EARLIER TERTIARY (EOCENE AND OLIGOCENE). 761 



J-K 13, L 13-14. GREAT PLAINS OF COLORADO, WYOMING, MONTANA, NORTH AND SOUTH 



DAKOTA. 



In the northern and central parts of the Great Plains area there are several 

 nonmarine formations lying near the boundary between the Cretaceous and Ter- 

 tiary, concerning the exact age and correlation of which there has been much con- 

 troversy. They are the Denver and Arapahoe formations of the Denver Basin 

 and the Lance and Fort Union formations of the Dakotas, Wyoming, and Montana. 

 The earlier history of the discussion concerning them, in which the more general 

 Laramie question is involved, was simimed up by Clark "^ in 1891. The Arapahoe 

 and Denver formations were first recognized as distinct from the Laramie by Cross 

 and Eldridge ^^^ in the course of their detailed study of the Denver Basin. The 

 following statement concerning their age is quoted from Cross : ^^^^ 



Until the discoveries which are described in preceding chapters were made the strata 

 of the Arapahoe and Denver formations had been uniformly assigned by geologists to the 

 Laramie, under the accepted definition of the latter as the uppermost division of the conformable 

 Cretaceous series; and not only had they been assigned to the Laramie, but no characteristics 

 of any kind had been mentioned, or apparently observed, by which these upper beds might be 

 even locally distinguished from the lower, coal-bearing horizon. This correlation was based 

 on the presence of the true Laramie below the beds in question, on the failure to notice their 

 peculiar and distinguishing characteristics, and on the assumptions regarding the unity of the 

 fossil flora, whose species were, however, collected from widely separated horizons in the Golden 

 section. 



In the earliest descriptions of these formations by Mr. Eldridge and the writer they were 

 assigned to the Tertiary. The reason for this assignment was the discovery that between the 

 Laramie and Arapahoe epochs there had occurred an orographic disturbance whose magnitude 

 was measured, for this locality, by the presence in the Arapahoe strata of pebbles of highly 

 iadurated clastic rocks, sandstones, conglomerates, etc., clearly belonging to various geological 

 horizons as far down as the Trias, representing erosion of 14,000 feet of strata, according to 

 the section of the formations in question in the Denver region. The lithological character of 

 the Denver beds showed that the interval of unlotiown duration between the Arapahoe and 

 Denver epochs had witnessed the occurrence of volcanic eruptions on a gigantic scale, and also 

 subsequent local erosion. 



Up to the time when these formations were thus identified, great orographic movements 

 in the Rocky Mountains had been commonly supposed to mark the ending of Mesozoic time, 

 and to be in great measure the cause of the wonderful changes that took place at this period, 

 especially in vertebrate life, as shown by the remains in the earliest known Eocene deposits. 

 The beginning of Tertiary time was also known to be widely characterized by great volcanic 

 outbreaks, recorded in the sediments of the Green River, Florissant, and other Eocene basins. 

 Hence it seemed natural to place the Arapahoe and Denver beds in the Tertiary, as, perhaps, the 

 earliest lake deposits of Cenozoic time. Examination of the paleontologic evidence available 

 at the time showed either that it did not controvert the assignment or, as in the case of the fossil 

 plants, was entirely untrustworthy because the floras of the distinct horizons involved could 

 not then be compared. 



The recent discoveries of fossil vertebrate remains are said by paleontologists to show 

 that the life of the epochs under discussion was much more nearly allied to Mesozoic than to 

 Cenozoic types, and in deference to this opinion the post-Laramie formations are classed in 

 this report' with the Cretaceous. But such a course raises at once the question as to the nature 

 and position of the boundary between M.esozoic and Cenozoic time in the Rocky Mountains 

 and broadens very materially the treatment which must be given to the problem. 



