Plant fossils can be found in almost any northern Illinois area where Pennsylvanian rocks are exposed (see [back cover]), but in some places they are much better preserved and more numerous than in others. Most of the well known collecting areas and a few of the lesser known ones are discussed below. Even though some of the localities were discovered many years ago, they may indicate areas that are still favorable for collecting.
Mazon Creek Area
Of all the fossils that have been found in Illinois, the most famous are the plant remains from the world-renowned Mazon Creek area in the northeastern part of the state. In this area in Grundy and Will Counties, plant fossils are found in ironstone concretions in the lower part of the Francis Creek Shale directly overlying the Colchester (No. 2) Coal.
Fossils were discovered in outcrops along Mazon Creek more than a century ago and collections later were made from scores of conical spoil heaps at underground mines. After coal stripping began in the 1920’s, great numbers of specimens were collected.
In the stripping operations, the concretion-bearing beds are commonly the last to be placed on the spoil heap. Weathering softens and removes the shales and leaves the nodules concentrated on the surface. Each season brings a new crop of concretions to the surface.
Plate 1
Calamites ⅓× Stigmaria ⅖× Lepidodendron ⅖× Calamites ¹/₁₀× Fern Stem ⅔× Caulopteris ¼× Sigillaria ⅖× Megaphyton ⅗× Calamites ⅗× Artisia ⅗× Sigillaria sub-bark ⅗×
The concretions generally are oval to elongate and range from less than an inch to a foot or more in maximum dimension. Only about one nodule in ten contains plant remains.