636 THE PLEISTOCENE PERIOD 



Not all of these several sheets of drift have been seen in super- 

 position, and the history sketched above is based on the relations of 

 the sheets of drift at different points. 1 Theoretically, the several 

 sheets of drift are imbricated as suggested by Fig. 525, but each 

 sheet of drift is discontinuous beneath the overlying one, and this 



__^___^_ __ _ ^ _ . - _ _ _ _ _ _ _ _ i 



Fig. 525. Diagram illustrating the imbrication of the successive sheets of 

 drift. The full lines represent the portion of the drift-sheets not overspread 

 by later ice-sheets; the broken lines represent the portions of the successive drift- 

 sheets which w'ere covered by ice at a later time, i corresponds to Jerseyan 

 or sub-Aftonian, which in general is less extensive than the Kansan, though 

 locally, as in New Jersey, it extended farther south than any other; 2 repre- 

 sents the Kansan drift, the southern margin of which is not covered by younger 

 drift; 3 and 4, respectively, represent the Illinois-Iowan, and Wisconsin 

 sheets of drift. 



discontinuity goes so far that beneath the Wisconsin drift, for ex- 

 ample, the several sheets are more commonly wanting than present. 



VIII. Glacio-lacustrine stage. In the course of the retreat of 

 the ice of the Wisconsin epoch, a complex series of lakes arose be- 

 tween the ice border on the one hand, and the higher land fronting 

 it on the other. Many of these lakes were temporary and shifting, 

 and had shifting outlets. Their history cannot be given here; but 

 a brief sketch of the history of the Great Lakes will indicate the na- 

 ture of the changes which took place. 



When the end of the Lake Michigan ice-lobe (Fig. 526) with- 

 drew a little from the southern end of the Lake Michigan basin, a 

 lake formed there, and discharged its waters into the Illinois valley 

 southwest of Chicago. The channel followed by the outflowing 

 waters has since become the site of the Chicago drainage canal. 

 The glacial lake (Lake Chicago) thus initiated was gradually extend- 

 ed northward (Fig. 527) as the ice-lobe was melted. 



A similar lake was formed about the head of the Lake Superior 

 ice-lobe. Lake Maumee developed about the end of the Erie ice- 

 lobe, and its waters flowed to the Wabash. A later stage of Lake 

 Chicago and Lake Maumee is shown in Fig. 527, when, finding a 

 lower outlet as the ice melted back, Lake Maumee sent its outflow 

 across southern Michigan to Lake Chicago. 



Later, the whole Erie basin, and a portion of that of Ontario, 



1 Jour. Geol., Vol. I, pp. 61-84. 



