Leverett — Weathering and Erosion as Time Measures. 365 



pean correlatives is also as conspicuous as in our Kansan. If 

 we consider that since the time when the Kansan drift and its 

 correlatives were deposited there have been repeated climatic 

 changes from cold to warm and probably also from humid to 

 arid, it may seem remarkable that weathering should have been 

 so similar in amount in these widely separated glaciated dis- 

 tricts. It is at least suggestive of worldwide influences in the 

 control of the climatic conditions that produced glaciation. 



Pre-Kansan Drift. 



The Pre-Kansan drift of the Keewatin field, so far as yet 

 known, lies entirely within the limits of the Kansan drift and 

 has exposures only beneath the Kansan. It is from these 

 exposures alone that one may learn the degree of weathering, 

 and they are inadequate to throw much light upon its erosion. 

 Recent studies in western Iowa, under the Iowa Geological 

 Survey, indicate that herbivorous animals had taken possession 

 of that region in the interval of ice recession between the Pre- 

 Kansan and the Kansan, so it is inferred that there was plant 

 growth such as comes only with a genial climate. Data bear- 

 ing directly upon the amount of leaching of the gravelly sur- 

 face portion of this drift were obtained by my assistant, P. T. 

 Chamberlin, in 1907, at exposures near Afton Junction, Iowa, 

 the type locality of the interglacial Aftonian exposures. Four 

 classifications of pebbles from near the surface of the deposit 

 show percentages of limestone pebbles ranging from 14 to 20 

 per cent, while two classifications of pebbles from underlying 

 till, not affected by leaching, show 38 and 52 per cent of 

 limestone pebbles. In other respects the pebbles of the gravel 

 and o*f the underlying till are strikingly similar. It thus 

 appears probable that the limestone content of the gravel was 

 originally about the same as that of the till, and that nearly 

 two-thirds of the limestone had been removed from the surface 

 portion of the Pre-Kansan drift before it was buried under the 

 Kansan. The limestone pebbles remaining in this surface por- 

 tion are in an etched and deeply weathered condition, so that 

 had there been but a moderate extension of the interglacial 

 stage the limestone might have been completely removed. In 

 places gravelly portions of the Pre-Kansan drift had become 

 firmly cemented in this interglacial stage, so that masses were 

 gathered up by the ice of the Kansan stage and incorporated 

 as bowlders in the Kansan till. The change which the Pre- 

 Kansan drift experienced probably approaches if it does not 

 equal that which the Illinoian has experienced, and appears to 

 be greater than that which has affected the Wisconsin drift. 

 It may therefore properly be referred to a distinct glacial stage. 



Am. Jour. Sci.— Fourth Series, Vol. XXVII, No. 161.— May, 1909. 

 25 



