Phareodus encaustus

Green River Formation - 

The composition of the Green River rock formation consists of a group of sedimentary rock layers formed in a huge system of lakes. The source of the Green River Formation happened mostly during the Eocene Period between 59-33 million years ago. It's also located along the Green River of today, covering parts of Utah, Wyoming, and Colorado. In this post, we'll concentrate on the various layers that contain fossil fish.

Streams exiting mountainous regions brought vast quantities of sand, silt, mud, and minerals dispersed into lakes that filled the intermountain foundation. Over time the lakes started to be infilled with sand, silt, and mud. The minerals dissolved also altered the reservoir waters' composition. Plentiful plants flourished in large swampy areas that formed along the lakeside margins.

Most of the Green River Formation fossils in southwest Wyoming are extracted from Fossil Lake. This region was divided into two 'fossil zones' both of which include well-preserved fossil specimens. One layer is referred to as the “Split fish layer”, which is loaded fossil fish as well as other fossils. This layer is about 6 feet thick. The sediment was heavily laminated in the Split fish layer, meaning it has many layers that are easily separated to expose fossils. The second fossil zone is referred as the “18 Inch layer”. The 18 Inch layer is not a laminated layer and as expected it is about 18 inches thick. This second layer also contains abundant fossils which are exquisite, but the fossils are much more difficult to expose and prep since the layer is much denser and not layered.

It can be very hard to determine the exact age for a rock structure. However, through the study of volcanic soil particles, the fossil bearing rocks of the Green River Formation were dated to be 53 to 48 million years ago. Volcanoes in what would become Yellowstone National Park to the north and the San Juan volcanic field to the south formed intermittent dust storms that fell in thin layers of volcanic ash into the still waters of the lake. These deposits of ash have been preserved and contain fine mineral grains that have crystallized mostly during a volcanic eruption. The researchers obtained specimens of these ash fall layers to date the Green River Formation fossils.