Leverett — Weathering and Erosion as Time Measures. 351 

 Table of drift sheets. 



Keewatiu 



Labrador 



Skandinavian 



Alpine 



Wisconsin 



Wisconsin 



Upper Diluvium 



Wiirm 



Iowan ? 









Illinoian ? 



Illinoian 



Middle Diluvium 



Riss 



Kansan 



Kansan ? 



Lower Diluvium 



Mindel 



Pre-Kansan* 



Jerseyan 





Gunz 





Wisconsin Drift. 





In the first attempts by Chamberlin and others to show the 

 complexity of the glacial deposits but two drift sheets were 

 recognized, the earlier and the later, and of these the later 

 embraces what is now known as Wisconsin drift. Its aspect 

 is everywhere much fresher or younger than that of the outly- 

 ing and underlying drift sheets, so that it can easily be distin- 

 guished from them. With the progress of the study it became 

 evident that considerable complexity of ice movement was 

 involved in producing the many moraines of this later drift. 

 The moraines do not form throughout a concentric series, such 

 as one would expect to find had the ice made a simple retreat 

 from its field, w T ith halts at places where moraines occur. 

 Instead, there are groups of concentric moraines which have a 

 trend out of harmony with other groups, resulting in a partial 

 overriding of the earlier by the later. From this it is inferred 

 that the ice made shiftings in its directions of movement, 

 such as call perhaps for some change in its center of accumu- 

 lation. 



The earliest group of these moraines of later drift has been 

 termed the Earlier Wisconsin, while the remainder of the 

 moraines are thrown together under the name Later Wiscon- 

 sin. The principal exposure of the Earlier Wisconsin is in 

 Illinois, but it has a slight extent beyond the Later Wisconsin 

 in Indiana and Ohio and possibly also in Wisconsin. Whether 

 it is exposed in states farther w T est has not been determined. 

 The amount of weathering and erosion which the Earlier 

 Wisconsin has undergone differs so little from that experi- 

 enced by the Later Wisconsin that we may feel sure that 

 no interval of great length separates them. The knolls 

 and basins along the moraines of the Earlier Wisconsin are 

 toned down by erosion and filling so that only the larger ones 

 are well-defined, whereas in the Later Wisconsin the small 

 hummocks and basins still preserve their sharpness of contour. 

 But in the Earlier Wisconsin as in the Later there are wide 

 areas not yet invaded by drainage lines, as may be seen by 

 reference to the Danville, the Urbana, and the Mahomet, 



* The Pre-Kansan is sometimes termed Sub-Aftonian. 



