The Green River lakes
Fish, birds and leaves preserved one annual layer at a time
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The Green River Formation is the deposit of three great lakes that covered parts of Wyoming, Utah and Colorado in the Eocene. The annual layers, varves, make Green River one of the few places where geological time can be counted directly rather than estimated. The fauna documents a subtropical North America: crocodiles in Wyoming, palms in Colorado. The formation also holds the world's largest known oil shale deposit, which for a century has been declared the energy source of the future at intervals without extraction ever proving economic.
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Great lakes covered parts of Wyoming, Utah and Colorado. The bottom mud settled in thin layers, one for each year, and anything that sank was pressed flat between them. The result is fossils of fish, birds, bats and leaves so clear that the feathers show.
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