The rocks in the Butte were laid down about 50 million years ago. Red rocks in the lower part are stream deposits named Knight Formation. Light colored beds in the upper part are of the Green River and are remnant of beds deposited in a lake once covering much of western Wyoming.

The fish-bearing layer and quarries are about half-way up the butte. The Green River Formation has yielded a great many varieties of fossil fish, including herring, perch, catfish and stingrays as well as fossil insects, birds and bat.