THE ICE-SHEETS AND THE LOESS 139 



cial and interglacial stages seem to have been longer than those of later date. 

 Sonae of the interglacial intervals were many times as long as the period which has 

 elapsed since the disappearance from Iowa of the great ice fields which charac- 

 terized the Wisconsin stage of glaciation. If the time since the Wisconsin is 

 taken as unity, the time since the Kansas is at least twenty. The history of gla- 

 ciation in Iowa is long; the records are exceeding complex." 



If the time elapsed since the Wisconsin be 8,000 years, as is now gen- 

 erally accepted by glacialists, the time since the Kansan, according to 

 the ratio given by Calvin, is 160,000 years. If allowance be made for 

 the Aftonian interglacial epoch and for the pre-Kansan glacial epoch, 

 I think it would be entirely safe to say that the first ice invasion inau- 

 gurating Pleistocene time in America advanced upon the temperate lati- 

 tudes at least two hundred thousand years ago. 



The Loess 

 the hypotheses as to origin 



It may readily be seen that from such a long and complex glacial 

 history there must have resulted a complex product. There were ad- 

 vance stages as well as stages of retreat, and the drainage waters of the 

 stages of advance were long continued, and probably as long continued 

 as the drainage waters of the stages of retreat. Each glacial stage dis- 

 rupted and rewrought within the zone of its activity the deposits of all 

 earlier stages, or buried them under its own drift sheet. It is by the 

 study of these successive phases of the drift of the country, and by a 

 collocation of the phenomena with the geographic distribution and with 

 latitude, that the records of the Pleistocene are slowly being interpreted 

 into known terms of science. Of all these phenomena, I will call your 

 attention specifically to but one, namely, the loess of the great valleys 

 of the interior. What was the manner of its formation and accumula- 

 tion ? 



The loess was first recognized in this country by Sir Charles Lyell, on 

 the lower Mississippi. By him it was considered an aqueous deposit 

 due to a flooded stage of the Mississippi river. It has been traced from 

 the lower Mississippi throughout the whole of its extent south of the 

 lowan drift, over nearly the whole of the valley of the Missouri, and up 

 several of the other great tributaries both from the west and from the 

 east. Although it varies somewhat in composition from place to place 

 and exhibits some peculiarities of topographic distribution, it has not 

 been found to be separable, so far as the latest surface deposit is con- 

 cerned, into different parts as to date or mode of origin. The opinion 

 of Lyell as to its mode of origin was unanimously accepted by geolo- 



