THE IOWAN LOESS. 153 



12 miles; it would embrace the belt of heavy loess west of Rock Creek, in 

 Whiteside County, at the points where the lobes made their closest 

 approach. A belt of this width would, when taken in connection with the 

 valley of the Mississippi River, apparently have afforded an adequate line 

 of discharge for the streams of the Driftless Area, and also for the waters 

 issuing- from the melting- ice lobes. If, on the other hand, the Iowa lobe 

 extended to the vicinity of Round Grove and Spring Hill, Illinois (see PI. 

 XII), and reached this culmination at the same time that the Illinois lobe 

 had its maximum extension, there may have been a coalescence of ice for a 

 breadth of perhaps 15 or 20 miles. This appears to represent the extreme 

 possibility of coalescence. Granting such coalescence, it remains question- 

 able whether a glacial lake could have been held in the region north of the 

 junction of the lobes. The existence of Lake Hennepin appears, therefore, 

 to rest upon exceedingly weak foundations so far as the influence of 

 coalescing ice lobes is concerned. It is doubtful if a body of water of 

 greater depth was jjresent in the district north of these lobes than in the 

 district south of them. It is generally supposed that the loess was deposited 

 under conditions of very imperfect drainage, and that the districts which it 

 covers were, in large part, under water for at least brief intervals. The 

 extent and duration of the flooding, both to the north and south of the 

 districts occupied by these ice lobes, is still largely a matter of conjecture, 

 .and a subject on which wide differences of opinion exist. With these 

 remarks we pass to the discussion of the Iowan loess. 



THE IOWAN LOESS. 



DISTRIBUTION. 



Deposits of silt, tentatively classified with the loess and supposed to be 

 of Iowan age, cover the entire ' surface of the lllinoian drift so far as it lies 

 outside the limits of the Iowan and Wisconsin drift sheets, not only in Illi- 

 nois, but also in Iowa and in States to the east as far as Ohio. These 

 deposits have been extensively removed by stream erosion on a consider- 

 able part of the slopes and in the valley bottoms, but still remain nearlv 

 intact on the uplands. They have been traced back several miles beneath 

 the edge of the Wisconsin drift in central and eastern Illinois, but the full 

 extent has not been ascertained. A portion of the Driftless Area in Illinois, 



