Leverett — Weathering and Erosion as Time Measures. 357 



drift displays, we find that where drainage gradients are good 

 it far exceeds that displayed by the Wisconsin drift. An 

 excellent topographic sheet for comparing the erosion of the 

 Illinoian with the Earlier Wisconsin is that of the Peoria, 

 Illinois, quadrangle, the western portion of which shows Post- 

 lllinoian erosion, while the eastern portion shows Post- Wiscon- 

 sin erosion, in studying this sheet, however, it should be 

 borne in mind that the Wisconsin drift area is morainic and 

 more elevated than the Illinoian, and thus has a great advan- 

 tage in the rapidity of shedding its water ; but in spite of this 

 advantage the extent of the erosion is far less than in the 

 Illinoian drift. 



Topographic sheets that lie entirely within the Illinoian 

 drift area are the Springfield, Eldorado and Belleville, Illinois, 

 sheets. The Belleville topographic sheet is of especial inter- 

 est since it exhibits the contrast in the work of streams under 

 different gradients. The western portion, a section of which 

 is here reproduced in fig. 3, illustrates the appearance of the 

 drainage when under a good gradient, while fig. 4, taken from 

 the southeast part of the same quadrangle, illustrates drainage 

 under a low gradient. It will be observed that Silver Creek 

 (in fig. 4) has a broad valley in which abandoned courses of 

 the stream testify to the sluggishness of the drainage and sug- 

 gest that aggradation rather than channeling is now in progress. 

 The entire Eldorado topographic sheet is in a region of low 

 drainage gradient, while the Springfield sheet represents a 

 region of more rapid drainage gradient. The series is, there- 

 fore, very instructive in setting forth the amount of variation 

 that may be found on a drift formation of a given age. 



To properly compare the Illinoian erosion with the Wiscon- 

 sin one must select areas in which drainage conditions are 

 similar, and also where no inequalities of drift surface, such as 

 moraines, etc., come in to give complexity. Figs. 2 and 3 

 represent fairly well the development of drainage under sim- 

 ilar conditions of slope. The conditions are perhaps more 

 favorable in the area represented by fig. 2 for the rapid devel- 

 opment of drainage than in fig. 3, yet the stage of drainage 

 development in fig. 3 is advanced far beyond that in fi.g. 2. 

 The writer has estimated that in the portion of the Illinoian 

 drift in western Illinois and southeastern Iowa, where the con- 

 ditions for development of drainage are the best, that approxi- 

 mately one-half the surface has been reduced below the origi- 

 nal level as a result of the drainage, while in the Wisaonsin 

 drift, as already indicated, scarcely one-tenth of the surface has 

 been thus reduced. It is evident that the time required for 

 developing the drainage found on the Illinoian is much greater 

 than that on the Wisconsin on the assumption that the climatic 



