What is Green River Formation?
[[File:Cockerellites liops Green River Formation.jpg|thumb|Fossil fish from Fossil Butte National Monument]] The '''Green River Formation''' is an [[Eocene]] [[rock formation]] on the eastern side of the [[Rocky Mountains]].<ref name=brad/> It gets [[sediment]] from a group of intermountain [[lake]]s. Its fine [[fossil]]s make this a [[lagerstätte]], a place of exceptional preservation. The various fossil beds of the Green River Formation span a five million year period, between 53.5 and 48.5 million years ago (mya).<ref>Smith M.E; Singer B & Carroll, A. 2003. 40Ar/39Ar geochronology of the Eocene Green River Formation, Wyoming. ''Geological Society of America Bulletin'', '''115'''(5), 549-565.</ref> During this period the climate moved from the moist early Eocene climate and the slightly drier mid-Eocene. The sediments are deposited in very fine layers, in pairs: a dark layer from the growing season, and a light-hue inorganic layer in winter. Each pair of layers is called a [[varve]] and represents one year. The sediments of the Green River Formation present a continuous record of six [[million]] years. The mean thickness of a varve here is 0.18&nbsp;mm, with a minimum thickness of 0.014&nbsp;mm and maximum of 9.8&nbsp;mm.<ref name=brad>Bradley W.H. 1929. The varves and climate of the Green River epoch: ''U.S. Geol. Survey'', Professional paper #158, pp 87–110.</ref> The sedimentary layers were formed in a large area named after the present-day [[Green River]], a tributary of the [[Colorado River]]. The formation is in three separate [[Basin (geology)|basins]] around the [[Uinta Mountains]] of northeastern [[Utah]]: * an area in northwestern [[Colorado]] east of the Uintas * a larger area in the southwest corner of [[Wyoming]] just north of the Uintas known as ''Lake Gosiute'' ** another area, in [[Fossil Butte National Monument]], Lincoln County, Wyoming is '''Fossil Lake'''. It has exceptionally well preserved fish fossils. The most productive zone — the split fish layer — is a series of laminated or [[