ALBERTAN STAGE 



529 



Along the border of an advancing ice-sheet, the same phe- 

 nomena were repeated, but the kames, valley trains, lake deposits, 

 etc., were incomplete because of the continually advancing ice, 

 which overrode them, and, it may be, ploughed them up, or buried 

 them under ground moraines, or otherwise modified them. The 

 surface deposits of a glaciated region are those made by the ice 

 and water during the final retreat. 



Evidently, a succession of glacial episodes of alternately en- 

 croaching and shrinking ice-sheets must produce an exceedingly 

 complex succession of stratified and unstratified drift, and it can 

 cause little surprise that interpretations of such obscure phenom- 

 ena should differ widely. If the successive deposits were sepa- 

 rated by long, truly interglacial periods, then the sheets of drift 

 must have been exposed to the denuding agencies for correspond- 

 ing lengths of time, and will exhibit the various stages of chemical 

 and mechanical disintegration appropriate to the length of expos- 

 ure. There should be a manifest difference in this respect, be- 

 tween the earlier and the later deposits of drift. 



For the reasons indicated, the chronological arrangement of 

 the various parts of the drift, and the correlation of the deposits, 

 glacial, lacustrine, and marine, of different regions of the conti- 

 nent are exceedingly difficult. The following classification has re- 

 cently been proposed by Chamberlin for the drift of the Mississippi 

 valley, but is only tentative and provisional. 



C 9. Wisconsin Till-sheets (earlier and later). 

 j 8. Interglacial deposits (? Toronto). 



1 7. Iowan Till-sheet. 



6. Interglacial deposits. 

 Illinois Till-sheet. 



4. Interglacial deposits (Buchanan). 

 Kansan Till-sheet. 



2. Interglacial deposits (Aftonian). 

 , 1. Albertan Drift-sheet. 



Glacial or 

 Pleistocene Series. 



The Albertan stage is typically displayed in the Canadian prov- 

 ince of Alberta, where the first formation of drift was due to the 

 extension of glaciers eastward from the Rocky Mountains. Far- 



