35-i Lever ett — Weathering and Erosion as Time Measures. 



tributary valleys. The tributaries, however, are in a very 

 incipient state of development. This map near West Jefferson, 

 Ohio, serves to indicate that fully nine-tenths of the drift plain 

 still stands at its original level, and this is true over a large 

 part of the surface of the Wisconsin drift. In fact there are 

 considerable areas in which less than one-tenth of the surface 

 has been lowered or reduced in any way by drainage lines. 



Areas in which the topography of the Wisconsin drift and the 

 Post-Wisconsin erosion may be studied without much compli- 

 cation with rock topography have already been mapped in 

 nearly every state from Massachusetts to North Dakota. 

 Thus a considerable part of southeastern Massachusetts, includ- 

 ing the islands of Marthas Yineyard and Nantucket, much of 

 Long Island in New York, as well as the greater part of the 

 plain east and south of Lake Ontario, nearly all the northwest 

 one-fourth of Ohio, the quadrangles mapped in southeastern 

 Michigan, northeastern Illinois and southeastern Wisconsin 

 (except the Broadhead, Janesville and Shoppiere sheets in 

 Wisconsin), the area around Minneapolis and St. Paul, Minne- 

 sota, the area north and west of Des Moines, Iowa, certain 

 sheets in the James Biver valley in South Dakota, and those 

 in the eastern part of North Dakota, may be drawn upon for 

 illustrations. It will be observed that the morainic areas of 

 southeastern Michigan, southeastern Wisconsin, and in the 

 vicinity of St. Paul, Minnesota, embrace much poorly drained 

 land represented as swamp, whereas such swamp areas are of 

 small extent in northwestern Ohio, northeastern Illinois, and 

 in the region bordering Lake Ontario. This should not be 

 interpreted to indicate that the less swampy areas owe their 

 condition to greater age. It results instead from a more favor- 

 able original condition for shedding the water or from a differ- 

 ent ground-water condition. 



There are extensive areas in New England, as also in New 

 York and in Ohio, where rock ridges are prominent, in which 

 the Wisconsin glaciation has served to produce what has been 

 termed by Salisbury superimposed youth.* In these areas the 

 drainage lines are often as poorly developed as in areas of the 

 Wisconsin drift, where the rock irregularities are completely 

 buried, and they indicate to the physiographer a similar youth- 

 fulness. Such areas, however, are not so satisfactory as the 

 smooth areas of heavy drift for determining the relative age of 

 the different drift sheets. 



The character of the glacial drainage of the Earlier Wis- 

 consin appears to have been different from that of the Later 

 Wisconsin, there being no extensive outwash gravel plains 

 associated with the Earlier Wisconsin moraines such as charac- 



* Journal of Geology, vol. xii, pp. 711-713, 1904. 



