Darton — Age of the Monument Creek Formation. 179 



Laranrie, Arapahoe and Denver formations were recognized, 

 and it was shown that the formation consists of two distinct 

 members separated by a well-defined break in deposition. The 

 lower member lies on an uneven floor of Denver formation 

 at the north and Laramie to the southeast. It displays "marked 

 regularity in the succession of its beds, excepting at the base, 

 where, owing to the uneven floor, the material varies from 

 conglomerate through sandstone to arenaceous shale. A 

 short distance above the base are two broad bands of green 

 shale separated by one of pink and capped by a fine grit, or 

 sandstone, which is soft and friable and easily disintegrates." 

 The thickness is estimated to be about 900 feet. The sand- 

 stones and grits of the lower member are mostly of granite 

 debris. The upper member consists of sandstones and shales, 

 with numerous beds of conglomerate, and between the two 

 there are local deposits of rhyolitic tuff, in places 40 feet thick, 

 which are quarried extensively for building stone near Castle 

 Rock. In the lower part of the upper member many frag- 

 ments of this rhyolitic tuff occur, a feature which is notably 

 displayed in the breccia and conglomerate capping the butte 

 known as Castle Rock. The thickness of the upper member 

 is estimated by Eldridge at about 400 feet. In portions of 

 the area, I have observed that in the lower member there are 

 extensive deposits of massive clay, very similar in appearance 

 and properties to the fullers earth which is characteristic of 

 the Chadron formation, or Titanotherium beds, of the White 

 River group in the Big Bad Lands of South Dakota and 

 elsewhere. 



In the general resume of the geology in the Monograph on 

 the Denver Basin,* Mr. Emmons suggests that the vertebrate 

 remains of Miocene age probably were from the lower mem- 

 ber of the formation and that the upper member might be 

 correlated with the Pliocene. This suggestion was based on 

 the fact that the uppermost Tertiary deposits in the eastern 

 portion of Colorado are of Pliocene age, and in the region 

 north of the Platte River they lie unconformably on White 

 River beds. Mr. Emmons recognized the fact that these beds 

 differ somewhat from Monument Creek beds in character, yet 

 this could be explained by the proximity of the Monument 

 Creek formation to shore lines along the mountain front. 



Two years ago, while examining the southern portion of the 

 Monument Creek area, I obtained from the conglomerate four 

 miles northwest of Calhan, the distal end of a large humerus 

 which Dr. F. A. Lucas has identified as Titanotherium. This 

 conglomerate is the upper member of the formation and caps 

 a long line of buttes and extensive plateaus. A number of 

 *Loc. cit., p. 39. 



