Lever ett — Weathering and Erosion as Time Measures. 353 



The erosion of the Later Wisconsin drift, compared with 

 that of the beds of the large glacial lakes which occupied 

 its surface during the recession of the ice from the St. Lawrence 

 basin, is sufficiently more advanced to be perceptible on many 

 of the topographic maps. The sections of the two maps here 

 introduced serve to bring out this difference (see figs. 1 



Fig. 2. 



Fig. 2. Part of London, Ohio, topographic sheet, illustrating the amount 

 of erosion ordinarily found on a plain of Wisconsin drift where the slope 

 is sufficient to favor a somewhat rapid development of drainage lines. The 

 incipient stage of development testifies to the recency of the Wisconsin 

 glaciation. Scale, 1 : 93700. Contour interval, 20 feet. 



and 2). Fig. 1, which represents a portion of the lake plain 

 in southeastern Michigan, shows the drainage lines to have 

 developed very few tributaries as well as to have cut only 

 insignificant valleys. Fig. 2, which represents the Wisconsin 

 drift in the western portion of Scioto basin in Ohio, shows a 

 slightly deeper valley cutting and the beginning of numerous 



