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The sediments were laid in a gigantic tropical estuary, a river delta with shifting channels and mud and sand bars. Much of the continent was then covered by a great internal epeiric sea, which sometimes swept over the area, and then withdrew.
Coal measures are formed like this. A low-lying tropical swamp or rain forest forms a peat swamp which is short of oxygen. Dead plant material builds up, and does not decompose entirely. The plant material gets buried as a peat bog. From time to time rivers bring down a load of sediment from nearby mountains, covering the swamp. Eventually layers of coal alternate with layers of sandy deposit. Pressure turns it all into hard rock. Later on, the rocks are uncovered by erosion, and fossils can be found by humans.
The huge growth of vegetation at this time produced coal measures, and the fossils were found in concretions (hard balls) in the spoil heaps of coal mines. The concretions are caused by iron carbonate, FeCO3, which makes the parcel of mud and organic material into hard balls.
Flora and fauna
Fossil specimens have been found which belong to these groups of plants and animals:
- 140 species of insects and other arthropods.
- Freshwater aquatic faunas include fish, amphibia, bivalves, crustacea & horseshoe crabs.
- Fauna from brackish water include ostracodes, lungfish scales, xenacanth sharks, polychaetes, crustacea, cephalopods with soft tissues, hydrozoa, siphonophores, chitons, sea cucumbers, eurypterids, echinoderms.
- One unique species cannot be placed in a taxon: the 'Tully Monster', Tullimonstrum gregarium, a soft-bodied marine invertebrate. It is the Illinois state fossil.
- 400 species of plants: Lepidodendron, horsetails, ferns, pteridosperms and primitive conifers and ginkgoes.
- Joanne Klussendorf (March 30, 1995), PDF (297 KB), National Park Service . PDF (497 KB)
- Bethoux, Oliver 2009. The earliest beetle identified. Journal of Paleontology 83, p931.
- Schellenberg, Stephen A. 2002. Mazon Creek: preservation in late Paleozoic deltaic and marginal marine environments. In Walter Etter, James W. Hagadorn, Carol M. Tang, David J. Bottjer eds Exceptional fossil preservation: a unique view on the evolution of marine life. Columbia University Press N.Y.