The Green River Formation records the sediments of great fresh water basins, covering a large area of Green River in Wyoming where the fossil herring below was found, Colorado, and Utah. Unlike most marine deposits, the freshwater strata are very fine layers, forming geological corridor of only six million years. The area in question is shown as an inset to the fossil map of Fossil Butte National Monument in Lincoln County, Wyoming, which is also known as Fossil Lake, because of the abundant fish fossils preserved in the rock. For an in depth discussion of the Green River Formation, press this link.
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