In the spring of 2021, the Earth Science Club of Illinois (ESCONI) was granted special collecting access to an early 20th century shaft mine spoil near Danville, Illinois. The coal mine opened there in 1904 and operated for forty plus years under four different companies. The goal was to extract Herrin Coal # 6: a seam averaging 6 feet in thickness, at a depth underground of over 200 feet. Above the coal layers in Illinois are roof shales that can host concretions containing fossils of the plants and animals that were buried by flooding sediments and rising sea level approximately 300 million years ago.
Photo: Bruce Lauer
Callipteridium neuropteroides in Red Dog Shale.
Illinois Carboniferous fossils are often best-preserved in hardened, iron carbonate concretions, but they also occur as soft carbon impressions in roof shales absent the siderite-forming process. Because of the fragility of the shale and the speed at which it decays when exposed to the elements, we generally see fewer of the compression fossils at collecting sites. However, there is a phenomenon in shaft mine overburden that occasionally preserves these delicate forms. It is a kind of “cooking” of the shale by spontaneous combustion deep inside the pile. When iron sulfide is exposed to water and air, it oxidizes and the intense heat can ignite the surrounding coals and other flammable ingredients. The result is a slow, often decades-long, bisque firing of the gray shale from friable to a more durable state. Its brick-like color has earned it the name, “Red Dog Shale” and it is presently used as landfill in a variety of construction settings.
ESCONI’s first trip into the old coal mine spoils was with a small group and mainly exploratory. What we discovered were exquisitely preserved shale fossils and a smattering of concretions containing mainly plant forms. The Herrin Coal roof shale flora has never been extensively researched, so we’ve begun a survey out of our field trips which we hope will provide a foundation for a more formal institutional study and catalog. This gallery showcases some of the better examples the club has found since our start at the Danville site. Jack Wittry, whose work on Mazon Creek flora has been published by ESCONI, has been instrumental in identifying many of the species we’re finding in the course of our survey. Below is a fossil index and links to pages that offer another step in organizing the amazingly beautiful and diverse specimens we’re finding.
ESCONI officers at the collecting site - April 2021.
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