The Green River Formation The Green River Formation contains the sediments from one of the world's longest-lived great lake systems, which included Fossil Lake, Lake Gosiute, and Lake Uinta, located within modern-day Wyoming, Utah, and Colorado. Within the formations sedimentary rocks, particularly those from the relatively small Fossil Lake of the early Eocene (between 50-55 million years ago), we find the best paleontological record of an early Tertiary aquatic community in North America and, possibly, the world. The fossil data recovered here strongly indicates a freshwater environment with tropical or subtropical conditions, similar to the present-day Gulf Coast. Today, in contrast, the area features a high mountain desert climate, where water is scarce and winters long and cold. Within the area of southwestern Wyoming that Fossil Lake once encompassed, different collecting localities correspond to both near-shore and deep-water environments, each with its own distinct set of organisms.