THE EOCENE PERIOD 777 



beds are inseparable from the Ft. Union formation (or series) , here- 

 tofore regarded as the oldest division of the Tertiary in the western 

 interior. During the Ft. Union stage, there was an extensive 

 area of aggradation in parts of North Dakota l and Montana, and 

 a still larger area in Canada. The Fort Union beds, composed of 

 sand, clay, etc., are said to be locally 2,000 feet or more thick, and 

 have usually been described as lacustrine. The presence of fresh- 

 water shells (unios, etc.), is consistent with this conclusion for 

 some parts of the formation; but the abundance of the leaves at 

 many places is indicative of subaerial aggradation for other parts. 2 

 Eocene formations of similar age are found in Colorado (Telluride 3 

 and Poison Canyon 4 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, 

 the deformative movements of the time, or the progress of the 

 gradational work of the streams, or both, may have been respon- 

 sible 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 in some other places. The beds of this stage have 



maximum thickness of several thousand feet, and are now 6,000 

 to 7,000 feet above the sea. About 77% of the fossils of this stage 

 are of terrestrial life. 5 



3. The third recognized stage of the Eocene of the west is the 

 Bridger* during which sedimentation was in progress in various 

 places. One area was in the Wind River basin, north of the moun- 



1 Wilder has recently called into question the separability of the Fort 

 Union and Laramie, in western North Dakota. Jour. Geol., Vol. XII, p. 290. 



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. 



3 Purington, Telluride, Colo. Folio, U. S. Geol. Surv. 



4 Hills, Science, Spanish Peaks and Walsenburg, Colo. Folios, U. S. Geol. 

 Surv. 



5 Loomis, Am. Jour. Sci., Vol. XXIII, 1907. 



6 The Green River group of Hayden and Powell, the Wind River group 

 of Hayden, and the Dinoceras beds of Marsh, are included here. 



