236 



MODIFICATION OF THE GREAT LAKES 



present water surfaces. It has been recognized at many points 

 about Lake Superior and the northern parts of lakes Huron and 

 Michigan, and measurements of its height show that its plane 

 has a remarkably uniform dip, at 7 inches per mile, in a south- 

 southwest direction, or, more exactly, S. 27° W. As will be seen 

 by the accompanying map, reproduced from Taylor, it crosses the 

 modern shore-line of Lake Superior near its western end, thereby 

 passing beneath the water surface ; and it similarly passes below 

 the surface of Lake Michigan near Green bay, and below the 



FIG. 3— THE NIPISSING GREAT LAKE (AFTER TAYLOR) 



Its boundaries are shown by the broken line 



surface of Lake Huron just north of Saginaw bay. The south- 

 ward tilting of the land, involving the uplift of the point of 

 outlet, increast the capacity of the basin and the volume of the 

 lake, graduall}^ carrying the coast-line southward in Lake Huron 

 and Lake Michigan until finally it reacht the low pass at Port 

 Huron and the water overflowed via the St Clair and Detroit 

 channels to Lake Erie. The outlet by way of the Ottawa was 

 then abandoned, and a continuance of the uplift caused the 

 water to slowly recede from its northern shores. This change 

 after a time separated Lake Superior from the other lakes, bring- 



