MODIFICATION OP GREAT LAKES BY EARTH MOVEMENT. 351 



equally clear. Before the Ontario Valley held a laud bound lake it 

 was occupied by a gulf of the ocean. Owing to the different attitude 

 of the land, the water surface of this gulf was not parallel to the 

 present lake surface but inclined at an angle. In the extreme north- 

 east, in the vicinity of the Thousand Islands, the marine shores are 

 nearly 200 feet above the present water level, but they descend south- 

 ward and westward, passing beneath the lake level near Oswego, and 

 toward the western end of the lake must be submerged several hun- 

 dred feet. This condition was of short duration, and the rising land 

 soon divided the waters, establishing Lake Ontario as an indepen- 

 dent water body. The same peculiarity of land attitude which made 



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THE NIP1SSING GREAT LAKE (AFTER TAYLOR). 



Its boundaiies are shown by the broken line. 



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 point of outlet and enlarging the expanse 

 of the lake. 



There is some reason to think that the upper 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 as could be desired. When the normal 

 lacustrine condition was established they were at first a single lake 

 instead of three, and the outlet, instead of being southward from Lake 

 Huron, was northeastward from Georgian Bay, the outlet river follow- 

 ing the valleys of the Mattawa and Ottawa to the St. Lawrence. The 



