560 EOCENE AND OLIGOCENE PERIODS 



on their fossils. These several stages are not readily correlated 

 with those of the coasts. 



1. Reference has been made (p. 539) to certain formations 

 (Denver, Raton, Lance, etc.), formerly classed as Cretaceous, which 

 probably should be regarded as early Eocene. Some of these beds 

 are inseparable from the Fort Union formation (or series), commonly 

 regarded as the oldest division of the Tertiary in the western inte- 

 rior. During the Fort Union stage, there was an extensive area of 

 aggradation in parts of North Dakota 1 , Montana, and farther north. 



The Fort Union beds are clastic and are said to be locally 2,000 

 feet or more thick. Parts of the formation may be lacustrine, but 

 parts are subaerial 2 as indicated by the abundance of leaves at 

 many places. The Fort Union series contains much coal, including 

 some that was formerly classed as Laramie. Eocene formations of 

 similar age are found in Colorado (Telluride and Poison Canyon 

 formations), New Mexico (part of the Puerco beds), and elsewhere. 



The sites of early Eocene deposition were finally shifted. In 

 so far as the sedimentation was in lakes, the basins may have been 

 filled or warped out of existence, and in so far as it was subaerial, 

 deformative movements, or the progress of the gradational work of 

 the streams, or both, may have been responsible for the shifting. 



2. During the next or Wasatch stage of the period, sediment 

 was being deposited over parts of Utah, western Colorado, and 

 Wyoming, and elsewhere. The beds of this stage have a maximum 

 thickness of several thousand feet, and are now 6,000 to 7,000 feet 

 above the sea. About 77% of the fossils are of land life. 



3. The third recognized stage of the Eocene in this region is the 

 Bridger, during which sedimentation was in progress in the Wind 

 River basin north of the mountains of that name, and another, a 

 little later, in the basin of the Green River, both in Wyoming, and 

 in Utah south of the Uinta Mountains. It may have been during 

 this stage, too, that the volcanic tuff (San Juan formation, 2,000 

 feet and less thick) of southwestern Colorado was made. This 

 last formation is of interest as an index of the vigor of volcanic 

 action in this region. Beneath it, glacial drift was found in 1913 

 by Professor Atwood. Its extent is undetermined, and it maybe 



1 Wilder. Jour. Geol., Vol. XII, p. 290, and Leonard, State. Geol. Surv. of 

 North Dakota, Fifth Biennial Kept. 



2 For criteria for distinguishing lacustrine and subaerial formations, see Davis, 

 Science, N. S., Vol. VI, p. 619, 1897, and Proc. Am. Acad. Arts and Sci., Vol. 

 XXXV, p. 345, 1900. 



