HISTORICAL REVIEW OF THE LARAMIE PROBLEM. 



21 



ology, and life of the "Monument Creek forma- 

 tion," which was of course the basis for the 

 general statement by Emmons. • Concerning 

 the stratigraphic relations Eldridge said: 



The Monument Creek formation occurs along the 

 southern edge of the Denver field in the steep slopes of 

 a high mesa and also stretches from its base prairieward 

 in thin sheets. The floor of the lake in which the Monu- 

 ment Creek was deposited was more or less irregular from 

 erosion and in one part or another consisted of the clays 

 and sandstones of the Laramie, Arapahoe, or Denver 

 formations. In the foothill region the Monument Creek 

 lies in contact with the Arapahoe; between Platte River 

 and Cherry Creek a few hundred feet of Denver beds 

 exist, which further to the east disappear. North and 

 east of Coal Creek, on the eastern edge of the field, both . 

 Denver and Arapahoe are wanting and the Monument 

 Creek rests directly upon the clays of the Laramie. 



Although the relations between the Arapahoe 

 and Denver and the "Monument Creek" were 

 somewhat obscure, the interpretation natur- 

 ally followed that the latter was structurally 

 above the former, for it was then supposed 

 that the lower division of the "Monument 

 Creek" was of Miocene age, while the Arapahoe 1 

 and Denver were referred to the Cretaceous. 



In 1902 W. T. Lee 83 discussed at some length 

 the "Monument Creek group" of Hayden. To 

 the lower portion (of Emmons and others) he 

 applied the name "Monument Creek forma- 

 tion" and described it as "composed of con- 

 glomerates, breccias, sands, and clays which 

 alternate -and intermingle and grade into each 

 other in the most lawless manner." Continu- 

 ing, Lee said: 



Above the Monument Creek beds lie masses of rhyolitic 

 tuff. This tuff has been noted by Hayden and others. It 

 forms more or le3s of a sheet, or sheets in some places, while 

 in others.it occurs intermingled with sand, gravel, and 

 clay. * * * Above the tuff occurs a sheet of glassy 

 rhyolite about 25 feet thick which forms the protecting 

 cap of several of the buttes near Castle Rock. 



To the "youngest formation in the Castle 

 Rock region" Lee gave the name "Castle con- 

 glomerate." This was the "Upper Monument 

 Creek" of Hayden and the so-called upper divi- 

 sion of Emmons and others. Concerning it 

 Lee said: 



These upper beds differ in character from those of the 

 lower division; they are separated from it by volcanic tuffs 

 and flows of rhyolite and by an unconformity representing 

 a period of erosion, as shown by the presence in it of the 

 material from the underlying rhyolite. It is therefore 

 separate and distinct from the lower division. 



m The areal geology of the Castle Rook region, Colo.: Am. Geologist, 

 vol. 29, pp. 101-103, 1902. 



Lee did not procure data bearing on the age 

 of the divisions of the "Monument Creek" 

 recognized by him, but three years later N. H. 

 Darton M obtained additional vertebrate evi- 

 dence as regards the upper division. At a 

 number of localities he collected remains of 

 Titanotherium and Hyracodon, concerning which 

 he wrote as follows : 



All this material appears to have been obtained from the 

 upper beds and it correlates these beds with the Chadron 

 formation of the White River group, or Oligocene. No 

 evidence was obtained as Jo the age of the lower member. 

 * * * The presence of the unconformity between the 

 upper and lower members suggests that the latter may be 

 of Wasatch or Bridger age. The nearest locality to the 

 Monument Creek area at which Oligocene deposits occur 

 in eastern Colorado is in the vicinity of Akron and Fre- 

 mont's Butte, where Titanotherium remains occur in 

 abundance. 



The. status of the "Monument Creek" prob- 

 lem remained as above indicated until 1910-11, 

 when G. B. Eichardson began a study of this 

 region preparatory to the preparation of a 

 report on the geology of the Castle Rock quad- 

 rangle, which is joined by the Denver Basin on 

 the north and by the Colorado Springs quad- 

 rangle on the south. As a result of this study 

 Richardson 85 stated that he found it necessary 

 to separate the "Monument Creek group" into 

 two formations on the basis of a well-marked 

 unconformity which separates beds of Eocene 

 and Oligocene age. The lower formation was 

 named the Dawson arkose and the upper one 

 the Castle Rock conglomerate. According to 

 Richardson the stratigraphic relations indicate 

 that the Arapahoe and Denver formations are 

 equivalent to the lower part of the Dawson 

 arkose, and this evidence is paleontologically 

 supported. 



The Castle Rock conglomerate may be first 

 considered. This' formation is the "Upper 

 Monument Creek" of Hayden and others, the 

 so-called "upper division of the Monument 

 Creek" of Emmons, Eldridge, Darton, and 

 others, and the Castle conglomerate of Lee. 86 

 It occurs in the south-central part of the Den- 

 ver Basin, where it crops out in detached areas 

 on the divides between the tributaries of South 

 Platte River from a point near Elbert to the 

 vicinity of Sedalia, a distance of about 40 miles. 



" Age of the Monument Creek formation: Am. Jour. Soi., 4th ser., vol. 

 20, pp. 178-180, 1905. 



bs Richardson, G. B., The Monument Creek group: Geol. So:. America 

 Bull., vol. 23, pp. 267-276, 1912. 



«« Lee's name was unavailable on account of prior usage. 



