Proceedings of the Ohio State Academy of Science 27 



of Geology, 1882) had a map of the drift area in which the ex- 

 treme southern border of the drift was shown, and within that 

 the morainic system of what we now know as the last or Wis- 

 consin drift ; but there is no suggestion of more than a single ice- 

 advance. But at the time Le Conte was writing, the geologists of 

 the upper Mississippi Valley were already engaged in the field 

 studies which were to show that the glacial series was more com- 

 plex than had at first been supposed. Chamberlin and Salisbury, 

 in their paper on the Driftless Area (1885), give an outline of 

 the glacial history of the interior, in which two epochs are recog- 

 nized, without emphasizing at all the length or significance of the 

 interval between them. In the years that followed an active dis- 

 cussion went on as to the meaning of this two- fold division. Does 

 the "interglacial epoch" signify a slight retreat of the ice, with a 

 later re-advance, in which the Wisconsin kettle-moraine was de- 

 posited? or does it mean the disappearance of the ice from the 

 region, perhaps from the continent, and later the invasion of the 

 northern states by an entirely new ice sheet? The outer till is 

 much more deeply weathered and more dissected, and in some 

 cases it is possible to show that it was in this condition at the 

 time of deposit of the inner drift. More than this, the differences 

 in the two drift sheets indicate that the ice-sheets that formed 

 them advanced in quite different manner, and over land surfaces 

 at unlike altitudes above sea-level. These things suggest a long 

 interval and changed conditions between the two ice advances, 

 and create a strong probability for separate ice-sheets. 



The same field study has brought out the further fact that 

 there are more than two drift sheets and ice advances. Cham- 

 berlin and Salisbury (Geology, III. 413) give six, which, com- 

 mencing with the most recent, are Late Wisconsin, Early Wis- 

 consin, lowan, Illinoian, Kansan, Sub-Aftonian. The genuine- 

 ness of the lowan has been recently questioned by Leverett, and 

 even among lowan geologists, there is doubt as to its real mean- 

 ing. Estimates of the relative weathering and erosion of the dif- 

 ferent sheets indicate roughly that, if the time since the com- 

 mencement of the retreat of the late Wisconsin ice-sheet is taken 

 as I, the time since the retreat of the early Wisconsin is 2, of the 



