278 



THE AGE OF MAMMALS 



Thus, as shown in the accompanying sections of the Pine Ridge (Fig. 

 138), of Sioux County, Nebraska (P^ig. 99), the Arikaree, 'Upper 

 Harrison,' and 'Upper Rosebud' beds, while geologically continuous with 

 the Harrison, and showing no unconformity or other evidence of a break 

 in time, yet do lack some of the mammals found in the Harrison and do 



By permission of the U.S. Geological Survey. 



Fig. 138.- 



■ Diagrammatic section of the Lower Miocene. Taken near Harrison, Nebraska. 

 After Peterson. 



contain some new mammals not found below. There are proofs of a long 

 interval of time, of several extinctions, and of some quite profound changes 

 of environment and of evolution. Further exploration may modify this 

 artificial line, and either lessen or intensify it. 



Ancient Physiographic Conditions 



The Plains Region 



Geologic conditions. — The conditions of deposition of these upper beds 

 east of the Rocky Mountains indicate the continuance of the same physio- 

 graphic features, namely, of great flood plains with a gentle slope in an 

 easterly direction, traversed here and there by river channels containing 

 coarser deposits. The typical Arikaree Formation of Darton,^ as observed 

 at Pine Ridge in northern Nebraska in 1899, is in the same region as the 

 'Rosebud' of Matthew. He observes (p. 176) that a large portion of the 

 high lands of western Nebraska and southeastern Wyoming extending 

 from Pine Ridge is occupied by the sands and soft sandstones of the 

 Arikaree Formation. It attains a thickness of over 800 feet in southeastern 

 Wyoming, and formerly extended far up the slopes of the mountains to 

 the north and west. As thus defined, the Upper Harrison of Hatcher, and 

 Upper Rosebud correspond to the upper part of the Arikaree Formation of 

 Darton. 



' Darton, N. H., Preliminary Report on the Geology and Underground Water Resources 

 of the Central Great Plains. U.S. Geol. Sun., Prof. Paper, no. 32, 1905, p. 174. 



