116 S. CALVIN — 10 WAN DRIFT 



at 15 or 20 times the length of the post-Iowan interval. The lowan sur- 

 face, with ver}' rare and insignificant exceptions, is free from loess, whereas 

 the Kansan surface is almost universally loess-covered, except where it 

 is overlain by other sheets of drift. 



lOWAN COMPARED WITH THE ILLINOIAN 



The differences between the lowan and the Illinoian are relatively not 

 so very great. The Illinoian surface, it is true, shows distinct effects of 

 erosion, oxidation, and leaching, but these processes have been carried 

 to a far less extent than in the case of the Kansan. The color of the 

 Illinoian till, where fresh and unweathered,is quite like that of the lowan, 

 and the boulders are somewhat similar, though very mucli inferior in 

 average size. The Illinoian surface, like that of the Kansan, is very gen- 

 erally covered with loess. Judging by the changes that had been wrought 

 in the surface of the Illinoian before the loess was laid down on it, this 

 sheet of till is at least 5 or 6 times as old as the lowan. 



lOWAN COMPARED WITH THE WISCONSIN 



The Wisconsin drift is represented in Iowa by the Des Moines lobe. 

 From Hardin county northward the eastern edge of this lobe overlaps 

 the lowan, and the relations are such as to offer ready facilities for 

 making comparisons between the two sheets of till. Compared with 

 the lowan the clay of the Wisconsin is paler or lighter yellow ; it is very 

 much richer in calcium carbonate and, in striking contrast with the 

 lowan, it is crowded with countless numbers of limestone pebbles. The 

 crystalline boulders, as a rule, are smaller, and they are never so numer- 

 ous as they are in some parts of the lowan area. The surface has suf- 

 fered no appreciable change since the withdrawal of the Wisconsin ice. 

 There has been practically no erosion. One may travel scores of miles 

 without seeing a definite stream channel of any kind. Numerous un- 

 drained kettle holes and depressions characterize the broadh' undulating 

 expanses of prairie. There are no detectable signs of even the incipient 

 stages of oxidation and leaching. Everything indicates that in point of 

 age the Wisconsin is even younger than the lowan, but is more nearl}^ 

 related to the lowan in this respect than is either of the other drift-sheets 

 of the Mississippi valley. It may safely be said that the lowan is not 

 more, or certainly not much more, than twice as old as the Wisconsin. 

 Both of these drift-sheets are young when compared with the Illinoian; 

 very young when compared with the much older Kansan. 



AVest of the Wisconsin area in northwestern Iowa there is a sheet of 

 till older than the Wisconsin and younger than the Kansan, which has 

 been regarded provisionally as the equivalent of the lowan east of the 



