THE PROBLEM TWENTY YEARS AGO AND TODAY 135 



stated at the outset, the location of Iowa with reference to the known ice 

 movements was exceptionally favorable. The two earlier sheets passed 

 across the state, covering the whole surface except the small part of the 

 Driftless area which lies in the northeastern corner. Terminal margins 

 of the known three later sheets come within the limits of Iowa, and the 

 movements were so distributed that no one of the ice lobes obscured the 

 records of its predecessors. There are, therefore, miles upon miles of 

 well defined border lines along which it is possible to compare directly 

 the characteristics of one drift sheet with another; and there are inter- 

 glacial deposits bearing testimony to the nature of the faunas and floras 

 and climatic conditions which characterized the central portions of the 

 continent during the long, mild intervals between the stages of glaciation. 

 It is the purpose of this address to set forth what the Pleistocene records 

 of such an area more or less definitely prove. 



First and Second Glacial Epochs and First interglacial Interval 

 general characteristics 



During the early Pleistocene, as already noted, there were two stages 

 of glaciation which affected larger areas in Iowa than any of those which 

 followed. So far as present knowledge goes, these were the real first and 

 second Glacial epochs of which we have discovered records in the interior 

 of the continent; but later investigations, as has happened before, may 

 change the ordinal positions here assigned them. No conceivable discov- 

 eries, however, can possibly modify the fact that these two invasions 

 were distinct glacial episodes, separated one from the other by a long 

 interval; for here are two sheets of drift distinctly different, recording 

 the coming and going of two ice caps, while soil bands, weathered zones, 

 buried peat bogs, forest beds, pond silts, and stream gravels lying between 

 the till sheets tell of interglacial conditions. The widely distributed 

 peats and forests of the intercalated beds furnish a record of the plants 

 which flourished during the interval ; and within the last four months the 

 gravels between the older drifts in Harrison, Monona, and other western 

 counties have contributed the first important information from the area 

 under consideration concerning the first interglacial fauna. 



The earlier of the two older drifts is known in the geology of Iowa as 

 the pre-Kansan or sub-Aftonian, and the later of the two, adopting the 

 name proposed by Chamberlin, is the Kansan. Whether the pre-Kansan 

 should be correlated with the Albertan of the north, or with the Jer- 

 seyan of the east, or with anything else now known in America or in 

 Europe, is a question that need not be discussed. It is enough for the 



