216 GEOLOGY OF THE BLACK HILLS. 



The relation of the rivers to the rock structure is entirely different. 

 The Belle Fourche, or North Fork, approaches the uplift from the west, and, 

 with a northeasterly course, runs directly across it — not at its highest point, 

 hut at a place where the height of the arch is more than 2,000 feet (see 

 Figure 26). In like manner the South Fork crosses the southern spur of 

 the uplift, and in its meandering passes in and out of the flank of the arch 

 in a manner that seems entirely independent of the dip of rocks. If the 

 reader will turn to the bird's-eye view, he will see that the South Fork, 

 which flows from left to right across the foreground, first crosses the series 

 of hog-backs in a direction opposed to their dip, and then on the oppo.site 

 side crosses the hog-backs somewhat in the direction of their dip. In its 

 middle course it traverses the arching plateau of the Dakota sandstone, 

 cutting a gorge several hundred feet in depth and passing out on the oppo- 

 site side ; and then, after having escaped completely from the obstruction, it 

 turns back, and in a loop several miles in length carves a second canon in 

 the same sandstone. 



In their relation to the rock structure these water-ways are manifestly 

 inconsequent, that is to say, their direction.s were determined by conditions 

 entirely independent of the uplift of the rocks. In Powell's classification 

 two kinds of inconsequent drainage are recognized, the one called "ante- 

 cedent," and the other "superimi)osed." Antecedent drainage is produced 

 when an ui)lift, occurring in a region the drainage system of which is 

 already established, progresses so slowly that the corrasion of the streams 

 keeps pace with it and their positions remain unchanged. Superimposed 

 drainage is produced when a system of streams which originally pertains 

 to one rock system is transferred vertically downward to a different and 

 unconforming rock system, the latter being brought to light by prolonged 

 denudation. 



If the Belle Fourche and the South Fork are antecedent to the Black 

 Hills uplift, then the region must have been dr}^ land when the uplift began, 

 and, if subsequently submerged, cannot have received enough sediment to 

 clog their channels. If, on the other hand, they are superimposed upon the 

 uplift, we may be equally confident that they acquired their courses on the 

 surface of a series of sediments laid down subsequent to the uplift and 



