The Green River Formation, which dates back to the Eocene Epoch, records the sediments of great fresh water basins, covering a large area of Green River in Wyoming, Colorado, and Utah, where the fossil herring below was found. Unlike most marine deposits, the freshwater strata are very fine layers, forming a 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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