GEOLOGICAL AND NATURAL HISTORY SURVEY. 29 



feet or less in''heig-ht. There is not so well-defined ridging as in 

 the main belt. Though the undulation is not sharp, it is sufficient 

 to give this belt a strong contrast to the very smooth till plain 

 which borders it on the south. The contrast with the till plain on 

 the north is not so striking, for that plain is dotted with low swells 

 ten feet or more in heigiht. 



Thickness of the Dn/it '.-^Situated as the Valparaiso moraine 

 is in a district over vi^hich there have probably been several suc- 

 cessive ice advances with intervening recessions, the drift can 

 scarcely be expected to belong solely to the advance which formed 

 this moraine. It is known that remnants of the partially ce- 

 mented sheets of the oldest stages of glaciation are present. 

 There is probably present also a considerable amount of drift of 

 the Early Wisconsin stage. The drift of the Early Wisconsin 

 stage is so similar to that of the Valparaiso moraine that it is 

 doubtful if it can be readily distinguished from it. There does 

 not appear to be a well-defined soil horizon at the base of the Val- 

 paraiso sheet to mark the line of junction such as occurs under 

 wide areas at the base of the Early Wisconsin drift. The thick- 

 ness of the Valparaiso drift sheet can perhaps be best estimated 

 b}- the relief, for it can scarcely be assumed to exceed greatly the 

 measure of the outer border relief of the moraine. The relief on 

 that border is estimated to average about sixty-five feet. The 

 average thickness is probably even less than the relief, since the 

 sheet is markedly thinner on the borders of the moraine than on its 

 crest. 



The combined thickness of the several sheets of drift here rep- 

 resented is also difficult to determine from the fact that the under- 

 lying rock surface is very uneven. In Cook County alone the rock 

 surface is known to range from about lOO feet above the level 

 of Lake Michigan to more than lOO feet below the surface of the 

 lake. In the table of thickness of drift which follows, the older 

 deposits as well as those of the Valparaiso moraine are included. 



