28 Proceedings of the Ohio State Academy of Science 



lowan 4, the Illinoian 8, and the Kansan 16. The estimates of 

 the time since the beginning of retreat of the late Wisconsin ice 

 vary between 20,000 and 60,000 years. Using the relative values 

 for the time of the earlier ice-sheets given above, this would make 

 the age of the Kansan between 300,000 and 1,020,000 years, and 

 even here we are not at the actual beginning of the Pleistocene 

 for this does not carry us back to the Sub-Aftonian. These facts 

 and values are suggestive only, but they are sufficient to show that 

 the ice age was a very long and a very complex period, the suc- 

 cessive ice advances being separated by long inter-glacial periods 

 in which the climate may have been as mild or milder than today ; 

 indeed, it is not certain that we ourselves are not living in an in- 

 terglacial period. Earlier glacial studies, proceeding on the 

 theory of a single ice age, placed together in many cases events 

 which we now know belong to different ice-advances, but 

 geologists have come to see that the unravelling of the events of 

 the Pleistocene means an immense amount of detailed work, 

 which will occupy many years to come. 



In eastern United State the late or Wisconsin drift extends 

 to the southern border of the glaciated area. It is not until 

 central Ohio is reached that any considerable width of the older 

 drift shows outside the Wisconsin area. This outer sheet of 

 older drift'^ widens to between 200 and 300 miles in the upper 

 Mississippi valley in Iowa, Illinois, and neighboring states. It 

 is the magnificent development of the older drifts in the interior 

 which made it inevitable that the geologists of these states should 

 take the lead in reaching a true understanding of the com- 

 plexity of the glacial period, in spite of the fact that in the east 

 and in New England long and detailed study had been given the 

 glacial deposits by able geologists. We have here another illustra- 

 tion of the control of local phenomena over the development of 

 the science. 



Former Glacial Periods. 



Ramsey, in 1855, in a paper "On the occurrence of angular, 

 sub-angular, polished and striated fragments and boulders in the 

 Permian breccia (of England) and on the existence of Glaciation 



