IOWAN DRIFT SHEET AND ASSOCIATED DEPOSITS. 141 



the Illinoian. These have all been observed by Buell, who has kindly 

 furnished a list of the localities and bearings for publication. They appear 

 in the list on page 88. 



PROBABLE EXTENT OF IOWAN DRIFT SHEET BENEATH THE 

 WISCONSIN. 



The extent of the Iowan drift beneath the Wisconsin can be deter- 

 mined only approximately, and chiefly by inferential reasoning. An infer- 

 ential determination may be based upon the occurrence of a series of drift 

 sheets sufficiently full to include the Iowan. For example, if there is but 

 one older sheet of drift near the southern or western border of the Wiscon- 

 sin, and if iipon passing back beneath the Wisconsin two distinct sheets of 

 older drift are found, it may be inferred that the additional sheet is either 

 the Iowan or is a sheet still older than the Illinoian. A series of well sections 

 have made it evident that there are three well-defined drift sheets in the 

 northern portion of eastern Illinois, two of which are older than the Wis- 

 consin. It may be difficult to determine whether the additional sheet is of 

 Iowan or pre-Illinoian age, since the data are based almost entirely upon 

 well sections, and the material thus obtained is scarcely adequate for a 

 satisfactory determination. The complicated series extends as far south as 

 southern Iroquois County, and may possibly extend into Vermilion and 

 Champaign counties, though the sections obtained in those counties do not 

 furnish such decisive evidence of the triple series as is found in Iroquois 

 County. 



The Wisconsin drift may usually be readily separated from the older 

 sheets which - underlie it. Its uniform blue color and the ease with which it 

 may be penetrated by auger or spade enables the well drillers to distinguish 

 it readily from the underlying deposits, which are of brown or gray color 

 and more difficult to penetrate. There is also in many places a black soil 

 at the junction of this sheet with the underlying drift which furnishes addi- 

 tional evidence of the passage from the Wisconsin to an older drift. The 

 well records obtained in the flowing well district of southern Iroquois 

 County have brought to light the occurrence of a second soil at a horizon 

 25 to 50 feet or more lower than the base of the Wisconsin drift. There is 

 often considerable peat at this lower soil horizon, and with it a large 

 amount of water-bearing sand from which the flowing wells are obtained. 



