Green River Formation
The Green River Formation is an Eocene geologic formation that records the sedimentation in a group of intermountain lakes in three basins along the present-day Green River in Colorado, Wyoming, and Utah. The sediments are deposited in very fine layers, a dark layer during the growing season and a light-hue inorganic layer in the dry season. Each pair of layers is called a varve and represents one year. The sediments of the Green River Formation present a continuous record of six million years. The mean thickness of a varve here is 0.18 mm, with a minimum thickness of 0.014 mm and maximum of 9.8 mm.
| Green River Formation | |
|---|---|
| Stratigraphic range: Eocene, | |
Green River Formation exposed in the cliffs at Fossil Butte | |
| Type | Geological formation |
| Lithology | |
| Primary | varied |
| Other | see text |
| Location | |
| Region | Rocky Mountains: Colorado Utah Wyoming |
| Country | United States |
| Type section | |
| Named for | Green River |
The sedimentary layers were formed in a large area named for the Green River, a tributary of the Colorado River. The three separate basins lie around the Uinta Mountains (north, east, and south) of northeastern Utah:
- an area in northwestern Colorado east of the Uintas
- a larger area in the southwest corner of Wyoming just north of the Uintas known as Lake Gosiute
- the largest area, in northeastern Utah and western Colorado south of the Uintas, known as Lake Uinta
Fossil Butte National Monument in Lincoln County, Wyoming is in a part of the formation known as Fossil Lake because of its abundance of exceptionally well preserved fish fossils.