268 G. B. RICHARDSON THE MONUMENT CREEK GROUP 



formable relations to the underlying Cretaceous strata, but because of 

 the fossil plants found in the group he suggested its equivalence to a 

 portion of the '^Lignitic" group. Later, influenced by Cope, Hayden 

 was "inclined to regard it [the Monument Creek group] as Miocene." 



In 1874, on the basis of vertebrate fossils. Cope concluded that the 

 age of the Monument Creek group was between Eocene and Pliocene, 

 and was probably Miocene. 



The authors of the Denver Monograph pointed out that two divisions, 

 marked by an apparent unconformity, could be distinguished in the 

 Monument Creek, which, nevertheless, they designated a formation in- 

 stead of using Hayden's name group. Emmons, referring to the fact 

 that the vertebrate remains on which the Miocene age of the beds was 

 based had not been definitely located, suggested that they probably carde 

 from the lower division, and that the upper might be Pliocene. Eldridge 

 provisionally accepted Cope's reference of the age of the Monument 

 Creek to the Miocene. In this connection it should be noted that Cope 

 doubtless considered the bones found in the Monument Creek to be 

 White Elver, which formerly was considered Miocene, but which now is 

 classed as Oligocene. 



Darton in 1906 announced the discovery, in the Monument Creek 

 "formation," of Titanotherium bones, "apparently from the upper beds," 

 which he correlated with the Chadrou formation of the White Eiver 

 group. 



PRESENT WORK 



The present writer, in connection with a study of the geology of the 

 Castle Eock quadrangle during the seasons of 1910-'ll, found it neces- 

 sary to separate the Monument Creek grou^D into two formations on the 

 basis of a well-marked unconformity which separates beds of Eocene and 

 Oligocene age. The lower formation is named the Dawson arkose and 

 the upper one the Castle Eock conglomerate. Stratigraphic relation- 

 ships indicate that the Arapahoe and Denver formations are equivalent 

 to the lower part of the Dawson arkose, and this evidence is paleonto- 

 logically supported. 



The map (figure 1) shows the distribution of the Dawson arkose and 

 the Castle Eock conglomerate, and also of the Denver formation and part 

 of the Arapahoe, the latter taken from the Denver Monograph, The 

 unconformity, on the basis of which the Monument Creek group is sub- 

 divided, is finely exposed on Castle Eock, north of the town of tliat name, 

 and is generally well marked throughout the area, being especially promi- 

 nent in the bluffs about Castlewood Eeservoir, in the vicinity of the 

 settlement of Bijou Basin, soutliwest of Elbert, and elsewhere. This 



