STRUCTURE OF THE DRIFT BORDER. 57 



which indicate that the earlier ice invasion, from the Iowa side, crossed the 

 Mississippi into western Illinois. Instances of a soil between till sheets have 

 also been- found in this county, as in the counties of southeastern Iowa just 

 discussed, and this soil is referred to the Yarmouth interglacial interval. 

 The distance to which the Illinoian sheet overlapped the earlier one is not 

 known. It is probable that the heavy deposits of drift found in central and 

 southern Hancock County should be largely referred to the earlier invasion. 

 A well made by William McCuen on the east slope of the terminal 

 ridge of the Illinoian drift, about 4 miles south of Hamilton, has the fol- 

 lowing- section : 



Section in well of William McCuen, about 4 miles south of Hamilton, Illinois. 



Feet. 



Yellow silt or loess (Iowan) 12 



Soil and gray subsoil grading downward into a pale till (Sangamon and Illinoian) 22 



Blue till (probably Illinoian) 8 



Peaty muck with wood (probably Yarmouth) 4 



Pebbly clay of bluish color (probably Kansan) 28 



Total depth 74 



Mr. McCuen reports that other wells in the neighborhood have pene- 

 trated a similar buried peat. It seems probable that this peat is at the base 

 of the Illinoian sheet, though it may possibly be interbedded with other 

 deposits. 



From near Carthage southward past Stillwell there is a filled valley 

 whose position is revealed by the deep wells, the filling being so complete 

 that there are no surface indications of the course. The artesian wells at 

 Carthage penetrate 214 feet of drift, and several wells between Carthage 

 and Stillwell reach a depth of nearly 200 feet without entering rock, and 

 one a depth of 220 feet. A well at Owen's mill in Stillwell enters rock at 

 207 feet. As the surface elevation at these wells is nearly 200 feet above 

 the Mississippi River, those which strike rock enter it at about river level. 

 Probably the deepest part of this filled valley is cut to a much lower depth, 

 for the rock floor of the preglacial Mississippi is 100 feet or more below the 

 level of low water in the stream. In nearly all these deep wells the drift 

 is mainly a blue till similar to that exposed in the Mississippi bluff near 

 Fort Madison, and, like that near Fort Madison, is probablj- older than the 

 Illinoian. In some wells the blue till is entered at a depth of only 20 to 

 25 feet, but in the majority it is struck at 35 to 40 feet. 



