352 Leverett— Weathering and Erosion as Time Measures. 



Illinois, topographic maps. If one should place these maps 

 besides those of quadrangles inside the Later Wisconsin, as for 

 example the Marion, Sycamore, Upper Sandusky and Arling- 

 ton sheets in northwestern Ohio, it would be difficult to dis- 

 cover any difference in the character of drainage. The amount 

 of weathering, however, is perceptibly greater in the Earlier 



Fig. 1. 



Fig. 1. Part of the Wayne, Michigan, topographic sheet, illustrating the 

 slight amount of erosion characterizing the beds of the great glacial lakes 

 Scale, 1 : 90000. Contour interval, 20 feet. 



than in the. Later Wisconsin, it being rare to find unleached 

 till on the plains of the Earlier Wisconsin at a depth of less 

 than a meter, whereas on the Later Wisconsin it is frequently 

 found at \ meter or less, and the average depth of leaching in 

 the Later Wisconsin scarcely reaches a meter. It is, therefore, 

 chiefly by the weathering and by toning down of the slight 

 inequalities that the Earlier Wisconsin is distinguished from 

 the Later. 



