522 THE ICE AGE IN NORTH AMERICA. 



as to give any conclusive evidences of complete departure of 

 the ice-sheets and their subsequent renewal to again spread 

 over nearly all of their former areas. Only in the peripheral 

 parts, broadly speaking, of these two immense ice-fields, are 

 proofs of successive stages of glaciation, divided by old land 

 surfaces and fossil-bearing stratified beds, either of the modi- 

 fied drift or of alluvial, lacustrine, or marine sedimentation. 



Minnesota, somewhat far back from our southern lim- 

 its of the maximum glaciation, has well ascertained proofs 

 of so great recession of the borders of the North American 

 ice-sheet as to uncover the south half of this state, succeeded 

 by renewal of the snowfall and ice accumulation until the 

 borders of our continental ice-sheet again reached southward 

 as far as the Iowan and the Wisconsin drift. The earlier 

 glaciation appears surely to have been of much longer dura- 

 tion than the later Iowan and Wisconsin stages. Thus our 

 Pleistocene Ice age was much diversified and even very com- 

 plex, yet I would now far more confidently ascribe all our 

 North American drift formations to one prolonged and con- 

 tinuous glacial period, with great fluctuations of the glacial 

 border, especially in the interior of the continent, than to 

 regard our ice age as two-fold or three-fold, in the sense of 

 having its vast ice-sheet wholly melted away, or even nearly 

 so, with ensuing renewal of the snow and ice-fields. 



Geologically very rare, an ice age would scarcely be dupli- 

 cated with so nearly the same limits of ice extension upon 

 half of our continent. The same general conclusion is also, 

 as I think, applicable to the European glaciation. Almost 

 inconceivable geologic duration divided the Permian and 

 Pleistocene Ice ages. In this most recent and geologically 

 short Ice age which has ended, as I surely believe, within the 

 last 10,000 to 5,000 years, at the threshold of the historic 

 period, I cannot think that the stupendous climatic changes 

 implied in the glaciation could permit complete repetition 

 of these continental ice-sheets in America and Europe, in 



