IMPORTANCE OF MAZON CREEK
Mazon Creek fossils is a generic name derived from the Mazon Creek, a tributary of the Illinois River where the concretions containing fossils were first found in the shale eroding from the creek’s banks.
The fossils are Upper Carboniferous in age and the fossils are found in the Francis Creek Shale deposit that is located right above a significant coal deposit. These fossil deposits are considered to be one of world’s great Lagerstätten. This location preserved many soft-bodied animals and plants rarely seen in the fossil record.
The Mazon Creek area contains another of the world’s great Konservat Laggerstätte. The fossils recovered in this area provide a glimpse into life in the Upper Carboniferous period approximately 300 million years ago preserving soft-bodied animals and plants.
Mazon Creek fossils were found as a result of the discovery of coal in Illinois. Coal was first discovered in 1673 by Marquette and Joliet along the Illinois River near Utica. This discovery resulted in substantial coal mining operations in the area from 1810 until the early 1970’s. Coal was first collected along the creek and then in shaft mines dug deep into the earth.
Later deposits were found closer to the surface and with the development of large machinery, strip mining was started. The concretions containing fossils are found in the Francis Creek Shale that was deposited right above the coal seams. Therefore the last waste from mining was the shale and this was deposited on top of spoil hills. Once the shale erodes the concretions are exposed.
The Mazon Creek area had many locations for collecting fossils. The map to the right shows the area in detail. Fossil collecting at the strip mines in Morris resulted in more terrestrial and fresh water fossils. The site known as Clam Flats or Chowder Flats was well known for the clams found in concretions. Some finds come from the banks of the Mazon Creek itself.
The most famous location is the marine area known as Pit 11. The famous Tully Monster comes from here. An old shaft mine at Braceville has a spoil hill that yields concretions. At one time there were spoil hills all over this area but most have been reclaimed or turned into sportsman clubs or cooling lakes for the nuclear power plants. Fossils were also found in central Illinois near Astoria.