​The Green River Fossil Formation

​The Green River Fossil Formation

Posted by Oak Rocks on 20th Jun 2023

The Green River Fossil Formation is one of the best known and most prolific fossil locations in the U.S.

About 55 million years ago, during the Eocene period, the Rocky Mountains were formed. At that time three lakes formed which covered western Wyoming, eastern Utah, and northwestern Colorado. Those three lakes were Lake Uinta, Lake Gosiute, and Fossil Lake.

The sediments deposited by these lakes became the rock comprising the Green River Formation. Fossil Lake, in southwest Wyoming, covered the smallest area and deposited the least sediment. These sediments however, have the greatest fossil concentration in the area.

The climate in the area at that time was most likely tropical and seasonal rains brought a deposit of fine mud which would bury the bodies of fish and plants on the bottom of the lake. In Fossil Lake a very fine layer of salt water kept the other fishes from scavenging these bodies. With the conditions just right, the bodies were prevented from decaying and fish and plants were preserved. That fine mud or silt created paper thin layers, that over many many years, slowly compacted, flattening the fish and plants. The organic material was carried away by groundwater, leaving the carbon which stained the bones and scales the variety of browns we see in the fossils today. Eventually these layers became limestone and accumulated into a thickness of up to 2000 feet in some areas.

There are documented records of fossils from the Green River Formation in the journals of early missionaries and explorers in the 1840’s. Geologist Dr. John Evans collected the first fossil fish, described as Clupea humilis (later renamed Knightia eocaena), from the Green River beds in 1856. And Edward Drinker Cope of Dinosaur bone collecting fame, collected extensively from the area and produced several publications on the fossil fish from 1870 onwards.

However, the first major discovery of this prolific fossil area occurred in 1868, when Union Pacific was building a railroad through the area. The railroad employees found fossil fish while excavating an area about 2 miles west of where present-day Green River is. Word quickly spread and many people flocked to explore the area to explore and collect.

Ranchers started settling in the area in 1884, and oil was discovered in the area in 1885.

Fossil Butte National Monument was established in Wyoming in 1972. It is 13 square miles and encompasses about 1% of the Fossil Lake area. It was designated to preserve and protect the fossils in the area. You may not dig fossils in the National Monument, but the surrounding area, known as Fossil Basin, (located in southwestern Wyoming, and includes the towns of Kemmerer and Diamondville), has some private quarries, and commercial quarries located on both state and private land, that allow visitors to dig for a fee.

Collecting these fossils is usually done with pry bars to split the layers. As the rock is split it is inspected several times for evidence of fossils. Once found the fossils are usually prepped with the aid of a special pneumatic tool.

Millions of fish fossils have been collected from the area, commercial collectors operating from legal quarries on state and private land have been responsible for the majority of Green River vertebrate fossils in public and private collections all over the world. Although most of the fossils collected from the Green River Formation are fish, many other fossils have been found including crocodilians, lizards, snakes, birds, bats, small horses, amphibians, gastropods, crustaceans, and insects. Some 35,000 fossiliferous rocks from the Green River Formation are housed at the Smithsonian Institution in Washington, D.C.

You can purchase some of these amazing   fossil fish right here on Oakrocks.

Thanks for reading and happy hunting!