178 Darton — Age of the Monument Creek Formation. 



Akt. XXI. — Age of the Monument Creek Formation '•* by 

 K H. Darton. 



This contribution is an account of additional evidence as to 

 the Oligocene age of the Monument Creek formation, or at 

 least of its upper member, afforded by the discovery of Titano- 

 therium and other fossil bones at several localities. 



On the high divide between the Platte and Arkansas drainage 

 basins, at the foot of the Rocky Mountains, there is an extensive 

 deposit of sands, gravel and clay to which F. Y. Hayden gave 

 the name of Monument Creek group. f This observer recog- 

 nized the fact that the group overlies the Laramie formation 

 unconf ormably, but apparently he included in its lower portion 

 more or less of the beds later separated, as the Arapahoe and 

 Denver formations in the Denver region. The opinion was 

 held that it was of early Tertiary age, but no precise correla- 

 tion was suggested. In 1873, Prof. E. D. Cope examined a 

 portion of the deposit and found a few bones in regard to 

 which he made the following statement :J 



"The age of the Monument Creek formation in relation to 

 the other Tertiaries not having been definitely determined, I 

 sought for vertebrate fossils. The most characteristic one 

 which I procured was the hind leg and foot of an Artiodaotyle 

 of the Oreodon type, which indicated conclusively that the 

 formation is newer than the Eocene. From the same neigh- 

 borhood and stratum, as I have every reason for believing, the 

 fragment of the Megaceratops coloradoensis was obtained. 

 This fossil is equally conclusive against the Pliocene age of the 

 formation, so that it may be referred to the Miocene until 

 further discoveries enable us to be more exact." 



Doubtless Professor Cope regarded the fauna as belonging 

 in the White River group, which is now generally considered 

 to be Oligocene. He added nothing regarding the precise 

 locality, or stratigraphic position of the fossils. So far as I 

 can find, no further paleontological evidence has since been 

 oifered, regarding the age of the formation. A brief account 

 of the Monument Creek formation was given by G. H. Eldridge, 

 in the " Geology of the Denver Basin. "§ The true strati- 

 graphic limits of the formation in relation to the underlying 



* Published by permission of the Director of the United States Geological 

 Survey. 



f Preliminary field report of U. S. Geological Survey of Colorado and New 

 Mexico, 1869, p. 40. 



X [7] Annual Report of the United States Geological and Geographical 

 Survey of the Territories, embracing Colorado, Report for 1873, by F. V. 

 Hayden, p. 430. 



§ United States Geological Survey, Monographs, vol. xxvii, pp. 252-254. 



