432 ANNALS NEW YORK ACADEMY OF SCIENCES 



mens and drawings, illnstrating the stages of development of tliis inter- 

 esting group. 



Miss Ogilvie said in abstract : Only abont thirty-five years ago the 

 controversy was going on as to whether there was really an ice age. That 

 question was hardly settled when the problem of two ice ages came to the 

 front. One by one ice ages were added, until at the beginning of the 

 present century we had six described and named, and naturally there 

 were corresponding interglacial stages. The six ice ages, with their 

 corresponding interglacial ages, have been incorporated in our text-books 

 and seemed to be passing into tradition when in 1909 Mr. Frank Leverett 

 raised the question as to whether one of them really existed. He did not 

 specifically attack the lowan age, but in a paper on the correlation of 

 American and European glacial deposits he left it out. In the various 

 papers which he and others have written there have been various lines of 

 attack^ and various conclusions have been reached as to what the lowan 

 drift is. Briefly, these were : that it is the weathered top of the Kansan ; 

 that it is contemporaneous with the Illinoian, coming from the Kewatin 

 center at the time that the Illinoian came from the Laurentide ; that it 

 is an interglacial deposit, formed contemporaneously with the loess. The 

 controversy was complicated by the fact that the only complete glacial 

 maps of the region were those published bj^ McGee in the Eleventh An- 

 nual Eeport U. S. Geological Survey, in which only two ice ages were 

 recognized. These upper and lower drifts are in some places the sub- 

 Aftonian and the Kansan, and in others the Kansan and the lowan. 

 Samuel Calvin, to whom we owe most of our knowledge of Iowa Pleisto- 

 cene geology, always defended the existence of the lowan as a separate 

 age, his death unfortunately occurring before the question was settled. 



Feeling that this question of the number of ice ages is the most im- 

 portant problem in glacial geology to-day, I visited the typical lowan 

 area. Having seen it, I feel convinced that, whatever the lowan is or 

 is not, it is not identical with the Kansan. The Kansan drift is blue and 

 clayey, the lowan yellow and powdery, but with huge granite boulders. 

 They often occur together, with the Buchanan gravel between, and there 

 is no question but that two drifts are there. The topography of the 

 lowan surface is 5^oung; it is for the most part a nearly flat plain with 

 very gentle undulations. The Kansan (in places where it was never 

 covered by the lowan) is much more deeply eroded, rivers having cut 

 through it and made gorges in the rock below. The lowan drift is unac- 

 countably absent from many places, and is never thick. In the various 

 cuts and gravel pits where I saw it, it was never more than seven feet in 

 thickness, and usually less. Its borders grade into loess, and the origin. 



