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Smithsonian National Museum of Natural History

Department of Paleobiology


The Mazon Creek area, located in the northeastern portion of Illinois (Figure 1), was named for the stream in whose bed and walls the first fossils were found. These fossils were destined to make this area the best known Pennsylvanian flora in North America. With representative specimens found in all the great natural history museums of the world.

       Figure 1

In 1857, concretions containing plant fossils found in the bed of Mazon Creek(a.k.a. river today) and from small scale coal mines were sent to the noted paleobotanist Leo Lesquereux. His subsequent classification and description of 140 species—which around 50 were new—brought this area to the attention of the scientific community. The earliest settlers in the area mined coal from stream banks and outcrops. It was not until commercial mining—shaft mines about 1870 and strip mines in the 1920s—produced spoil piles did large scale collecting of fossil-bearing concretions become possible.

Stratigraphy and Age

The Mazon Creek area fossils are found in siderite concretions, and occasionally as compression fossils on shale. They occur in the Francis Creek Shale Member above Colchester No. 2 coal, in the Carbondale Formation. The formation is Middle Pennsylvanian in age and assigned to the late Moscovian Stage in the latest global Carboniferous System (ICS) or Desmoinesian Stage (formerly called series) in the older North American system (Heckel, 2006). The Mazon Creek flora contains elements considered to be comparable to that in the early Cantabrain Substage of western Europe (Cleal et al., 2003), which makes it about 307 million years old.

The Mazon Creek flora is most similar to the Radstockian flora of the Bristol/Somerset area and that of the Household Coals of the Forest of Dean in England (Wagner, 1972). And in North America it most closely resembles the fossil flora from the Croweburg Fm. near Knob Noster, Mo. and the uppermost beds (upper Ptychocarpus unitus Zone) of the Sydney coalfield in Nova Scotia (Bell, 1938).


Figure 2 (click to expand)

Swamps composed of giant club mosses; seed ferns, true ferns, horsetails and early seed plants such as cordaites dominated the tropical wetland flora of the Middle Pennsylvanian Period in Illinois. Fresh water transported plant remains and deposited them in layers of silt. Through geobiochemical mechanisms concretions formed around the plants and hardened to form coffin-like rock structures that encased impressions of plant and animal swamp life.

Like modern day tropical forests, the Mazon Creek area was frequently visited by thunderstorms that generated the rainfall needed for the coal swamp plants to thrive. The combination of the high level of oxygen in Earth’s atmosphere (nearly 30%, which is one of the highest levels in Earth’s history, vs. the 21% of today), and the abundance of plant matter for fuel, resulted in frequent forest fires. It is thought that during this ancient time, given the presence of high levels of atmospheric oxygen, even wet plants would burn easily. The evidence for these forest fires is the charred plant remains that can be found in the rocks today as fossil charcoal or fusain (fusinite).

Within the Mazon Creek area two biotas have been discovered. One, called the Braidwood Biota is of terrestrial and fresh-water fossils. It contains most of the plant fossils and occasionally terrestrial animals, such as millipedes, arachnids or insects. The other biota, named Essex, is of a near-shore brackish or marine environment, containing mostly marine fossil animals, like sea jellies, annelids, fish and Illinois’ state fossil the Tully Monster. The plant fossils in this area are mostly small and poorly preserved. The dividing line in some areas is very clear; in other places the distinction is difficult to determine. Figure 2 shows a rough sketch of the known boundary.

Figure 3

In the Mazon Creek area, fossil evidence for plant and animal interactions is abundant. Some of it is circumstantial. Many plants species grew tiny hairs on leaves and scales on their stems. This was possibly to act as a barrier to repel arthropods, as it is in modern species. Other evidence is more direct. Here can be found early examples of folivory (animals eating live plants). Leaves will show chew damage that occurred while the plant was alive. There is no clear indication to what type of animal made the damage, but some believe that a “protorthopteroid” insect distantly related to modern grasshoppers and crickets was likely the cause. While some animals ate live plants, others were still eating plant debris on the forest floor. The results of this can be seen in their fossil fecal droppings. Fossils of this type are called coprolites and most cannot be matched with the animal that produced them. Coprolites often contain the fossilized remains of what the animal ate. Figure 3 is an interesting specimen because it not only shows the contents of the meal (lycopod cone), but from its large size, it probably was produced by the only plant-eating animal known to be large enough at that time; a giant (up to 2 meters) relative of the millipedes named Arthropleura.

The Event: Before and After

The world was a much different place 307 million years ago. The days were shorter than in modern times, only lasting 22 hours, and a year was 398 days long. Northeastern Illinois was located about 2600 miles farther south, within 10° of the paleoequator, on the eastern coast of a marine bay-estuary, and just southwest of the ancient Appalachian mountain range. At this time Illinois was situated at the mouth of a large flood plain, towards the eastern end of a vast tropical coal swamp. This swamp extended throughout England, northern Europe and north Africa and even farther to islands in the west that form part of modern day northern China. In total, this ancient coal swamp covered about 20 million sq. km (7.7 million sq. mi.). Only 30 degrees latitude to the south was a massive ice sheet, possibly one of the most extensive in the Earth’s history.

At this time the world was experiencing a global climate change and thought to be set in motion by the Earth’s orbit becoming more concentric. The present ice age was coming to an end, and a new interglacial period was starting. Sea levels were rising from ice melt and the coal swamps were starting to flood. In the equatorial areas the climate was now seasonal, with very wet and semi-dry periods.

The coal mined in the Mazon Creek area, like most Pennsylvanian age coal deposits, is made up of about 85% arboreal lycopsids and 10% calamite plant matter. These quantities have been determined though spore count studies made from coal balls. In addition, complete trunks of lycopsids found in situ from Pit 11 show that these trees were growing out of the coal bed and standing erect. It is thought that the lycopsids grew in the poorly drained, peat/coal forming areas, to the near exclusion of all other types of plants. What this tells us is that given that the plants growing in the Mazon Creek fossil collecting area directly over the coal beds were primarily arboreal lycopsids, the overall diversity of plant life was low. However, it is interesting to note that lycopsids as macrofossils are a relatively small component of any large Mazon Creek fossil collection. It turns out this fact is a clue to what happened to cause the fossils to form in the first place. Another clue comes from studying the Mazon Creek fossils themselves. It becomes quite apparent that there are certain biases in how and what fossils are preserved. For example, some species, notably Pecopteris unita, are found with fertile structures nearly half of the time while others like the more common true fern, Crenulopteris acadica, are rarely found with fertile structures preserved at all.

Most all of what is found as plant fossils in the Mazon Creek area did not grow on top of the peat/coal forming areas, but rather were transported in from better drained non-peat forming lowland areas. The “event” that buried the Mazon Creek area’s coal in fine silt and ultimately caused the formation of the fossils, was a catastrophic flood which at first rapidly covered the area with silt, then continued for many decades, depositing in the order of about one meter per year as sea levels continued to rise. The sediment came from areas denuded of dead and dying vegetation caused by climate change. The initial sediment pulse phase was from accumulated deposits, which had become bound up in rivers and streams. A massive monsoonal-like rain flooded the area and transported the accumulated sediment along with the plants towards the bay and buried them. By this time, most of the lycopsids had already died off as the climate became only seasonally wet. This scenario explains the diverse makeup of the plant fossil species and the relative lack of lycopsid fossils. The relative geologic brevity of the event also can explain the preservational biases seen in some species. The flood event had occurred when these plants were undergoing different parts of their seasonal life cycles.

The plants caught in the flood did not make up the natural biota of the area, but instead represent plants growing within a large geographic area and within a variety of microenvironments which drained towards the ancient Mazon Creek deltaic floodplains. Were it possible to explore this rather large area before the flood event and catalogue the plants from that time, you would find the majority of the plant species—approximately 40 species—belong to the now extinct, but diverse group of plants collectively known as seed ferns. True ferns made up of both tree and scrub-like plants are the next most diverse group, represented by at least 27 species. In disturbed soils out of the flood plane, but near to water, grew roughly 14 different types of sphenopsids. Then, in and along the floodplains, grew another 12 types of lycopsids, likely dispersed amongst them, were some of the 20 species of tree ferns. These different groups, added together with some other more poorly understood plants, would total approximately 105 biological species. In comparison, modern rainforests have about 7000 plant species in a sq. km. The relatively low level of diversity can be better put into perspective if you consider the fact that the vast majority of living plants are flowering plants, and did not evolve for another 180 million years after the Mazon Creek plants lived.

Mazon Creek area fossil collecting

Coal mine spoil piles that once covered acres in the counties of northeastern Illinois are rapidly being leveled because of environmental concerns and land development. The good old days of the concretion collector are gone. Numerous field guides and newspaper articles written in years past detailing locations and ease of access are no longer valid. Small patches of strip and shaft mine spoil piles are still to be found but permission of landowners is required before searching for concretions. There is some good news about the future of fossil collecting in this area. The state of Illinois has been leasing all the northern part of Pit 11, now Braidwood Lake and has recently signed a new 30-year lease. This will allow fossil collecting to continue under state regulations for the life of the lease. The southern portion of Pit 11 is now owned by the state and is part of Mazonia Braidwood State Park.In the park, the state has of now occasionally taken up the turning over of portions of the old strip-mined areas to encourage fossil collecting. Unfortunately for fossil plant collectors, the park contains only fossils from the previously mentioned Essex biota. It is important that collectors abide by all state regulations in the park to help keep collecting alive. As of this writing, a day permit and reporting system is in place for fossils found in the park and a collecting season has been established from March 1 through September 30. It is recommended that collectors check with the park headquarters for the latest regulations at; Mazonia Fossil Permits Office, P.O. Box 126, Braceville IL. 60407 or by calling (815) 237-0063 or to this website

Once collected, concretions should not be hammered too vigorously; many a specimen has been shattered by an impatient collector. Alight tapping may produce a crack which splits open to reveal an impression. However, if tapping fails, the concretions are best treated by the freeze-thaw method. They may be placed in plastic, water-filled pails over winter. The freeze-thaw process of the seasons over several years should open a good proportion of the concretions. Each spring, remove the open concretions from the pails. For those who are less patient, quicker method is to place the concretions in sturdy water filled containers such as plastic bleach or anti-freeze bottles. A few days in the freezer, a few days out to thaw. Check the contents after several thaws and remove the opened concretions. About a dozen such sessions are usually sufficient. Concretions still unopened are best left to the wallop technique, mentioned above. Cleaning fossils is not always desirable and can render specimens useless for paleontological study. For this reason museums generally do not clean specimens. However, if you wish to clean them, wash them in soapy water and/or white vinegar. Soak them in undiluted white vinegar for no more than 15 minutes to avoid damaging the fossil, followed by a thorough water rinse. After drying, place paper toweling between the halves to prevent scratching or disfigurement. Avoid covering the fossil with any sealer. If the concretion is broken, use a small amount of a clear-drying “white” glue on the broken ends and carefully reassemble the fragments, ensuring that the glue does not bubble up along the fracture. Then identify and write a unique number on the specimen with permanent ink. This number should be stored separately along with any collecting data in a safe place, prefer-ably in a hard copy form e.g. index cards or a notebook.

Mazon Creek Links:
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