PERIPHERAL AND INTERLOBULAR STREAMS 39 



The lowering of the divides so as to allow the ice to transgress them 

 and the extent of the ice in these lateral valleys toward the south afford 

 further evidence that they were lower south of the quartzite ridge, as we 

 have before suggested and as is indicated on the map for the early Pleis- 

 tocene. 



PERIPHERAL AND INTERLOBULAR STREAMS 



Such were the changes accomplished, when the ice was reaching and 

 occupying the First or Altamont moraine. At the same time several 

 minor streams were established, or at least much enlarged and deepened. 

 These may be grouped in two classes, those which flowed parallel with 

 the edge of the ice, usually just outside of the moraine, which we may 

 call peripheral, and those originating between two adjacent lobes of ice, 

 which we may call interlobular. 



Of the former class there are (passing around the ice from east to west) 

 Battle creek, Brule creek, Box Elder creek, Chapelle creek, Little Chey- 

 enne creek, and lower Swan Lake creek. 



Of the latter class, in similar order, upper Big Sioux, upper Skunk 

 creek, Turkey creek, upper Crow creek, the upper Box Elder, the lower 

 Medicine creek, and the upper part of Swan Lake creek. These are 

 marked on the map. 



Similar streams of less importance, particularly of the first class, were 

 developed in connection with the formation of the later moraines. Of 

 these we may name, for the Second or Gary moraine, the upper Ver- 

 milion, Turkey Ridge creek, Clay creek, and the Firesteel-Enemy- 

 Twelvemile creek, which combined the upper courses of these several 

 streams of the present time. It also had several successive courses shift- 

 ing to the northeast as the ice receded ; also Medicine creek, the Blue 

 Blanket, and Spring creek. 



And for the third or Antelope moraine, Marsh creek, Sand creek, upper 

 Snake creek, and portions of Willow and Elm creeks. 



An interesting incident of the recession of the ice-sheet of the Wiscon- 

 sin epoch was the gradual formation of a shallow lake which has been 

 named lake Dakota. It extended from the vicinity of Huron past Red- 

 field and Aberdeen to Oakes, North Dakota. Its greatest extent was 

 probably when the ice occupied the fourth moraine. It was filled quite 

 evenly with fine silt. Smaller lakes continued the series to Mitchell and 

 the red quartzite sill before mentioned at Rockport. Its primary cause 

 may be found in the greater erosion of the clays by the glacier north of 

 that sill* 



* Bulletin no. 144, U, S. Geological Survey, p. 52. 



