282 THE ILLINOIS GLACIAL LOBE. 



ridges appear to mark the shores of a lake-like expansion of the Illinois 

 River, and represents two stages differing about 30 feet in level. 



In northern McLean County a tract embraced between the inner border 

 of the main ridges of the Bloomington system and the Mackinaw River 

 Valley is gently undulating and is also dotted by occasional well-defmed 

 knolls 20 or 30 feet in height. It has a marked northward descent, the 

 elevation along the Mackinaw River being 50 to 100 feet below the inner 

 border of the moraine. North from the Mackinaw River, as already noted, 

 there is a well-defmed ridge (Cropsey Ridge) crossing the country in a 

 AVXW.-ESE. direction. From this ridge there is a gradual northeastward 

 descent toAvard the Vermilion River. The greater part of the surface is 

 plane or but gently undulating. There are, however, in southern Living- 

 ston County a few knolls and ridges of sand 10 or 20 feet in height. 

 These knolls and ridges are probably the result of wind action rather than 

 glacial features. 



East from the reentrant angle of the Bloomington morainic system in 

 Ford and southeastern Livingston counties there is a plain which covers the 

 greater part of Iroquois County and extends slightly into bordering' counties. 

 The plain is bordered at the north by the Marseilles morainic system and at 

 the east by the Iroquois moraine, a moraine of late Wisconsin age. It 

 extends into the State of Indiana only a few miles, in northwestern Benton 

 County. This plain descends toward the north, its altitude at the inner 

 border of the Bloomington morainic system being about 700 feet and in 

 northern Iroquois County only about 625 to 650 feet. It is crossed nearly 

 centrally from east to west by a gently undulatory belt, discussed above as 

 a possible continuation of the inner member of the Bloomington system. 

 Aside from this belt the drift surface is nearly plane. There are, however, 

 a few low sandy ridges in the western part of Iroquois County and numer- 

 ous dimes in the eastern part. Some of these ridges appear to be beaches 

 of a temporary lake, as indicated below. The dunes are, in all probability, 

 a result of wind action upon the sand deposits of the lake bottom. 



THICKNESS OF DRIFT. 



There are present beneath these intermorainic tracts a sheet of fresh 

 drift of Wisconsin age and older deposits of Iowan and Ulinoian age. The 

 thickness of the Wisconsin drift may be ascertained at many places by the 



