THE LAKE BORDER MORAESIO SYSTEM. 407 



STRUCTURE OF THE DRIFT. 



The drift beneath the Illinois portion of these till plains, like that of 

 the ridges, consists largely of a soft blue till, beneath which are remnants 

 of a hard till of earlier age. The tills appear to be of direct glacial depo- 

 sition, even in portions of the plain which lie within the limits of the 

 beaches. The clayey matrix of the soft till seems to- be less thickly set 

 with stones than that of the underlying hard till, but in both tills many of 

 the. stones are glaciated and show little evidence of water abrasion. The 

 rocky constituents grade from bowlders several feet in diameter down to 

 minute pebbles. These, in the upper or later till, are made up in large 

 part from the local upper Silurian rocks, probably less than 10 per cent 

 being from the pre-Cambrian Canadian rocks. Fragments of Devonian 

 rocks, apparently from ledges outcropping to the north of Chicago, are 

 sparingly represented. 1 The clayey matrix is highly calcareous, and under 

 the microscope it is found that angular or but slightly rounded grains of 

 limestone constitute a large proportion of the fine material. With the 

 minute limestone fragments there appear quartz grains, bits of shale, and 

 fragments from crystalline rocks of various kinds. Whether the rock con- 

 stituents of the older till differ markedly from those of the newer has not 

 been ascertained. Its situation immediately upon the Lockport (Niagara) 

 limestone would, in all probability, result in the incorporation of an even 

 lai-ger proportion of fragments from this rock than appears in the upper till. 



One of the most conspicuous instances of the occurrence of the lower 

 till within this area is that brought to light in the excavation of the Chicago 

 Drainag*e Canal. Immediately east from Summit the canal for about a 

 mile extends a few feet into a very hard, partially cemented till, apparently 

 of early glacial age. Its hardness compared with that of the overlying till 

 is so marked that the contractors who engaged to excavate this part of the 

 channel were obliged to abandon the steam shovel which had been used in 

 the soft till and to resort to blasting. It is probable that this old drift fills 

 depressions in the rock quite extensively in this district, but as no special 

 attention has been given its identification the instances recognised are not 



1 On the microscopic structure of certain bowlder clays, and the organisms contained in them, 

 l>y Dr. George M. Dawson: Bull. VI, Chicago Academy of Sciences. 



