130 B. SHIMEK PLEISTOCENE OF SIOUX FALLS AND VICINITY 



field previously investigated in "Woodbury, Monona, Harrison, and Potta- 

 wattamie counties, Iowa. Unexpected results, reaching beyond the limits 

 of the original assignment, were obtained, and they are here presented 

 with some hesitation, because they contradict several of the conclusions 

 which have been reached concerning the geology of the area under dis- 

 cussion by the experienced geologists whose works have been cited. It is 

 comforting to know, however, that one of these contradicting conclusions, 

 namely, that the Wisconsin is wholly absent from the western part of 

 Lyon County, Iowa, was also reached independently and coincidently by 

 Professor J. E. Carman, of the Iowa Geological Survey. 



A brief description of the topography of the region may assist in the 

 interpretation of its geology. 



Topography 



Topography no doubt played an important part in determining the 

 conclusions which had been previously reached concerning the drifts of 

 the Sioux Falls region, for it presents peculiarities and variations of such 

 nature that were topographic factors made the sole criterion by which the 

 age of a drift sheet is determined they would cause a division of the area 

 between two distinct Glacial epochs, and this division would not coincide 

 with stratigraphic structure which may be determined in sections. 



These topographic peculiarities have already been shown on the map, 

 plate III, accompanying Wilder's report (1900), and on the maps in 

 Todd's Elkpoint folio (1908), but a brief description of them may make 

 them more striking in their relation to the geological problems involved. 



The Big Sioux Eiver forms the boundary between Iowa and South 

 Dakota in the greater part of the territory under discussion. (See map, 

 figure 1.) 



