THE EIVEES SUPEEIMPOSED. 217 



reposing imconformably on its flanks. Knowing as we do that the White 

 River Miocene was deposited after the completion or approximate comple- 

 ton of the uplift and that it lay against the flanks of the Hills (upon the 

 south at least), we cannot hesitate to decide that the rivers were superim- 

 posed. If they existed before the formation of the Hills they were com- 

 pletely blotted out by the Tertiary lakes and their sediments, and the 

 assumption that they did so exist is entirely gratuitous. Whenever the 

 water of the lakes finally retired from the region of the Hills, the rivers 

 began to flow in what were then the lowest lines of the surface and there 

 they have flowed ever since. 



The courses of the creeks were fixed and their work of erosion was 

 commenced when the uplift first exposed its summit to the rain. They 

 are precisely as old as the Hills, and they have been the agents of the 

 degradation of the uplift At first they may have descended directly to a 

 sea or lake, but when the White River conglomerate was formed their 

 lower courses were upon a plain which bordered a lake. Afterward the 

 lake rose and carried its shore to the flanks of the Hills, and some or all 

 of them discharge their water directly into the lake. When at last the lake 

 was withdrawn, the branches of the Cheyenne River came into existence 

 upon its bed, receiving the discharge of the creeks and completing the 

 present drainage system. 



The coincidence of the directions of creek courses with the directions 

 of the dip must be understood to be only approximate. Each creek in the 

 course of its history has sunk alternately through soft and hard strata, and 

 while in the soft strata it has deviated from its original place by the process 

 of meandering. In a few cases streams of considerable size have so far 

 shifted their courses as to form unions with each other before escaping from 

 the foothills, but such instances are rare. There is, however, one general 

 exception to the rule of cataclinal drainage which deserves fuller mention. 



When the displacement began its summit was necessarily a divide or 

 water-shed, from which the water flowed in all directions. In the absence 

 of any disturbing cause this water-shed would remain as steadfast as the 

 drainage lines all through the period of degradation. Knowing of no dis- 

 turbing cause, we may assume that the existing water-shed in the Hills holds 



