Green River Formation
Fossil Fish and Oil Shale
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Fossil Butte in southwestern Wyoming.

At first glance it looks like another badlands covered sparsely by vegetation. The hills have steep ravines. The summers are hot and dry and the winters are cold and windy. Now examine the rocks of Fossil Butte. They tell of a time when the climate and the terrain were much different from today. There are fossils of palm fronds and of the bones of crocodiles and turtles. And there are fossils of fish. Millions of specimens of fossilized fish have been recovered from Fossil Butte and the surrounding area. All of this indicates a time when the climate was tropical and the region was covered by huge lakes. And there is something else here: One of the greatest deposits of hydrocarbons in the world.

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The rocks exposed at Fossil Butte are those of the Green River Formation. It is a geologic formation that covers substantial parts of Utah, Wyoming and Colorado. They are the deposits of ancient lakes. At Fossil Butte, the deposits were laid down as paper-thin layers. There are several million such layers. And within these layers have been the millions of specimens of fossil fish.

The Green River Formation also contains the largest oil shale deposit in the world. Oil shale is a misnomer. It is composed of neither oil nor shale. What seems to be shale is a type of limestone that formed from the accumulation of tiny algal shells. The organic remains of the algae are in the form of a axe hydrocarbon nown as kerogen. If kerogen is subjected to suitably high pressure and high temperature over the correct length of time, then oil is produced. Whether it will ever be economically feasible to dig up the oil shale and, artificially, convert the kerogen into oil is yet to be determined.


Fossil fish from the Green River Formation at Fossil Butte. (Photo courtesy of the National Park Service)