Fossils Give Clues to Wyoming's Paleo-Past

Giant cypress trees growing today in swamps (or forested wetlands), such as these found in Louisiana's Pointe Lake, used to grow in Wyoming back when it was a warm, subtropical swamp - about 55 million years ago during the Late Paleocene epoch. Some of these ancient trees were buried under sediment and turned to stone. the three petrified trees located here were found during coal mining operations at the Eagle Butte Mine near Gillette. They were generously donated by Alpha Coal West, Inc., and transported to this site by the Wyoming Department of Transportation.

Coalification: Turning Swamps into Coal
Vegetation thrives in warm wetlands. As these plants die, they accumulate in a layer of material called peat. Peat generally occurs below the water line where bacteria turn the plants into a dark gel. Over time, the gel gets buried deeper under new layers of plant batter and peat. high temperatures within the earth essentially cook this material over millions of years, driving off moisture and converting the plant residue into the organic, sedimentary rock that we call coal.

Petrification: Turning Wood into Stone
While some trees that die in swamps turn into soft coal, others turn into hard stone. Submerged underwater in the muddy, low-oxygen conditions of a swamp, some tree trunks are protected from decay. Silica dissolved in groundwater soaks into the wood. Through a chemical process called petrification, this and other minerals replace the plant tissues, making a crystalline cast of the tree. The orange and red colors seen in these examples indicate the presence of iron oxide, along with silica.

Rings Reveal Age of Tree
The petrified tree next to the front door of the Visitor Information Center has visible tree rings. If you count them, you can estimate its age to be about 300-500 years old - when it died millions of years ago. it died young. The petrified tree near you is much larger and probably lived much longer, but its rings are harder to read. mature cypress trees in this area reached between 1,200 and 1,500 years of age.

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