Mazon Creek

Mazon Creek is a lagerstätte in northeast Illinois. It is a site of exceptional fossil preservation from the Pennsylvanian sub-period of the Carboniferous, about 309 million years ago.
The area was declared a National Historic Landmark in 1997. 
The site's greatest find is the oldest-known beetle.
Palaeoecology.
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: