GEOLOGY OF THE TORONTO REGION 
thus far no deposits formed in these interglacial 
lakes, except perhaps near Toronto, have been iden- 
tified. 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 evi- 
dence 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. 
Laxe Iroquors. 
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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