712 B. SHIMEK MINGLING OF PLEISTOCENE FORMATIONS 



is located on the west side of Dakota avenue, north of West Third street, 

 in Sioux Falls, South Dakota. It presents another example of inter- 

 stratification which is of interest in this connection. 



Briefly, the section shows a mass of gray silt lying between a lower 

 stratum of Kansan drift, of which 7 to 8 feet are exposed, and an upper 

 stratum of the same drift thickening northward to 10 or 12 feet. 



The Kansas drift (plate 41, figure 2, &). — Both drift strata present 

 the usual characteristics of the western Kansan. The heavy bluish joint 

 clay is very calcareous and contains scattered pebbles and boulders. 



The silt (plate 41, figure 2, a). — The silt is gray, calcareous, occa- 

 sionally mingled with broken fresh-water mollusk shells of the pond 

 typ'es. The mass is exposed for about 75 feet, and distinctly dips down- 

 ward toward the south. Its greatest thickness is 3 feet, and at the north 

 it appears as if crowded or folded. Smaller masses of the same silt occur 

 in the lower part of the upper drift stratum. 



Another section, on McClellan street opposite Main avenue, shows the 

 folding and crowding much better. Here a mass of silt, 20 feet long and 

 4 to 5 feet deep, imbedded in and completely surrounded by Kansan drift, 

 shows, on its northern side, distinct evidence of pushing and folding. 



The conclusion that these masses of fossiliferous silt are older than 

 the Kansan, and that they were brought to their present position by the 

 Kansan ice, is certainly warranted. The silt is evidently Aftonian. 



Conclusion 



If narrow vertical sections of the two exposures here discussed were 

 made they would appear somewhat as follows : 

 The Des Moines section, west side: 



Wisconsin drift, 10 to 12 feet (with bands of gray loess). 

 Gray, fossiliferous loess, 2 to 4 feet. 

 Yellow loess, 5 feet. 

 A normal section should show these loesses reversed and the larger sec- 

 tion here shows how the reversal had taken place. 

 The Sioux Falls section : 



Kansan drift, 10 to 12 feet. 

 Fossiliferous silt, 3 feet. 

 Kansan drift, 7 to 8 feet. 

 In this case it would be easy to jump to the conclusion that the silt is 

 interglacial, separating two distinct drifts. The larger section shows 

 that the Kansan is continuous and that the silt is intraglacial. 



Manifestly, in both cases such sections would not show the true rela- 

 tion, and both show the need of great care in drawing conclusions from 

 sections of limited lateral extent. 



