36tt Lever ett — Weathering and Erosion as Time Measures. 



The best sheets to illustrate the Post-Kansan erosion are the 

 Atlanta, Edina, and Kahoka, Missouri, quadrangles, not only 

 because the drift there is sufficiently thick to dominate the 

 topography but also because the maps are better executed than 

 older contour maps of eastern Iowa and eastern Nebraska. 

 The DesMoines, Iowa, and Waukee, Iowa, topographic sheets 

 are, however, recent and of a high standard, and they serve to 

 exhibit the contrast between the erosion of the Kansan and 

 the "Wisconsin, the southern edges of the maps being within 

 the limits of the Kansan drift, while the remaining, much less 

 eroded portion is in the Wisconsin drift. 



As to the length of the Post-Kansan period of weathering 

 and erosion compared with that of the Post- Wisconsin, only a 

 rough approximation is likely to be reached by measurements 

 of the changes effected in the two drifts. Dr. H. F. Bain 

 has made a computation of the relative size of valleys in the 

 Kansan and in the Wisconsin drift in the vicinity of Des 

 Moines, Iowa, and ascertained the capacity of the Post-Kansan 

 valleys to be about seventeen times that of the Post- Wisconsin.* 

 While a considerable time must necessarily be allowed for a 

 drainage system to get well started and thus correspondingly 

 lengthen the Post-Wisconsin time and reduce the difference in 

 the age of the Wisconsin and the Kansan drift, the periods of 

 aridity through which the Kansan drift has passed prior to the 

 Wisconsin stage of glaciation may fully offset this lengthening 

 of the Post-Wisconsin time. 



The question of a correlative of the Kansan drift in the 

 Labrador field is considered below in connection with the dis- 

 cussion of the Pre-Kansan drift. 



European Equivalents of the Kansas Drift. 



. In each of the glaciated districts of Europe a drift sheet is 

 found in which the weathering is strikingly similar to that of 

 the Kansan drift. In the Alpine region the apparent correla- 

 tive is the Mindel drift ; in north Germany it is the Lower 

 Diluvium, the drift formed at the maximum extent of the 

 Skandinavian ice held, when it reached the recesses of the 

 mountainous districts of Silesia and Saxony ; in Great Britain 

 it is the chalky bowlder clay of the southeastern counties. Not 

 only is there a similar depth of leaching but also a similar 

 degree of oxidation, and a deep penetration of weathered pipes 

 and seams. In the Cromer sections on the east coast of Eng- 

 land the weathered seams are conspicuous to depths of 10 to 15 

 meters, and the leaching of the chalky bowlder clay is generally 

 thorough to a depth of about 2 meters. The reddish tinge of 

 the upper part of the weathered drift in these supposed Euro- 



* Geology of Polk County, Iowa Geological Survey, vol. vi. 



