TIME DELATIONS OK GLACIAL SUCCESSION. 21 



Stage 10. Fifth interval of recession, unnamed; shown by shifting of ice lobes. 

 Stage 11. Late Wisconsin drift sheets. 



Substage 1. Great bowlder belts and accompanying moraines, including, perhaps, the 

 Minooka till ridge. 



Substage 2. Valparaiso morainic system. 



Substage 3. Lake border morainic system. 

 Stage 12. Lake Chicago submergence. 

 Stage 13. Emergence of plain covered by Lake Chicago. 

 Stage 14. Partial resubmergence of plain covered by Lake Chicago. 

 Stage 15. The present stage of Lake Michigan. 



The outline just presented differs from the one last presented by 

 Chamberlin 1 in the separation of the Wisconsin drift series and lake history 

 into the several substages, and the introduction of names for three inter- 

 glacial stages. Except for the introduction of names for the interglacial 

 stages, it is essentially the same as an outline presented by the writer in a 

 recent bulletin of the Chicago Academy of Sciences. 2 



Mr. J. B. T3Trell of the Canadian Geological Survey, who has studied 

 widely in western Canada, favors the separation of the Albertan and sub- 

 Aftonian stages. In a letter to the writer dated July 9, 1897, which 

 discusses the outline given in the bulletin just referred to, he makes the 

 following statement : 



The Albertan drift sheet of Dawson is older than any till of the Keewatin 

 glacier seen in the plains of northwestern Canada, while the Upper and Lower 

 Bowlder Clay of our Reports seem to correspond closely with your Kausau and sub- 

 Aftoniau. I should, therefore, completely separate the Albertan and sub-Aftonian. 



In the present stage of investigation the correlation of the Upper and 

 Lower Bowlder Clays of the Canadian Reports with the Kansan and sub- 

 Aftonian of southern Iowa is not worked out satisfactorily. The full extent 

 of either the Kansan or sub-Aftonian in districts lying between Iowa and 

 the Canadian boundary is not determined. It therefore may be hazardous 

 to venture definite correlation, though the balance of probabilities, as sug- 

 gested by Tyrrell, seems to favor this correlation and the transference of 

 the Albertan to an earlier stage. 



The complexity of the glacial history is still further increased by the 

 occurrence of more than one gathering ground or center of dispersion of 

 the ice. The explorations of the Canadian Survey have shown that there 



'Jour. Geol., Oct.-Nov., 1896, Vol. IV, No. 7, pp. 872-876. 



- Pleistocene features and deposits of the Chicago area, by Frank Leverett : Bull. Chicago Acad. 

 Sci. No. 2 ; issued May, 1897. 



