The Green River Formation is a geologic site that dates back more than 50 million years. The formation occurred due to mass sedimentation in a group of intermountain lakes in three basins along the current Green River in Colorado, Wyoming, and Utah. Covering an area of more than 65,000 square kilometres, the site was formed by lakes on the intermountain foundation being gradually filled with sediment, silt, mud, and dissolved minerals from streams draining the steep slopes. Wide wetlands that formed along the lakeshores were home to a variety of vegetation, including ferns and palm fronds.

It has been found that the uppermost layer of rock holds the most extensive record of fossilised freshwater fish in North America, with complete fossils of over 20 different fish species, 100 different kinds of insects, and a large variety of plants. 

The formation has allowed palaeontologists to learn about the habits of ancient lake inhabitants. For example, mass mortality layers are found, where many specimens of the same fish species are preserved in a small area, indicating schooling.