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thus far no deposits formed in these interglacial lakes, except perhaps near Toronto, have been identified. Each retreat of the ice must also have been followed up by bodies of water whose outlet was blocked toward the east. The last ice retreat, that of the 'Wisconsin sheet, has left unmistakable evidence of a succession of glacial lakes which covered much of the region. Of these lakes two are of greatest importance—Lake Iroquois, which occupied the basin of Lake Ontario, but at a higher level ; and Lake Algonquin, which covered most of the great upper lakes with a single vast sheet of water.

These ancient lakes have been somewhat carefully mapped, and their shores are as mature and often almost as well preserved as those of the present Great Lakes, though now, of course, covered with vegetation.



The shores of Lake Iroquois, with their wave-cut cliffs and well-defined gravel bars, have been traced almost all round Lake Ontario, by Gilbert and Fair-child in New York, and by Spencer and Coleman in Ontario; but there is a wide gap at the north-east end of the basin, where no shore has been found. The shore in that quarter was of ice. Since the St. Lawrence outlet was blocked, Lake Iroquois emptied through the Mohawk Valley in New York into the Hudson. The beach is continuous except where cut by river valleys and at Scarborough



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