356 NATURAL HISTORY BULLETIN. 



of loess than in districts remote from the ice margin." But 

 he continues, "the ice-lobe which extended south westward into 

 northern Illinois has a less marked thickening of the loess 

 near its border," and then points out on pp. 155-6, the well- 

 known fact that there is a similar thickening along the main 

 drainage lines of our larger rivers! These thickenings along 

 the Missouri and Mississippi rivers extend in some cases to 

 points several hundred miles from the border of the Iowan 

 drift-sheet, and no direct connection between them and the 

 latter has yet been shown. It is true, as has been repeatedly 

 noted, that there is an accumulation of loess without the border 

 if the Iowan drift in Iowa which thins out more or less in the 

 outlying sections, and fades out in large part within the Iowan 

 drift area. But it is not true that the heaviest deposits of loess 

 in the state lie along the Iowan drift-border, for that dis- 

 tinction belongs to the loess in the Missouri river drainage, 

 which is not directly associated with the Iowan. 



The writer has already expressed the opinion* that the 

 greater thickness of the loess along the Icwan border is due 

 in large part to the fact that it follows the larger streams in 

 the eastern half of the state, f especially along those portions of 

 their courses which have broad valleys. In such valleys the 

 winds had, and now have, free sweep over the numerous mud 

 and sand bars which abound at low water; and the bordering 

 highlands were then, as now, clothed with a vigorous vegeta- 

 tion which retained the dust. This would be possible only 

 after the glacier had wholly retreated and the climate was 

 sufficiently tempered to support such a vegetation. After the 

 recession of the glacier the surface which had been ice-covered 

 was no doubt barren for a time, and even after vegetation ap- 

 peared it was probably at first tufted like that of sand-dunes 

 and sand-bars, leaving comparatively large bare areas. 



Hence wind and water easily shifted the finer materials, re- 

 moving them from the higher Iowan areas to the river-valleys, 



*See p. 338 of this Bulletin. 



tSee map, PI. Ill, showing position of Iowan border, in Iowa Geol. 

 Sur. , vol. viii. 



