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> How the Fossils Were Formed

How the Fossils Were Formed

Fossilization Process
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Many animals lived in the shallow marine bays. More plants and animals lived in the swampy areas along the rivers. As animals in the bay died they fell to the bottom of the bay. They were joined by plants and animals that died along the river and were washed into the bays.

When the remains of these plants and animals sank to the bottom of the bays, they were rapidly buried by the mud washing in from the river(s). This process protected the remains from being destroyed. Bacteria began to decompose the plant and animal remains in the mud. The action of these bacteria produced carbon dioxide in the sediments around the remains. The carbon dioxide combined with iron from the groundwater around the remains forming siderite (ironstone). The siderite protected the remains from further damage.

The combination of rapid burial and rapid formation of siderite resulted in excellent preservation of the many animals and plants that ended up in the mud.

Concretion Zones
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Concretion Zones
Courtesy of Stephen LeMay
Copyright 2003, Illinois State Museum Society