38 J. E. TODD — HYDROGRAPHIC HISTORY OP SOUTH DAKOTA 



in this case is the distribution of boulders and trace of the upper limit 

 of drift like a water line around the Bijou hills and south of the mouth 

 of White river, west of the Missouri, about 590 feet above the Missouri or 

 1,880 above the sea;* also evidences of a series of rapids marked by 

 vast accumulations of boulders above the mouth of Platte creek and 

 the canyon-like character of the trough of the Missouri at the Bijou hills, 

 though these are less conclusive. 



It seems not improbable that because the Grand and Cheyenne rivers 

 were dammed before the White, and because the ice did not reach its 

 maximum extent until some time after it had dammed the White, that 

 meanwhile there may have been a waterway over the lower land east of 

 the Bijou hills, past lake Andes and Choteau creek to the Niobrara ; but, 

 if so, it was not occupied long enough to excavate much of a trough. 



DISPLACEMENT OF THE NIOBRARA 



The ice-sheet flowed over a sag in the divide between the James and 

 the Niobrara into the valley of the latter stream so as to fill it from Bon 

 Homme to the vicinity of Yankton, forcing that stream, reinforced by 

 contingents farther north, to cut a new channel between Bon Homme 

 and Yankton. This may partly explain the present rugged character of 

 the region known as the " Devil's nest," in Knox county, Nebraska. 



DIVERSION OF THE BIG SIOUX 



Again, on the east of the main valley the divide between the James 

 and the Vermilion, perhaps attenuated by the latter receiving during 

 the preceding epoch the copious drainage of the ice-flow, was also over- 

 ridden, and the ice filled its valley to the vicinity of Vermilion. This 

 so dammed the Big Sioux, which was a tributary of the Vermilion, that 

 another small lake was formed over the present site of Sioux Falls. 

 This found an outlet into the Split Rock and eventually around the edge 

 of the ice into Rock river, near the present town of Hudson, South 

 Dakota. 



The evidences of such changes, besides those already mentioned, as 

 indicating the former course of the Big Sioux are the fluvio-lacustrine 

 beds about Sioux Falls, including some aqueous till, old soils and silts, 

 and the canyon-like character of the Big Sioux between Canton and 

 Hudson, South Dakota/)" 



* Bulletin no. 158, U. S. Geological Survey, p. 47. 



■j- Compare the author's paper, American Geologist, vol. xxv, p. 96. Further examination has, 

 however, renewed a former conclusion as given above. Some of the evidence is given. Bulletin 

 no, 158, U. S. Geological Survey, pp. 102, 103, 



