that are the byproduct of mining operations. Turns out the fossil-rich layer of shale harboring Paleozoic concretions was sitting on top of a seam of coal. The shale was stripped away to get at the coal below, and it’s these spoil piles that fossil hunters comb through looking to strike pay dirt of their own. To date, the Mazon Creek deposit has coughed up nearly 800 different species, any one of which, either in whole or in part, could be entombed inside a concretion. Maybe a leaf, an anemone, a spider or, quite often, nothing.
