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.

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.