50,000,000 years ago ancient Fossil Lake existed in what is now southwest Wyoming. Of its estimated maximum size of 930 square miles, roughly 230 square miles across the center of the ancient lake-bed contain exceptionally fossiliferous sediments and associated geologic features including deltas, beaches, springs, and rocks from the center and nearshore environments.
The unusual chemistry of Fossil Lake prevented decay and scavenging of dead organisms while millimeter-thick layers of alternating limestone matter slowly accumulated. The result is laminated limestone that contained the highest concentration of fossil fish in the world. These fish, aquatic insects, and associated geologic features make Fossil Lake the world's best Paleogene record of a freshwater lake ecosystem.