152 S. CALVIN PKESENT PHASE OF PLEISTOCENE IN IOWA 



and it is to this fact that we owe the possibility of making estimates as 

 to relative age. The oldest glacial deposit in which the accumulating 

 effects of continuous time is recorded is the Kansan; the youngest is the 

 Wisconsin; and between the two the differences in age seem almost im- 

 measurable. Comparing the weathering and erosion in the central, inter- 

 morainic part of the Wisconsin with the corresponding evidences of 

 change in the Kansan, the differences must be expressed by a number 

 greater than one hundred. It cannot be affirmed that changes have 

 progressed at a uniform rate in each bed of drift or during all parts of 

 Pleistocene time ; but, making every possible allowance, there is no escape 

 from the conclusion that the Pleistocene was a long, long period, com- 

 pared with which the recent period, or post-Glacial time, would have to be 

 represented by a very small fraction. The Yarmouth, or even the Sanga- 

 mon interval, was long as compared with the post- Wisconsin. 



Conclusion 



The facts presented in this address show that the history of the re- 

 markable period we call the Pleistocene was vastly more complex than was 

 suspected by any one twenty years ago; but, complicated as this history 

 now seems, it is possible that we have just made a beginning in recover- 

 ing the leaves of the fuller and larger history which will include Pleisto- 

 cene details at present unknown and undreamed — details which will 

 clearly illuminate every successive step in the evolution of the world as 

 we know it today from that which existed at the close of the Tertiary. 

 In the presidential address on Pleistocene history which will be delivered 

 before this Society twenty years hence there will probably be descriptions 

 of drift sheets — now unknown because completely buried under younger 

 deposits — dividing the long Yarmouth and Sangamon intervals; there 

 will be more interglacial phases, fuller discussions of interglacial faunas 

 and floras, more significant details of every sort and kind. Some who 

 are here today may have the privilege of listening to that address and of 

 joining with the younger men of the time in expressing surprise at the 

 meagerness of the knowledge of Pleistocene history possessed by geolo- 

 gists during the first decade of the twentieth century. 



