THE 10 WAN LOESS. J 55 



that very detailed study is necessary to determine its limits. The general 

 extent of the loess and associated silts may be seen by reference to the 

 glacial map (PL VI). 



VARIATIONS IN THICKNESS. 



The variations in thickness of the loess apparently depend largely 

 upon the relation (1) to the ice margin and (2) to the principal lines of 

 drainage. On the margin of the Iowan ice sheets, especially the one which 

 occupied eastern Iowa, there is a thicker deposit of loess than in districts 

 remote from the ice margin. Also in Carroll and Whiteside counties, Illi- 

 nois, there is a belt of loess extending from the hypothetical margin of the 

 Iowan ice sheet above outlined northward to the south border of the 

 Pecatonica Basin and eastward to Elkhorn Creek, in which the loess has a 

 thickness of 15 to 20 feet, while in the districts immediately north and east 

 its average thickness does not much exceed 5 feet. The features here as 

 well as elsewhere seem to indicate that the outwash from the Iowan ice 

 was much heavier within 10 or 15 miles from its border than at greater 

 distances. The ice lobe which extended southwestward into northern Illi- 

 nois has a less marked thickening of the loess near its border, though Mr. 

 Hershey reports a perceptible thickening on the borders of the Pecatonica 

 lobe. On the south border of the Green River Basin there is, however, a 

 marked thickening of the loess at the line supposed to mark the southern 

 limits of the Iowan ice sheet. The thickness is nowhere much less than 25 

 feet and in places is fully 40 feet. Upon passing southward the thickness 

 soon decreases to 12 or 15 feet, and about this thickness is maintained over 

 much of the area between the Illinois and Mississippi rivers. 



The thickening of the loess along the borders of the main drainage 

 lines is well illustrated by the Mississippi. On the borders of this valley 

 in northern Illinois there is 25 to 40 feet of loess, but within 5 or 10 miles 

 east of the bluff the thickness usually decreases to less than 10 feet. 

 Following down the Mississippi, there is found to be a heavy deposit of 

 loess on its immediate borders, passing through the area which was occupied 

 or closely bordered by the Iowan, as well as in districts to the north and 

 south, a feature which seems to indicate that the loess deposition occurred 

 after the withdrawal of the ice lobe to the west side of the river. Upon 

 passing into the district south of the limits of the Iowan ice lobe the loess 



