The Green River Formation is a rock unit formed from sediment which accumulated in three interconnected lake basins in the region where Wyoming, Colorado and Utah meet. Fifty million years ago, this was a lush, sub-tropical area, very different from the high desert climate found in much of the area today. We know this because of the many fossils found in the Green River Formation.
The south and western lake basin is now called fossil Lake Uinta. This area, where attempts have been made to extract oil from the Green River Formation and related oil-shales, has preserved a record of much of the plant and insect life of the time. Sycamore forests were once predominant. The area surrounding the lake must have looked like a southeastern U. S. hardwood forest. The layers which contain mostly insects give us another perspective of the otherwise attractive scenery.
The basin to the north and east is now called fossil Lake Gosiute. The deposition here ranges from highly organic oil-shales, to beds of hard limestones and sandstones. A large variety of lake environments are preserved in Lake Gosiute. It was from some of these beds, along the Green River in Sweetwater County, Wyoming, that the first Green River Formation fossil fishes were recovered about 150 years ago.
Fossil Lake, in Lincoln County, Wyoming is the source of most of the familiar Green River Formation fossils. Though Fossil Lake was much smaller than Lake Uinta or Lake Gosiute, it was much deeper, and it is here that the best preserved and most abundant fossil fishes are found. Associated with the fishes are occasional rarities such as spectacular palm leaves, crocodiles, birds, and even bats.
The Institute has numerous fossil fish for sale. These include specimens not only from the Green River Formation, but also from the country of Lebanon and Niobrara Chalk Formation of Kansas.