262 O. W. WILLCOX 



It is a matter worthy of remark that the immediate surface of the 

 Iowan drift-sheet seems to be much poorer in soluble sulphates 

 than is the case with the Wisconsin. The Iowan has its "alkali 

 spots," to be sure, but their occurrence is due to the preservation 

 of favoring topographic conditions. Where natural drainage has 



long been established, as in 

 the vicinity of the master- 

 streams, the proportion of 

 soluble sulphates in soils of 



~Z T - ~~Z ! T~. 3 the Iowan drift is quite 



_ Former Sandy Beach of the Lakelet T . 



BTill rapine Silt normal. It appears to be 



Fig. 2.— Section through an "alkali spot." otherwise with the Wiscon- 

 sin drift, even in the broken 

 area adjacent to the streams which flow across the tip of the Des 

 Moines lobe south of the Gary moraine; in this region is to be 

 found that part of the Wisconsin drift which, in Iowa at least, 

 has been longest exposed to weathering, since it was the first to be 

 left bare by the retreating ice. Close examination of hillside surfaces 

 in this area after several days of fine weather often reveals the presence 

 of an efflorescence similar in appearance to that found in the "alkali 

 spots;" the presence of soluble sulphates is easily shown by scraping 

 about twenty grams of earth from the surface, extracting with pure 

 water and adding a solution of barium chloride, whereupon a copious 

 precipitate of barium sulphate will be obtained. This appears to 

 be a point of difference between the surfaces of the two drift-sheets. 



It will be recalled that in estimating time ratios within the glacial 

 epoch much weight has been laid upon the obvious differences in the 

 present condition of the upper zones of the various drift-sheets. The 

 absence of lime, the decay of the felspars, the complete oxidation of the 

 iron content so unmistakably patent in the ferretto of the Kansan clearly 

 stamp this till as of greater age than that made by later ice-sheets. 

 No such obvious differences set off the later tills one from the other. 

 The difference between the surface zones of the Iowan and Wisconsin 

 tills in regard to lime content is not particularly noticeable, and the 

 same may be said of the degree of oxidation of the iron in these tills; 

 the granite bowlders of the Wisconsin are not noticeably fresher 

 in appearance than those of the Iowan. The chemical criteria 



