EVIDENCE FROM SOUTH DAKOTA 479 



The conclusion from these facts seems simple and "unquestionable. 

 Before the Wisconsin ice came, Whiite River flowed through this valley 

 into the James River. When the ice dammed up the White River the 

 waters rose north of the Bijou Hills until they found a way around east. 

 Some trace of such a channel is marked by a shallow depression occupied 

 by a string of lakes and lake beds. When filled with water, there are few 

 places east of the Bijou Hills where it can be crossed. When this outlet 

 was stopped by the farther advance of the ice, the water rose until it 

 found an outlet along the line of the present Missouri River. Since that 

 time it has cut down about 600 feet. This rapid erosion is greatly assisted 

 by the soft and slippery character of the Pierre formation, which occupies 

 nearly the whole height of the gorge (see figures 3, 4, and 5). 



FOX HILLS DIVIDE 



Lake Ariharee. — This is the most complete and complicated of all. As 

 one approaches from the south, he sees stretching across the course of 

 the Missouri River a high, massive highland. It seems to be nearly flat 

 on top and somewhat higher toward the west, while eastward it becomes 

 more broken, for it is well covered with glacial deposit. The higher 

 portion west of the river was named by early explorers as Fox Ridge, 

 although its top was nearly flat for a breadth of 10 miles. Bast of the 

 river are prominent points in the Altamont moraine about Gettysburg. 

 The southern side of this ridge is more abrupt than the northern, which 

 is traversed on the west by Moreau River and by Swan Lake Creek; flow- 

 ing west. The basin of the latter is limited in extent, but it must have 

 been considerably higher to allow for the glacial erosion to which it was 

 subjected without being reduced in height lower than Fox Ridge on the 

 west. The latter rises to 2,300 feet within 10 miles of the Missouri 

 River. East of the river are exposed points of older rocks, which reach 

 a height of 2,000 feet, while the morainic points may reach 2,100 to 

 2,200 feet. 



At the maximum stage of the ice, which had reached apparently little 

 more than the Altamont moraine, the water derived from the Moreau and 

 Grand Rivers, and other streams seeking their way around the edge of 

 the ice-sheet, gradually filled the present valley of the Missouri north of 

 Fox Ridge into a lake 30 or 40 miles wide and extending northward in- 

 definitely. This hypothetical lake has been named "Lake Arikaree," 

 after a tribe of Indians which occupied the region when first discovered. 



Glacial Boulders without Till. — The main evidence of the former ex- 

 istence of this lake consists of the absence of glacial, till west of the 

 Missouri River and of the general distribution of scattered boulders over 



