The Green River Formation is a world-renowned fossil site, offering an exceptional window into an ancient world. It preserves an incredible diversity and quality of fossilized life, providing a detailed glimpse into a prehistoric ecosystem. The formation’s unique conditions created a fossilized landscape unlike almost any other on Earth.
The Ancient Lake System
The Green River Formation represents the remnants of an expansive system of intermountain lakes, not a single river. These ancient lakes existed across what is now Wyoming, Colorado, and Utah. This lake system thrived during the Eocene Epoch, approximately 53 to 49 million years ago. During this period, the uplift of the Rocky Mountains created basins where large freshwater lakes, such as Lake Gosiute, Lake Uinta, and Fossil Lake, collected sediments.
A Perfectly Preserved World
The Green River Formation contains exceptionally preserved fossils, including a wide array of life forms. Fish fossils are abundant, with species like Knightia and Diplomystus commonly found. The fine-grained sediments also preserved delicate organisms such as insects, spiders, and complete palm fronds. Rare finds include birds, reptiles like crocodiles and snakes, and early mammals such as bats, some with intact skeletons and wing membranes.
This exceptional preservation is attributed to unique conditions within the ancient lake beds. Fine-grained carbonate sediments, primarily limestone, settled gently, capturing minute details of organisms as they sank. The deep waters were often anoxic, lacking oxygen, which prevented scavenging and slowed bacterial decay. This combination of rapid burial in fine sediment and oxygen-poor environments created a lagerstätte, a geological term for a site of remarkable fossil preservation.
What the Fossils Reveal
The fossils from the Green River Formation offer scientists insights into the Eocene Epoch’s environment. The presence of subtropical plant fossils, such as palms and sycamores, alongside reptile fossils like crocodiles, indicates a warmer and wetter climate than exists today. Crocodiles, for example, cannot tolerate frost, underscoring the mild temperatures of the Eocene. The formation preserves a nearly complete ecosystem, offering a rare snapshot of species interactions, including evidence of fish eating other fish, which provides direct information about ancient food webs.
Finding and Owning a Piece of History
A distinctive aspect of the Green River Formation is the accessibility of its fossils to the public. Unlike many strictly protected fossil localities, numerous privately-owned commercial quarries operate within the formation. These quarries legally excavate fossils for sale to museums, private collectors, and individuals.
Many commercial operations offer “pay-to-dig” experiences, allowing tourists and hobbyists to search for their own fossils. Visitors can split layers of shale to uncover common fish fossils, such as Knightia, and sometimes insects or plant specimens. These operations often provide tools and guidance, making the experience educational and rewarding.