WASHINGTON ACADEMY OF SCIENCES 73 



The discovery was followed by many others of similar nature until 

 the complete sequence involved no less than five distinct till-sheets. 

 Today the Iowa classification of the Glacial period is accepted the 

 world over. 



Since diversity of the Glacial period is a necessary consequence of 

 CrolFs now celebrated hypothesis of climate and time, this first seri- 

 ous attempt to establish glacial complexity as opposed to glacial 

 unity premised by Agassiz, was soon shown to have its astronomical 

 dates too far apart satisfactorily to explain the vicissitudes of the 

 Ice age that were even then known. In the meanwhile direct obser- 

 vations in the field were beginning to tell heavily against the unity 

 idea then generally held by scientific men. Professor Edward Orton 

 had noted (1870) a soil separating the Ohio drift into two parts. Al- 

 ready Professor T. C. Chamberlin had argued for the duality of the 

 Glacial period (1876) . It remained, however, for Doctor McGee actu- 

 ally to demonstrate for the first time an orderly succession of glacial 

 deposits in a definitely circumscribed area. 



The arguments for a dual Glacial period, and at the time of its sug- 

 gestion for a multiple Ice age, were for many years based mainly upon 

 the fact of the presence in some till-sections of thin black soil streaks, 

 replaced here and there by old peat-beds. The latter were plausibly 

 accounted for in other ways. That there might be extensive inter- 

 glacial sands, or clay-deposits, was not thought of. Yet they were 

 actually described and a complete record made of their attendant 

 relationships a full decade before the phenomena were properly in- 

 terpreted. 



At the time when Doctor McGee's little memoir appeared the sub- 

 ject of the complexity of the Glacial period was too new for a ready 

 and correct apprehension of the facts which were accumulating with 

 such amazing rapidity. The interest of glacial workers was centered 

 too intensely upon the possible dual character of the Great Ice invasion 

 to permit of the entertainment of any further complications. In 

 Iowa, where afterwards the five great till-sheets were made out and 

 shown to be separated from one another by extensive soil zones, thick 

 deposits of loess, and vast accumulations of sands and gravels, Doc- 

 tor McGee (then) had but recently gone over the ground, and had 

 found everywhere the same succession of only two tills parted by fine 



