36 J. E. TOED HYDROGRAPHiC HISTORY OF SOUTH DAKOTA 



cene. Another is about 150 feet lower at the same point, which may 

 correspond to the one 150 feet above the depot at Rapid City, and to one 

 200 feet above the junction of the stream with the Cheyenne. Another 

 about 30 feet above the creek at Rapid City may be the one 100 feet 

 above the stream at the junction. The second may date from the late 

 Pliocene and the other from the early Pleistocene; but much more 

 study is necessary before we can be sure of it. 



The streams of the hills, as they deploy on the Cretaceous plain, very 

 generally have shifted to the right, according to Ferrel's law, leaving 

 their terraces more extended on the left side. There are, probably, a 

 few cases of change of course by abstraction. 



An important feature in James river at the present time is a broad 

 sill of red quartzite at Rockport, in Hanson county. It is now 1,200 

 feet above sealevel, but before the erosion of the glacier was doubtless 

 much higher. This no doubt long served as a barrier to the erosion of 

 this valley. The ice-sheet was unable to remove it, though it carried 

 away mai^ thousand cubic yards and distributed it in the form of 

 boulders farther south. From the depth of the till south of this barrier 

 we find some reason for believing that there existed rapids at that point, 

 as indicated on the map. If our view thus far has been correct, this 

 ridge of quartzite exercised an important influence on the baselevel of 

 James river and its tributaries above that point. Though the Niobrara 

 river and Vermilion had evidently cut down to nearly their present 

 depth before the deposition of the till in the southeastern part of the 

 state, we find no trace of such deep channels having been formed in 

 James river above Rockport or in any of the larger tributaries from the 

 west above that point. 



We may believe also that the Big Sioux descended by several rapids 

 from its present altitude of 1,400 feet above the falls to the valley of the 

 Vermilion, a little over 1,200 feet in altitude. 



During the later Pleistocene 

 formation of the missouri 



The early Wisconsin epoch witnessed a great change in the drainage 

 of the state. The main cause was the advent of the ice-sheet from the 

 northeast. If our reasoning hitherto has been correct, the overflow of 

 water in the previous epoch opened the way for the entrance of the ice 

 at this time. 



The glacier rapidly filled the James River valley 1,000 to 2,000 feet in 

 depth and as far south as Gayville. It flowed up the valleys of the 



