Green River Formation
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The Green River Formation is an Eocene Lagerstätte , a rare place where conditions were right for a rich accumulation of fossils. The sedimentary layers were formed in a large area of interconnecting lakes, named for the Green River, a tributary of the Colorado. The area covers the northwest part of the state and neighboring Utah, in places like Bonanza and Vernal, and the southeast corner of Wyoming. The stone matrix is so fine-grained that fossils include rare soft parts of complete insects and fallen leaves in spectacular detail. More than 22 orders of insects are represented in the collection at the Smithsonian Institution in Washington, D.C. alone.
The Green River fossils date about 48 mya, but cover several million years, including the transition between the moist early Eocene climate and the slightly drier mid-Eocene. The climate was moist and mild enough to support crocodiles, which do not tolerate frost, and the lakes were surrounded by sycamore forests. As the lake configurations shifted, each Green River location is distinct in character and time. The lake system formed over underlying river deltas and shifted in the flat landscape with slight tectonic movements, while the Rocky Mountains were being thrust upwards to the west. The lagerstätten formed in anoxic conditions in the fine silt that formed in the lakebeds, some of which lasted so long that the accumulated siltstone has formed varved strata 2000 feet thick. Lack of oxygen slowed bacterial decomposition and kept the usual scavengers away, so leaves of palms, ferns and sycamores, some showing the insect damage they had sustained during their growth, were covered with fine-grained silt and preserved. Insects were preserved whole, even delicate wing membranes and spider spinnerets.
Vertebrates were preserved too, besides the scales of Boreasuchus, the crocodile that was an early clue to the mild Eocene climate of Western North America. Fish are common. The fossils of the herring-like Knightia, sometimes in dense layers, as if a school had wandered into anoxic water levels and were overcome, are familiar to fossil-lovers and are among the most commonly available fossils on the commercial market. Approximately 60 vertebrate taxa in all have been found at Green River. Besides fishes they include at least eleven species of reptiles, and some birds and one slothlike mammal, Brachianodon westorum,. with some scattered vertebrae of others, like the dog-sized Meniscotherium and Notharctus one of the first primates. There is the earliest known bat, already full-developed for flight. Even a snake, Boavus idelmani found its way into a lake and was preserved in the mudstone.
The high quality of preservation of these fossils have led some Creationists to decide that they were laid down by Noah's Flood [1] (http://www.answersingenesis.org/Docs/213.asp).
External links
- Green River Formation (http://www.ucmp.berkeley.edu/tertiary/eoc/greenriver.html)UCMP Berkeley website
- Eocene fossils from the Green River Formation (http://www.coloradomtn.edu/campus_rfl/staff_rfl/kohls/eocene.html); color photos
- Fossil fish of the Green River Formation (http://www.fossilmuseum.net/Fossil_Sites/fossil_Fish%20of%20the%20green_river_formation/fossil_Fish%20of%20the%20green_river_formation.htm)