MODIFICATION OF THE (iREAT LAKES 



23:> 



parallel to the present lake surface but inclined at an an{j;le. In 

 the extreme northeast, in the vicinity of the Thousand Islands, 

 the marine shores are nearly 200 feet above tlie present water 

 level, but they descend southward and westward, jjoasing be- 

 neath the lake level near Oswego, and towanl the western iMid 

 of the lake must be submerged several hundred feet. This con- 

 dition was of short duration, and the rising land soon divided 

 the w^aters, establishing Lake Ontario as an independent water 

 body. The same peculiarity of land attitude winch made the 

 original Erie a small lake served to limit the extent of Ontario, 

 but the restriction was less in amount because of the steeper 

 slopes of the Ontario basin. Here again the southward tilting 

 of the land had the effect of lifting the i)oint of outlet and en- 

 larging the expanse of the lake. 



TorontjO, 



Oswego 



FIG. 2 — ANCIENT AND MODERN OUTLINES OK LAKE ONTARIO 



The broken line shows the original extent of the lake 



There is some reason to think that the ui)per lakes, Huron. 

 Michigan, and Superior, were at first open to the sea, so as to 

 constitute a gulf, but the evidence is not .so full Jis could l>e de- 

 sired. When the normal lacustrine condition was esUiblisht 

 they were at first a single lake instead of three, and the outlet, 

 instead of l)eing southward from Lake Huron, was northeastward 

 from Georgian bay, the outlet river following the valleys of the 

 Mattawa and Ottawa to the St Lawrence. The trii)le lake is 

 known to us chieflv through the labors of F. B. Taylor, who lia.s 

 made extensive studies of its shore-line. This line, called the 

 Nipissing shore-line, is not wholly submerged, like the old 

 shores of lakes Erie and Ontario, but lies chietly ab..ve the 



