In the spring of last year Tom Testa made the painful decision to sell off a piece of his legacy–a 300-million-year-old fossilized chiton called Glaphurochiton concinnus, an oblong mollusk whose modern relatives graze on algae that cling to wave-swept rocky shores. Back then, in the middle Pennsylvanian period, Testa’s chiton crept along the muddy floor of a shallow inland sea whose long northern coastline arced through what is now Kankakee, Will, and Grundy counties, about 50 miles southwest of the Loop.
Amateurs still hunt in the Mazon Creek area for fossils and still find amazing things. But many collecting spots are disappearing or already gone–bulldozed flat, hauled away for landfill, overgrown by vegetation, or submerged underwater. Someone who wants a Pennsylvanian-period chiton will usually have to wait until a serious collector like Testa decides to sell one.
Testa is still adding to his collection. He says the scientific community thinks all the important collecting at Mazon Creek has already been done but insists the fossils he continues to find are proof that it hasn’t. He believes that only a fraction of Pit 11’s fossils have been recovered and estimates that hundreds of thousands more could be.
Flowering plants hadn’t yet evolved, so the forests were dominated by primitive fernlike plants that grew up to 130 feet tall. Cockroaches, dragonflies, spiders, scorpions, horseshoe crabs, primitive amphibians, and six-foot millipedes swarmed amid the vegetation. The fresh, brackish, and salt waters teemed with crustaceans, mollusks, fish, jellyfish, bivalves, and worms. Geologists have theorized that a series of major storms caused the sea level to rise and inundate the swampy forest, while torrents of water swept down the rivers, breaching the delta floodplains and engulfing millions of creatures in mud.
The Mazon, particularly a stretch just southeast of Morris, provides the only natural exposure of ironstone concretions in Illinois. As the river wore through the shale 10,000 years ago, the concretions spilled out of the banks. The water saturated the concretions, and as it repeatedly froze and thawed with the changing of the seasons, the concretions often split where they were weakest–along the plane where the organism lay.
By the mid-50s the Peabody Coal Company was mining Pit 11, and collectors soon discovered huge numbers of concretions tumbling from its spoil piles. Peabody welcomed the collectors as long as they stayed clear of the rumbling machinery, and amateur geology groups such as the Earth Science Club of Northern Illinois (ESCONI) began taking field trips there.
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In the 50s Pit 11 was a huge moonscape that covered several square miles, and a collector with a hammer and bucket could wander over the spoil piles and not see another person all day. Fossils were so plentiful that collectors frequently picked up only concretions that had weathered open on their own. They noticed that certain spots tended to yield particular kinds of organisms–chitons on one hill, shrimp on another. Helen Piecko, a housewife and legendary collector who first picked the spoil heaps in 1956 with her husband, led rookies to her favorite spots for a while, but then the mines got crowded and the collectors competitive. She, like other experienced collectors, started giving the novices the slip. “We were coming out with bags full of rocks, and they were all there with cars parked by ours,” she says. “They never did find out where we were collecting.”
