I04 ELIOT BLACKWELDER 



and to measure its duration are not feasible on the basis of the facts 

 obtained in this district alone. The youngest beds involved in the 

 folding are of the Montana group, the age of which is well estab- 

 lished by marine fossils as middle Upper Cretaceous. Still younger 

 strata, which now stand in a nearly vertical attitude in the canyon 

 of Buffalo Fork, have yielded plant remains, identified by Dr. 

 F. H. Knowlton as belonging to the Fort Union flora. Elsewhere 

 in Wyoming it is reported^ that the Fort Union and other formations 

 which are correlated with it have been folded to the same degree 

 as the underlying marine Cretaceous strata and are overlain in 

 angular unconformity by the Wasatch Eocene and equivalent 



Fig. 4 



formations. However, in some of these districts the Cretaceous 

 rocks are decidedly more folded than the Fort Union beds. This 

 has been most clearly worked out in southwestern Wyoming by 

 Veatch and Schultz.^ The Fort Union formation and contempo- 

 raneous deposits elsewhere are generally referred to the ''basal 

 Eocene" or Paleocene,^ although both stratigraphically and pale- 



^ Max W. Ball and E. Stebinger, U.S. Geol. Survey, Bull. 381, 1910, p. 194; 

 D. E. Winchester, U.S. Geol. Survey, Bull. 471, 191 2, p. 47. 



^ A. C. Veatch and A. R. Schultz, " Geography and Geology of a Portion of South- 

 western Wyoming," U.S. Geol. Survey, Prof. Paper 56, 1907. 



3 Although the term "Paleocene" has not yet been sanctioned by the U.S. Geo- 

 logical Survey and is not in use by all American geologists, it is so much needed in this 

 instance in order to discriminate between "Lower Eocene" and other early Tertiary 

 rocks stiU older than "Lower Eocene" that it will be freely used in this paper. The 

 term "Eocene" will refer in these pages to those strata now commonly included in 

 "Lower," "Middle," and "Upper Eocene," or, in terms of Rocky Mountain stratig- 

 raphy, the sequence from Wasatch to Uinta formations. 



