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Three hundred million years ago the landmass that we know as North America occupied a place on the globe straddling the equator. Its climate was consistently warm and it received about 10 inches of rain per month throughout the year. Under these conditions the landscape of what’s now Illinois was dominated by lush, peat swamp forests where the waterlogged soil kept dead plant material from decomposing fully.
But changes were in the offing. The glaciers nearer the earth’s poles were retreating, giving rise to alternating wet and dry seasons in the tropics, which included Illinois. This meant that the waters emptying into the shallow sea nearby were charged with great loads of sediment. When a major earthquake south of Danville suddenly dropped a section of peat swamp below sea level and the water rushed in, this sediment accumulated at an astonishing rate, maybe as fast as ten feet in one year. As the sediment coalesced into shale, many of the plants that it covered were preserved, and a fossil forest was formed.
Despite the fact that the fossil forest now lies 250 feet below ground, scientists have been able to see it in recent years--in the ceiling of the Peabody Energy Company’s Riola coal mine. As miners dig out the coal seam—which is all that ancient undecayed plant matter, aged and compressed—they have opened up access to the underside of the lowest layer of the fossil forest. A person looking up at the ceiling in the Riola mine sees a slice of ancient forest floor, a worm’s-eye view, if you will.
Geologists John Nelson and Scott Elrick with the Illinois State Geological Survey say that at four square miles the fossil forest south of Danville is the largest preserved coal-age forest yet documented.
This forest lived and died long before dinosaurs roamed the earth, so there are no T. rex bones to be discovered there, and none of the 50 or so species of fossil plants found there is new to science.
Of course that doesn’t make them any less fascinating.
Largest and among the most abundant are the giant lycospids, which are also known as “scale trees” for the scale-like patterns on their bark.
Reaching heights of more than 100 feet, giant lycospids grew like leaf-covered poles, developing a branching crown only at the end of their lifespan. [Photos by Howard Falcon-Lang: Part of the trunk of one type of giant lycospid, showing scale-like bark pattern, and tree fern branches with leaves attached.] Tree ferns, which are not closely related to modern ferns, made up the other most abundant and widespread group of plants in the fossil forest.
Tree ferns were characterized by a large crown of feathery fronds, some of which are preserved with stems and leaves all still connected in the ceiling of the Riola mine.Because it represents such an extensive sample, examination of the fossil forest has allowed the geologists and paleobotanists collaborating with them to answer the important question of how the mix of plants varied across the landscape of a coal-age forest. Previously, researchers could only speculate about the composition of forests at that time, since they had been able to study only isolated fragments of them.
You can’t go look at the ceiling of the Riola mine for yourself, but you can see a fossil covered slab of shale taken from it on display in the coal-mining exhibit at the Museum of Science and Industry in Chicago. You can see also see more photos of it online at http://fossilforest.notlong.com.
