150 THE ILLINOIS GLACIAL LOBE. 



in thickness to scarcely half that depth within a few miles east or north, 

 beyond which for many miles it continues thin. The distribution and 

 general relations of this loess in reference to the district west of it present 

 some points of similarity to the loess borders of the Iowan drift in north- 

 eastern Iowa where the loess is considered an outwash or overwash apron 

 from the ice sheet, formed while it occupied the neighboring tracts of Iowan 

 drift, which are nearly free from loess. The thinning out of the loess upon 

 passing a few miles back from the hypothetical ice margin, as well as the 

 abrupt border next the ice margin, may here find illustration just as in 

 Johnson County, Iowa. 1 



The remaining feature bearing upon the question of the Iowa invasion 

 into northwestern Illinois is that of canoe-shaped ridges of loess, with 

 parallel shallow troughs, having a WNW.-ESE. trend. Such ridges and 

 troughs are best developed in a belt of thick loess lying between the strip of 

 thin drift just considered and the valley of Rock Creek in western Whiteside 

 County (see PI. XVIII). There are other well-defined ridges, as already 

 noted, south and east of Morrison, in the lowland tract nearly destitute of 

 loess, and a few have been found south of Rock River in northwestern Henry 

 County. There is a faint development of this class of ridging in northern 

 Rock Island County, Illinois, and in southern Scott and eastern Muscatine 

 counties, Iowa. It is, perhaps, significant that they are. best developed in 

 the district lying between the well-defined phases of the Iowan drift of the 

 two ice lobes. While the origin of this class of ridges like that of the loess 

 sheet is in all probability attributable to a combination of aqueous and 

 seolian agencies the precise mode of action and relation of the two agencies 

 have as yet received no adequate explanation. These ridges and bordering 

 troughs were apparently developed before the present drainage lines had 

 been opened in that region, if not while the ice occupied the neighboring 

 drift plains on which the loess is a scanty deposit, The length of the 

 ridges ranges from a fraction of a mile to two or three miles, but the width 

 seldom reaches one-eighth of a mile. In height they range from 5 feet or 

 less up to about 50 feet. While usually made up of typical loess they 

 occasionally include fine sand, as noted above (p. 139.) 



Aside from the canoe-shaped ridges of loess there are found other forms 

 of loess and sand aggregation in the region under discussion. On the east 



1 See Calvin, Iowa Geol. Survey, Vol. VII, 1897, pp. 86-90 ; also map of .Tohnsou County, p. 92. 



