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.
Note: To zoom in and out, click on the photo below: