PLANT FOSSILS are the remains of prehistoric plants. Woody structures of
plants aid preservation just as hard parts of animals do. Leaves and
plants without much woody material generally were well preserved only if
they were buried quickly in fine, soft sediment.
The most famous Illinois plant fossils are those from the Mazon Creek
area in Grundy and Will Counties of northeastern Illinois. The plant
material acted as a nucleus around which iron minerals accumulated to
form concretions. Many good fossils—of trunks, branches, leaves, and
seeds—are found in coals and in shale directly overlying coals.
Descendants of “Coal Measures” plants, such as ferns, mosses, and
rushes, are still living today, but they no longer thrive as they did in
the warm, moist climate of the Pennsylvanian forests.
Some plants of Pennsylvanian age are petrified, and occasionally such
trees or stumps are found. Petrified trees are found also in the upper
Mesozoic deposits of southern Illinois. Fossils of “Ice Age” plants
closely related to forms living at the present time are occasionally
found in peat bogs or scattered throughout glacial deposits.
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