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The most productive zone—called the split fish layer—consists of a series of laminated or varved lime muds about 6 ft (1.8 m) thick, which contains abundant fish and other fossils.
These are easily split along the layers to reveal the fossils.
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 lagerstätten formed in anoxic conditions in the fine carbonate muds that formed in the lakebeds.
The lithology of the lake sediments is varied and includes sandstones, mudstones, siltstones, oil shales, coal beds, saline evaporite beds, and a variety of lacustrine limestones and dolostones.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.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 formation of intermontane basin / lake environments during the Eocene resulted from mountain building and uplift of the Rocky Mountains (late Cretaceous Sevier orogeny and the Paleogene Laramide orogeny).