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Field & Street 

These Parts

What I needed was a mentor. Starting my first-ever fossil hunt, I was wandering without coherent aim, staring at the muddy ground with a hopeless expression. Wanting this to be easy, certain that it would be very hard, I felt like a man looking behind the bureau for his car keys when he knows perfectly well he left them locked in the car. If you want to find fossils, I thought, attach yourself to somebody who knows how to look for them. There must be a technique, a knack, a secret that separates the adept from the ignorant.

Did I mention that it was raining? Very hard, in fact. When it rains, the bare earth on the steep slopes of piles of strip-mine spoils gets very slippery. But the bare slopes are where the action is, because there is nothing green and leafy to block your view of the fossils.

The bare slopes were striated with parallel gullies running almost straight up and down. The rain continued to fall, and the gullies became rushing streams. The waters were gray, colored by the sand and clay suspended in them. The eroded beds of these tiny, precipitous streams were where the most rocks would be exposed, and the fossils were all inside rocks.

I had gotten this far on my own when Paul Uzureau and Tom Browne, two gentlemen from Flossmoor, started to work the same hillside I was staring at. I was totally without tools. They had hammers for splitting open rocks, buckets and plastic bags for specimens, and they were going at the problem in a systematic way, working gully by gully across the face of the hill, starting at the foot of the slope and going up as far as the slippery mud would allow, digging out promising stones with the claws of their hammers. I started watching them because they seemed to know what they were doing. "That's the important thing," Jim told me later. "To seem to know what you are doing." They were actually on their second-ever fossil-hunting trip, but that still made them learned in my eyes.

We were all part of a group of 22 fossil hounds who were scouring the mine debris. We were on an outing sponsored by the Field Museum, which had provided five staff members to keep us happy and well-informed. We were a dedicated bunch. When the rain started really coming down, one person took refuge with a Stephen King novel in the chartered bus that had transported us to the mines. The rest of us stayed out, slipping and sliding on the bare slopes and amassing large collections of rocks.

After watching Paul and Tom collect promising stones for a while, I began to recognize the fossiliferous rocks sticking out of the tailings pile. Looking at the rocks gave me both more knowledge and more hope, and soon I found several of my own.

We were digging in mounds of strip-mine debris because these heaps of earth and rock are part of Mazon Creek, one of the premier fossil beds in the world. Three hundred million years ago, in the Pennsylvanian Era, the area around the towns of Braidwood, Morris, Coal City, and Essex was the edge of a river delta. The river drained lands to the north and east, and it met the sea around Braidwood. Coal formed in the coastal swamps of that delta. Silt in the ancient river flowed over the delta and out to sea. The deepening silt buried the bodies of plants and animals, buried them so soon after death that even the outlines of the soft parts of their bodies are now visible. Since the Pennsylvanian Era, the geology of northeastern Illinois has been peaceful enough to leave the delicate fossils undisturbed.

To paleontologists, the Mazon Creek fossil beds take in about 70 square miles in Will, Grundy, and Kankakee counties. The name comes from the fact that the first fossils were discovered in small bedrock outcrops along Mazon Creek (or Mazon River on contemporary maps). There weren't many such outcrops. Most of the bedrock is buried under a deep layer of glacial debris, so it wasn't until coal mining began around Mazon Creek that the richness of the fossil life came to be appreciated.

Coal miners sank shafts through the glacial stuff and then through a layer of soft, dark rock called the Francis Creek shale. This late Pennsylvanian shale lies directly over the coal beds, so when the first mines were dug at Braidwood, Illinois, in 1855 and in nearby Coal City in 1858, the miners pulled large piles of shale up to the surface and heaped them around the pitheads.

Local fossil collectors combed through the heaps of rock and dirt and then passed their findings along to the professional paleontologists, who published as quickly as possible. The first academic paper on Mazon Creek fossils appeared in 1864. Since then all sorts of distinguished scientists, including Edward Drinker Cope, one of the founding lunatic geniuses of paleontology in America, have published hundreds of papers on the Mazon Creek fossils. To date, about 325 species of animals and perhaps 350 species of plants have been found in Mazon Creek sites.

They have been found inside concretions, rocks that formed around the fresh dead bodies of 300 million years ago. The concretions are made of siderite, a combination of iron and carbonates that produces a reddish brown rock with a rather gritty texture. Concretions are usually flattened disks or ovals with rounded edges. They may be an inch long, or they may be a foot long.

So I searched through the streambeds looking for the flat, symmetrical concretions in the jumble of stones exposed by the running water. The stones are a considerable jumble, combining the native shale and its concretions with granites, gneisses, and other fancy igneous and metamorphic rocks dragged here from the Canadian Shield by the glaciers. We were digging in the rain in Pit 11 south of Braidwood. Commonwealth Edison had strip-mined this in 1928. Now the lakes that filled the old mines are cooling ponds for the Braidwood nuclear reactor. We had to sign a release stating that if we got zapped, irradiated, or otherwise damaged on the trip, we wouldn't blame Com Ed for it.

The scientists got a big boost when strip-mining replaced deep mining in the region beginning in the 20s. Instead of digging a small hole through the Francis Creek shale, the strip miners pulled it all up, acres of it, and dumped it in great piles that paleontologists could rummage through.

The discoveries are extraordinary. Jellyfish are rarely found in the fossil record. In Mazon Creek there are not only two species but also hundreds of specimens of each. Mazon Creek also gave us the world's oldest isopod fossil. Isopods are small crustaceans that are still with us today. The world's oldest squid came from there, and the only known fossil lamprey. Other soft animals that are beautifully preserved include worms, sea cucumbers, and larval fish.

The most famous of Mazon Creek's fossils is the Tully monster, a wormlike creature of, as they say in the journals, uncertain affinities. The first fossil of Tullimonstrum gregarium was pulled from Pit 11 about 30 years ago by Francis J. Tully, a local man who was into fossil collecting as a hobby. He took his find to Dr. Eugene Richardson at the Field Museum, and Richardson published a description in 1966, naming the fossil in honor of its discoverer.

A Tully monster could be as much as 14 inches long. It has a flattened tail and a long proboscis carrying teethlike projections. It has been shoehorned into various phyla since its discovery. Lately it is being proposed as an ancestor to the vertebrates, and it will probably be proposed as a lot of other things in the future. For the moment, it may be officially placed among the Problematica.

The second word in the Tully monster's scientific name is revealing. The Tully monster is gregarious. It likes to hang out with the other monsters. We can believe this because hundreds of fossils of this unique creature have been found in Pit 11. There were whole schools of monsters in these waters in the Pennsylvanian Era, and parts of many of them were preserved in the amazing Francis Creek shale.

The fossils at Mazon are so numerous and varied that it is possible to think of conducting a census of ancient life. Gordon C. Baird of the Field Museum collected 80,000 concretions to take a census of plants and animals over the whole fossil bed. Mapping the occurrences of animals, Baird found that marine animals dominated the southern section of the beds. A freshwater fauna occupied a narrow band of coastal marshes, streams, and estuaries. Inland-- toward the north and east--the coal swamps covered the land of the delta.

Stephen J. Gould, in his book Wonderful Life, applied the term Lagerstatten to Mazon Creek. Lagerstatten means "rich lode" or "mother lode" and refers to the handful of places in the world where the record of ancient life is abundant and comprehensive enough to show us not just what lived then but how these various animals lived. The preservation of the soft parts of so many animals is a major factor in the value of Mazon Creek. According to Gould, only three real Lagerstatten exist: the Burgess shale from the Cambrian Era in British Columbia, a bed from the Devonian Era in Germany, and Mazon Creek.

I certainly have a fine collection of concretions from my collecting trip. However, I don't know for certain that they have fossils in them. You can stand a concretion on edge and bang it with a hammer. If it is ready to split, it will split along the plane of weakness created by the fossil. So you bang a couple of times, but if it shows no signs of splitting you stop hammering. Further beating will just crush the whole rock.

To open the strong rocks--like all of those I collected--you need cycles of freezing and thawing, of water seeping into cracks in the rock and freezing. The sudden expansion widens the crack. After the next thaw water seeps into the wider crack and then freezes and expands again.

This is God's way of breaking rocks, but he doesn't have any deadlines. I am trying to speed up the process. I tossed my collected rocks in a bowl of water and froze them. Once they were solid, I took them out and thawed them. And then froze them again. Someday I will find out if I have a sea worm, a fern leaf, or a species of shrimp utterly new to science. It has been known to take two years to pop a rock even with accelerated freezing and thawing. In paleontology, you have to be patient.

For information on the Joliet-Braidwood area, see the Visitor's Guide in this issue.

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