THE BLOOMINGTON MORAINIC SYSTEM. 267 



able gravel, however, seems to be scarcely adequate to supply ballast for the 

 wagon roads of the region traversed by the moraine. The composite belt 

 with which this morainic system connects at the northeast is much better 

 supplied with surface gravel. 



As noted above, the outer face of the moraine in Lee and northern 

 Bureau counties is heavily coated with sand, which apparently has been 

 drifted by the wind from the Green River Basin on the west. From this 

 sand belt southward through central Bureau County the surface of the 

 outer ridge is in places coated with sand or a sandy loam. The texture 

 of the moraine itself is also exceptionally sandy in that portion of the belt. 

 Sand is found in the form of dunes along the east bluff of the Illinois 

 River, from the bend of the river at Hennepin southward to the inner border 

 of this morainic system. Sand deposits were also noted on the inner slope 

 of the moraine in northern Vermilion County and west from the reentrant 

 angle in southeastern Livingston County. It is probable that the sand 

 deposits in both these localities are attributable to the presence of temporary 

 glacial lakes held in front of the retreating ice sheet, whose waves worked 

 upon the surface of the till sheet and formed the sandy beds there present. 

 As noted below, sand in places assumes the characteristic features of beaches 

 or shore lines in portions of the plains north of this morainic system. 



The surface of this morainic system from the vicinity of Bloomington 

 northward to Dekalb County is generally coated with a loess-like loam or 

 silt to a depth of 2 to 4 feet. East from Bloomington this surface silt is so 

 thin as scarcely to conceal surface bowlders. The silt also extends over 

 the plain east of this morainic system in counties bordering- the Illinois 

 River, and has often a depth of 6 or 8 feet on these plains. It is especially 

 prominent on the plain between the outer and inner belts in the Bureau 

 Creek Basin, its average thickness being not less than 6 feet. From the 

 inner belt eastward, in Bureau and Lasalle counties, it is less conspicuous 

 than in the Bureau Creek Basin, being scarcely 2 feet in average thickness. 



This surface silt was apparently deposited within a short time after the 

 retreat of the ice sheet, for the underlying till sheet appears to have suffered 

 no leaching prior to its deposition. The origin and mode of deposition of 

 this silt or loess-like loam, like that on adjacent portions of the Shelbyville 

 sheet, are as yet not clearly understood. The loess-covered plains on the 

 west seem to afford a source of supply, and the prevailing winds, if in the 



