Mazon Creek Fossil Flora
The Mazon Creek fossil deposit extends over a wide area of northeastern Illinois. The fossils are best known from concretions or nodules of siderite, an iron carbonate mineral, which generally must be fractured to expose a plant or animal fossil within. These nodules occur in a geological unit known as the Francis Creek shale, a wedge-shaped body of gray shale that lies immediately above the Colchester (No. 2) coal bed in northeastern Illinois. The Francis Creek shale is of Middle Pennsylvanian age, approximately 309 million years old, which is part of the late Moscovian Stage in the latest international terminology for the Carboniferous System (ICS), or the Desmoinesian Stage in regional North American terminology. At the time of Francis Creek shale deposition, the Mazon Creek area was near the equator and the present day continents formed one large land mass, Pangaea.
The Pennsylvanian Subperiod of the Carboniferous was a time of intense polar glaciations, particularly in the southern hemisphere, known as Gondwana. These glaciations resulted in a series of glacial-interglacial cycles much like those the Earth has experienced over the past million years or so. The Francis Creek shale appears to have been deposited as the Earth transitioned from one of these glacial intervals to an interglacial. This transition was accompanied by rising sea-level, which flooded the surface of the vast coastal peat-forming swamp that became the Colchester coal. During this time, climate also was in transition from very wet when the peat/coal bed formed, during glacial times, to increasingly seasonally dry during the interglacial. The plants that were deposited in the Francis Creek shale were part of the waning wet climate that accompanied drowning and flooding of the tropical lowlands.
Plant and animal fossils have been excavated from the banks of Mazon Creek (River) since the 1840s. About 1855, papers describing the plant and animal fossils found in these concretions began to appear in scientific journals. Preservation of the land animals, insects, and other arthropods equaled or exceeded anything known at that time. This association of fauna and flora (the Mazon Creek biota) became world famous. Concretions and their contents became paleontological prizes, subjects of intensive research. At the turn of the century, shaft mines and then strip mines, permitted the collecting of fossils from Will to LaSalle counties, roughly paralleling the Illinois River for over 50 miles. Concretion collecting developed into a popular hobby and tens of millions of these fossiliferous rocks have been rescued from weathering and/or reburial by collectors.
Large collections of Mazon Creek plants and animals are held by many major museums around the United States and in many foreign countries. The largest U.S. collection is housed in the Field Museum of Natural History in Chicago. The plant fossils from that collection have recently been re-examined and a catalogue of the flora has been published by Jack Wittry (Wittry, J. 2006. The Mazon Creek Fossil Flora. ESCONI Associates, Downers Grove, Illinois. 154 p.). The Smithsonian has a modest sized, but excellent collection of Mazon Creek plants and animals, many acquired by donations made by private collectors. Especially fine and noteworthy are the Langford and McLuckie collections.