326 THE ILLINOIS GLACIAL LOBE. 



which leads northward to the Marseilles moraine and passes over that 

 moraine, as already noted, in the vicinity of the valley-like gap east of 

 Yorkville. The bowlders occupy the gap and also the portion of the moraine 

 to the east. To the north from this point the broad gravel plain along Fox 

 River interrupts the belt for a space of 3 or 4 miles, but at the north 

 border of this gravel plain bowlders again become numerous and abound 

 along the west side of Fox River throughout the interval between this 

 gravel plain and the composite belt of moraines above described. 



The bowlders are unevenly distributed, there being small tracts and 

 narrow strips where they are so numerous as to constitute a serious 

 obstruction to the cultivation of the soil, occasionally numbering several 

 hundred per acre, but throughout most of the belts there are only a few 

 per acre. The belt taken as a whole probably carries ten times the number 

 i >f bowlders found in neighboring districts. 



The sand along the south border of the Kankakee is usually so heavy 

 that the bowlders, if present, would be obscured. In places where the sand 

 is thin, bowlders are usually abundant. These oasis-like tracts in the midst 

 of the sand area are in some cases difficult to account for. The heaping of 

 the sand into ridges is probably due, in part at least, to wind action, and 

 the wind may also have been influential in sweeping the sand away from 

 parts of the surface. It seems well, however, to introduce au alternative or 

 supplementary explanation, though the applicability has not been fully 

 tested. By the alternative interpretation a causal relationship is suggested 

 between the occurrence of bowlders and the absence of sand, and the 

 explanation found in the persistence of ice where the bowlders occur until 

 the sand had accumulated in practically its present depth and topography. 

 This alternative explanation would also make the ridging of the sand partly 

 the result of glacial molding which has been intensified by subsequent 

 seolian action. This interpretation has been suggested by features found in 

 the old Lake Kankakee district, discussed below. 



It should perhaps lie stated that the bowlders here or elsewhere in the 

 belt can scarcely be considered a residue from erosion of the till, for they 

 often occur on plains where there has been scarcely any opportunity for 

 erosion. Furthermore, they differ in constitution from the coarse rocks of 

 the till, being almost wholly of crystalline rocks of distant derivation, while 

 the till abounds in local rock fragments, both coarse and fine. 



