IVA VE-LIKE PROGRESS OF AN EPEIROGENIC UPLIFT. 387 



loess to have been deposited while the ice-sheet that spread the 

 upper portion of the early till was melting away. The very 

 remarkable paha of that district, which are eskers of loess, were 

 accumulated while the waning ice-sheet walled them in at each 

 side.' That the later part of the loess deposition was contem- 

 poraneous with the formation of the Altamont moraine, belong- 

 ing to the later drift and marking its limits, I ascertained in 

 northwestern Iowa, where this moraine along a distance of seventy- 

 five miles, from Guthrie county northwestward to Storm Lake, is 

 bordered on its west side by an expanse of loess as high as the 

 crests of the morainic hills, while its elevation above the expanse 

 of till eastward is from fifty to seventy-five feet. During the 

 time of deposition of this part of the loess the ice-sheet reached 

 to the Altamont moraine and was a barrier preventing the waters 

 by which the loess was brought from flowing over the lower area 

 of till that reaches thence east to the Des Moines river. ^ On 

 three widely separated tracts the loess, as elsewhere the coarser 

 portions of the modified drift forming sand and gravel plains, 

 was in progress of deposition upon successive areas as fast as the 

 ice-sheet supplying these stratified drift beds receded. Imme- 

 diately after the land was bared by the retreat of the ice, and 

 even while the ice itself occupied the adjoining land, the loess 

 was being laid down, contemporaneous successively with the 

 early till on the southern border of the drift, with the till of 

 intermediate age in northeastern Iowa, and with the later till 

 enclosed by the Altamont moraine. The loess deposition I be- 

 lieve to have been m.ainly continuous, accompanying the gradual 

 and widely extended but wavering departure of the ice-sheet 

 from its farthest boundary to this outermost of the conspicuous 

 morainic belts. 3 



' U. S. Geol. Survey, Eleventh An. Rep. for 1889-90, pp. 435-471. 



^Geoi. and Nat. Hist. Survey of Minnesota, Ninth An. Rep. for 1880, pp. 307- 

 314, 338. 



3 Xhe interpretation of the loess and glacial history of the Mississippi basin which 

 I here present differs widely, it must be acknowledged, from the opinions of Professors 

 Chamberlin and Salisbury, and Messrs. McGee and Leverett, to whom we owe so much 

 of the critical investigation of this area. These observers have been led by their 



