24 TWELFTH REPORT. 



GLACIAL AND INTERGLACL^L STAGES. 



The studies of the glacial deposits have shown that thej do not con- 

 sist of a single sheet but embrace three and in places four distinct drift 

 sheets. There is evidence that these drift sheets are the product of re- 

 peated glaciation at Avidely separated intervals. They are separable by 

 beds of peat and deposits made by lakes and streams. These deposits 

 contain remains of forms of life similar to those now existing in the 

 same regions, and thus they indicate a temperate interglaclal climate. 



It is probable that after the first glaciation as well as after each 

 succeeding one there were lakes in the basins of the present Great Lakes. 

 These would be invaded and completely filled by ice in each recurring 

 glacial stage. Evidence of such interglaclal lakes is largely destroyed 

 by the glacial occupancy and must be at best very fragmentary. Yet 

 there are not wanting features that point to their occurrence. For ex- 

 ample, at the southern end of the Lake Michigan basin, and also on 

 the eastern side where dunes ilow occur, the moraines formed by the last 

 glacial stage are found to be more sandy than their continuation on 

 the western side of the lake basin. This sandiness it is thought may be 

 due to the incorporation of the interglacial dune material in the drift. 



The history of the lakes in connection with and subsequent to the 

 last glaciation is so com] ilex that it alone will be outlined at this time. 

 The complexities here outlined are likely to be but a recurrence of those 

 of the interglacial times. The interglacial times differ from the post- 

 glacial in that they involve a readvance of ice and extinction of any 

 lakes that may have been present in the basins. The interglacial times 

 were also much longer than the postglacial and there were corresjiond,- 

 ingl}' gTeater filling and modification of the lakes. We are, therefore, 

 presenting in this postglacial lake history a succession of events less 

 complete than probably occurred in each int(>rg]acial stage. 



In the last or ^Msconsin stage of glaciation, and also in the next 

 preceding, or Illinoian stage, the Great Lakes region was covered by 

 ice moving south west Avard from a' center in Labrador. In a still earlier 

 glacial stage, the Kansan, there appears to have been a southward move- 

 ment across the (Jreat Lakes region from a center in central Canada 

 knoAvn as the Keewatin. At least, copper was distributed from the Lake 

 Superior region as far southeast as central Ohio probably by ice moving 

 southward through the Huron basin and across the Avesteru portion of 

 the Erie basin. The lake history is likely to have been different at the 

 withdrawal of the Keewatin ice sheet from the basin it had filled than it 

 was at the withdrawal of the Labrador ice slieet in subsequent stages of 

 glaiciation. It would not, bowever, be an easy matter to unravel that part 

 of the Great Lakes history, for any moraines or features that Avould throw 

 light on the outline of the ice border have been overridden and com- 

 pletely concealed by the later invasions from the Labrador center. 



FEATURES CHARACTERISTIC OP LAKE SHORES. 



Inasmuch as the predecessors of the Great Lakes stood at higher 

 levels than the present bodies of water and covered certain outside 

 territory, the features characteristic of lake shores have been discov- 

 ered in this outside territory and used as a basis for interpreting the 



