32 J. E. TODD — HYDROGRAPHIC HISTORY OF SOUTH DAKOTA 



been before the early Pleistocene, but considering the erosion of the 

 Cheyenne since that time and the height of the divide between it, and 

 the headwaters of Bad and White rivers, it seems likely that the change 

 was accomplished in the Pliocene. 



The reasons for the Cheyenne overreaching its fellows may be found 

 in three conditions : 



First, the greater rainfall in the northern hills. Though we may not 

 understand the conditions which produce this at present, we see no 

 reason why the same should not have existed in the Pliocene. This 

 greater rainfall must have increased the erosion in the main portion of 

 the stream and rapidly deepened it. 



Second, the thinness of the Tertiary beds and the more easy erosion 

 of the Montana beds underlying. Though the Tertiary beds have been 

 mainly removed, it is probable that they extended to the Cheyenne, in 

 its middle course, and they certainly crossed it near the Black hills. 



Third, the probable cutting back of the stream at an early date so 

 as to capture the headwaters of a stream flowing north, possibly a 

 branch of the Little Missouri and now the upper course of the Belle 

 Fourche. 



By the united influence of these conditions we can understand how 

 the Cheyenne would outstrip its fellows in reaching baselevel. More- 

 over, the streams flowing eastward from the Black hills having their 

 upper courses lying on the easily eroded Montana clays would have 

 rapidly widened their valleys until the divides separating them would 

 have been much narrowed if not completely broken at some points. In 

 case of floods, which were probably more frequent at that time because 

 of the gentler slope, overflows may have early extended into adjacent 

 streams. In view of these relations it is not difficult to see how a tribu- 

 tary of the Cheyenne should cut back, along the line of junction between 

 the Tertiary and the Montana, southward, nor is it difficult to believe 

 that in a comparatively short time all. the streams draining the Black 

 hills were united in one system, as at present. This having been ac- 

 complished, the Cheyenne became the principal stream in size of valley 

 and amount of water. 



THE ANCIENT MINNECHADUSA 



A few other less important particulars should be noticed. During 

 the Pliocene it is likely that the south branch of White river, follow- 

 ing the common easterly trend, entered the Niobrara through the 

 Minnechadusa at Valentine. Though this channel has not been fully 

 explored, it is known that from the abrupt bend in that stream there 

 extends a broad valley toward the Minnechadusa, and above that point 



