580 THE ICE AGE IN NORTH AMERICA. 



composition of the material upon the extreme border of the 

 glaciated region as compared with that of the kettle-moraine 

 in Wisconsin, and what is considered to be a moraine of corre- 

 sponding age in the regions both east and west. 



A striking evidence of the reality of this difference in 

 oxidization is related by Prof essor Penck. When Mr. Frank 

 Leverett was visiting him in Germany the two went out 

 together into the Alpine fields where Professor Penck had 

 distinguished three well marked stages of glaciation of 

 increasing amounts of oxidization and erosion. These suc- 

 cessive periods of glacial and interglacial episodes he had 

 named after three streams in the foothills of the Alps in south- 

 ern Germany, where the deposits are typically present; 

 viz . , Mindel, Riss and Wiirm.* In every instance Mr. Leverett 

 was able to correlate these with the three divisions which he 

 had made in America; viz., the Kansan, corresponding to 

 the Mindel period; the Illinoisan to the Riss; and the Wiscon- 

 sin to the Wiirm. But he did not recognize the Iowan. All 

 these identifications were made in the field without previous 

 knowledge of Professor Penck's determinations. But with 

 reference to this evidence it is to be noted : 



1. That the more complete oxidization of the glacial 

 debris along the southern border and the greater decomposi- 

 tion of the granitic bowlders and pebbles distributed over this 

 border, are naturally accounted for by the obvious fact that 

 for the most part the material along the southern border, 

 and for some distance back from it, was that which was first 

 picked up by the advancing ice, and was probably already 

 oxidized and partially decomposed by the long-continued 

 action of preglacial agencies when the ice began its removal. 

 Its oxidization, therefore, may not be any true indication of 

 the remoteness of its transportation and deposition. It is 

 evident that every successive period of movement from the 

 north would operate upon lower strata of rock and upon the 

 masses which had been less affected by secular agencies of 



* See above page 459. 



