The Green River Formation is an Eocene rock formation on the eastern side of the Rocky Mountains. It records sediment from a group of intermountain lakes. Its fine fossils make this a lagerstätte, a place of exceptional preservation.
The various fossil beds of the Green River Formation span a 5 million year period, dating to between 53.5 and 48.5 million years ago (mya). During this period the climate moved from the moist early Eocene climate and the slightly drier mid-Eocene.
The sediments are deposited in very fine layers, in pairs: a dark layer from the growing season, and a light-hue inorganic layer in winter.
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 after the present-day Green River, a tributary of the Colorado River. The formation is in three separate basins around the Uinta Mountains of northeastern Utah: