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.
Approximately 25 to 30 species have been found in this region. The productivity of the area was shown by George Langford, Sr., a well known midwestern fossil collector. He and his son split about 250 thousand concretions during a 140-day period and obtained some 25 thousand plant specimens. Fine specimens still can be collected in a few hours.
The plant collecting localities in Will and Grundy Counties along Mazon Creek, four to six miles southeast of the town of Morris, were the first to be well known. Ferns are especially abundant. Fossils of insects, crustaceans, worms, and salamanders also have been found. Collecting conditions vary considerably from season to season, and fossils are not as easily obtained there as from the strip-mine spoil heaps.
Fossiliferous concretions may be found in a number of the strip mines in the area, although probably most have come from the Northern Illinois Coal Corporation mine between the towns of Braidwood and Wilmington.
In earlier years good collections were made from the spoil heaps of underground mines. Especially notable are the mine dumps of the Wilmington Star No. 7 mine, 2¼ miles west of Coal City, and Skinner No. 2 mine, two miles northeast of Braidwood.
In the vicinity of Morris on the northwest edge of the Mazon Creek area, fossil ferns have been found along the north side of the Illinois River and in the banks of the Illinois-Michigan Canal. About a mile north in an area of strip mining, fossil-bearing concretions have been found in shale and irregular sandstone layers.
Plate 2
Sphenophyllum ⅗× Lepidostrobophyllum ⅗× Annularia ⅗× Sphenophyllum 1× Lycopodites ⅗× Cordaites ⅗× Asterophyllites ⅓×
Fossils in concretions also have been collected from a shaly limestone at the south end of the Kankakee River bridge along the Grundy and Will county line.