EARLIER TERTIARY (EOCENE AND OLIGOCENE). 775 



For the east side of the Bighorn Basin Washburne *" gives the following section : 

 Siratigraphic column on the east side of the Bighorn Basin, 'Wyoming. ' 



System, 



Formation. 



Characteristics. 



Tertiary (Eocene). 



Wasatch formation 



-Unconformity 



Fort Union formation 



-Unconformity (?) 



Bright-colored clays, with a few thin lenses of sand- 

 stone. Contains workable coal in the central 

 part of the basin. 



Dark-colored shale and massive sandstone. Con- 

 tains workable coal. 



Cretaceous. 



[For the Cretaceous see p. 690.] 



Loomis '^^ has discussed the conditions of deposition in the Bighorn Basin 

 and has shown that the strata are mainly fluviatile, using the term to cover all kinds 

 of flood-plain deposits, in contrast to lacustrine. Osborn ^^'^ says : 



Geologically tlie section is 2,391 feet thick, divided into lower, middle, and upper levels, 

 all showing flood-plain rather than eoUan characteristics, but indicating different rates of depo- 

 sition and consequent longer or shorter exposure of the deposits to the sun and air. Only the 

 middle or red beds are decidedly fossiliferous, and they seem to have been exposed longest to 

 the air, leaving the bones of terrestrial animals on the flats; they contain the typical Wasatch, 

 Coryphodon and Eohippus fauna. OccasionaUy truly aquatic animals, such as crocodiles, fishes, 

 and turtles, becoming stranded or inclosed in lagoons far from the river, mixed their remains with 

 those of the land animals. Loonais's approximate analysis of the natural habitat of the total 

 vertebrate fauna is: Aerial, 3 per cent; terrestrial and arboreal, 77 per cent; amphibious, 12 

 per cent; aquatic, 10 per cent. 



The section compiled by Osborn *^^'' from Loomis gives the Lambdotherium 

 zone as the highest fossiliferous horizon of the Bighorn Basin, with 730 feet of 

 unfossiliferous clays above it. Lambdotherium occurs at the base of the Wind 

 River section and thus serves to correlate the two. 



L 12. WESTEBN MONTANA. 



The areas in southwestern Montana -on the headwaters of the Missouri, which 

 are distinguished by the "Later Tertiary" color on the map, include possibly 

 Eocene and certainly Oligocene deposits. Douglass ^^^ describes the Sage Creek 

 beds (Eocene?), White River (Oligocene), and Fort Logan beds (upper Oligocene, 

 John Day?). 



The Eocene (?) "occurs on Sage Creek about 7 milfes northeast of Lima^ 

 Beaverhead County." The fossils found are considered to represent Heptodon?, 

 Hyrachynus priscus Douglass, and Metamynodon?. 



The deposits of White River age are shown to be lacustrine, in contrast to those 

 of the Great Plains, and " appear in the main to represent the Titanotherium and 

 Oreodon beds of South Dakota. " 



The Fort Logan beds of Douglass are the lower Deep River beds of Scott. '^^^ 

 They occur on Smith River, formerly known as Deep Creek, southwest of the Little 



