126 B. SHIMEK PL*EISTOCENE OF SIOUX FALLS AND VICINITY 



Pleistocene formations 145 



Nebraskan drift 145 



Aftonian iuterglacial deposits 146 



Aftonian in the Sioux Falls sections 147 



Kansan drift , 148 



Characteristics of the two types 148 



The Kansan plain 149 



The Altamont moraine 150 



The gravels of the river terraces 151 



The loesses 153 



Conclusions 1S4 



Introduction 



The region which surrounds Sioux Falls, South Dakota, particularly 

 that part lying south and east on both sides of the Big Sioux Eiver and 

 extending south to Fairview, South Dakota, has received much attention 

 from students of Pleistocene geology. This is due to the fact that it 

 displays two drifts and was supposed to lie partly in the sinus of a mar- 

 ginal lobe of the Wisconsin drift border, including ridges which were 

 thought to be portions of the Altamont moraine. It also contains fossil- 

 iferous iuterglacial silt and gravel beds which have attracted attention. 



Work and Conclusions of previous Observers 



The region was first carefully studied by Todd, whose preliminary 

 report on the geology of South Dakota^ contains a description of four 

 moraines, of which the first or outer moraine^ is described (page 116) as 

 extending "to a high point south of Canton. It is then feebly developed, 

 or entirely absent, from that point to the west side of the Big Sioux oppo- 

 site the northwest corner of Iowa. Tliere it forms a ridge running west, 

 south of the great bend of the Big Sioux near Sioux Falls." In the same 

 paper (page 110) he provisionally refers certain ^^eds of clay and sand 

 . . . which are clearly preglacial," along the valley of the Big Sioux 



2 J. E. Todd: South Dakota Geological Survey, Bulletin No. 1, .1895. 



8 This had been named the Altamont Moraine by Dr. T. C. Chamberlin in the prelimi- 

 nary paper on the terminal moraine of the second Glacial epoch. Third Annual Report 

 of the U. S. Geological Survey, 1883, p. 388. The map. plate xxxv. represents this 

 moraine as passing east and west into the great bend of the Big Sioux River east of 

 Sioux Falls, and thence south along the Big Sioux River for more than 50 miles. Dr. 

 Chamberlin's later map, published in Geikie's "The Great Ice Age." 3d edition. 1805, 

 gives the Wisconsin border somewhat the same location, but represents the Kansan as 

 occupying a strip along the west sidf of the Big Sioux near and south of Sioux Falls, 



