‘a er 
THE GEOLOGICAL HISTORY OF NEW YORK STATE Q7 
lake basins were appreciably deepened. Even so, however, we have 
not yet accounted for the present closed basins. In the writer’s 
opinion the two most important phenomena which have contributed 
to the formation of the closed basins of the Great Lakes are the 
great drift accumulations along the south side and the tilting of 
the land downward on the north side of this region. The deep 
drift deposits must certainly have been very effective in damming 
up the south or southwesterly-flowing preglacial streams of the 
region. For example, the deep channel of the so-called Dundas 
ic. 30 The first stage in the formation of the Great Lakes, 
when most of the region was still buried under the ice. 
After Taylor & Leverett 
fiver (see figure 27) has been drift-filled as proved by many well 
borings, and a distinct moraine extends around the southern half 
-of Lake Michigan. The great dumping ground of ice-transported 
materials from the north was in general along the southern side of 
the Great Lakes and southward. Late in the Ice age the land on 
the northern side of the Great Lakes region was lower than it is 
today as proved by the tilted character of certain well-known 
beaches of extinct glacial lakes (see below). Such a differential 
tilting or warping of the land must have helped to form the closed 
basins by tending to stop the southward or southwestward drainage 
