Field Book
PENNSYLVANIAN PLANT FOSSILS OF ILLINOIS
Charles Collinson and Romayne Skartvedt
Plants that flourished 200 million years ago have made Illinois one of the best known fossil collecting sites in the world. The unusual abundance and preservation of these fossils in the northern part of the state have brought collectors to Illinois from many countries, and prized specimens from that area may be seen in science museums throughout the world.
The remarkable fossils represent plants that lived during the geologic period called the Pennsylvanian or Coal Age and are the result of special geologic conditions that occurred repeatedly during the period.
At the beginning of the Pennsylvanian Period, Illinois was part of a vast lowland that stretched for hundreds of miles to the north, south, and west, and was bordered on the east by highlands. At times much of the plain was swampy and, because the climate was relatively warm and moist, great jungles of fast growing trees, shrubs, and vines covered the landscape. As successive generations of plants lived and died, plant material fell into the swamp waters and, protected there from decay, accumulated.
Frequently during the period, seas spread over the swampy lowlands, submerging the forests and covering them with mud. Each submergence lasted only a short time, geologically speaking. When the seas withdrew, the deposits of sand and mud left behind were cut by streams that carried fresh sand and mud from the eastern highlands. The streams eventually became clogged with sediments and when the lowland was again depressed swamp conditions returned and forests grew afresh. Such a cycle of deposition was repeated again and again during Pennsylvanian time, and after burial each layer of plant material gradually lost most of its liquids and gases and was slowly converted into one of the numerous coal beds presently found in Illinois.
Reconstruction of Pennsylvanian Coal-forming Swamp
In some places in the state conditions existed that were especially favorable for preservation of plants, and there delicately preserved fossils are found in great numbers. In the most favorable areas, such as in northern Illinois, the plants are preserved in stony nodules called concretions, but they also may be found separately as molds, casts, or petrifactions.