56 THE ILLINOIS GLACIAL LOBE. 



are exposures of drift 180 feet in height. The upper 50 or 60 feet consists 

 of alternations of till with sand or gravel, and is referred mainly to the 

 Kansan. The remaining 12.0 feet is a nearly solid mass of dark-blue till, 

 which is, however, characterized by stony parts that give it the appearance 

 of being interbedded with gravel and cobble. These bands of stony material 

 are verv nearly horizontal. It is not certain that this should be referred to 

 the Kansan; possibly it is pre-Kansan. Many bowlders are accumulated 

 along the base of this bluff. A strip covered with these bowlders, having 

 a length of 8 rods and a width of 3 rods, was carefully examined. It in- 

 cluded 107 bowlders, with an average diameter of about 3 feet. The larg-est 

 three exceed 6 feet in diameter; the smallest included in the count were at 

 least a foot in diameter. Upon classifying the bowlders it was found that 

 red granite greatly predominates, there being 85 sjjecimens. Of the gray 

 or dark-colored granite onty 6 were found. The greenstones are repre- 

 sented by 10 specimens. The two remaining specimens were limestone. 

 About one-fourth of the bowlders were glaciated on the sides exj;>osed to 

 view; probably many others are glaciated on the under side. The pro- 

 portion of red granite is exceptionally large for this region, though it is 

 probable that at least half the bowlders are of this class. These bowlders 

 are probably largely from the dark-blue till. 



The artesian wells in the Mississippi Valley at Fort Madison enter rock 

 at a level about 135 feet below low water in the Mississippi, or at about 

 365 feet above tide. The drift is mainty blue till, such as is exposed in the 

 neighboring bluff. It is older than the Illinoian, and possibly is pre-Kansan. 

 This blue till is covered to a depth of 10 to 40 feet by alluvial sand, which 

 in places extends to near the level of the river bed. 



The artesian well on the Mississippi bluff at Mont Clare, about 12 

 miles southwest of Fort Madison, penetrated 305 feet of drift. It is 

 reported to be mainly through clay for a depth of 250 feet, beneath which 

 there is sand extending to the rock. This well, it should be noted, stands 

 just outside the limits of the Illinoian drift. The drift, therefore, like that 

 at Fort Madison, belongs to an earlier sheet than the Illinoian. 



The Illinoian drift probably extends to the western limits of the upland 

 in Hancock County, Illinois, throughout the entire length of the county. 

 The drift, however, can not be referred entirely to the Illinoian invasion, for 

 there are southeastward-bearing- striae in the western part of this county, 



