AFTONIAN DEPOSITS 147 



often ferruginous, and frequently contain streaks and cloudings of ^InOo, 

 and may be consolidated to form conglomerate. 



In referring these deposits to the Aftonian the writer is conscious of 

 the danger of confusion which may arise from the presence of gravels, 

 sands, and silts in other Pleistocene formations. There are beds of sand 

 and gravel in the Kansan, and the mere fact that one finds Kansan drift 

 overlying a bed of gravel does not prove that the gravel is Aftonian. 

 However, in the region under discussion the bluff sections consistently 

 show the beds designated as Aftonian below the Kansan and above the 

 Nebraskan. That they do not belong to either of these drifts is shown 

 again by the presence of fossils in the Otis mill-site section and sections 

 2 and 4 of our series. These sections also clearly show the position of 

 the fossiliferous beds between two different drift sheets, the lower of 

 which is Nebraskan and the upper Kansan. In these sections the fos- 

 siliferous sections are so extensive and so regular that they can not be 

 regarded as masses incorporated in the drift during its advance, such as 

 those which occur in some of the Sioux Falls sections. In structure, 

 composition, and contents, as well as in stratigraphic position, these fos- 

 siliferous deposits are in all respects like the Aftonian beds of western 

 lowa.^* 



There is greater doubt concerning the identity of the bluff sands and 

 gravels which contain no fossils. In some cases, as noted, there is evidence 

 that the present position of the beds has been determined by the action of 

 the Kansan ice, and there are other bluff sections of limited extent in 

 which exact relations can not be satisfactorily determined. However, the 

 evidence of the fossiliferous sections, together with the consistent position 

 of the members of the fossiliferous and non-fossiliferous sections, is 

 sufficiently conclusive to warrant the conclusion that in all these bluff 

 sections the silts, sands, and gravels are Aftonian or, where disturbed by 

 Kansan ice, at least of Aftonian origin. 



Aftonian in the Sioux Falls sections. — It was the opinion of the writer, 

 expressed when the first draft of this paper was prepared, that the silt, 

 sand, and gravel beds in sections 13 to 15, at Sioux Falls, represent 

 Aftonian in place. Subsequent investigations led to a change of opinion. 

 Attention has been called to the remarkable confusion of Pleistocene 

 strata which is revealed in the sections south of the river. In the vicinity 

 of section 15 and eastward much evidence of mixing and plowing is pre- 

 sented. In the western part of section 13 the gravel bed is somewhat 



" See the writer's papers in the Bulletin of the Geological Society of America, vol. 20, 

 1910. pp. 399-408, and vol. 21. 1910. pp. 119-140. Reports of the Iowa Geological Sur- 

 vey, vol. XX, 1911 (1910), pp. 309-366, 



