96 NEW YORK STATE MUSEUM 
Great Lakes history. The Great Lakes certainly did not exist 
before the Ice age, but instead the depressions in that region were 
occupied by stream channels. During the very long erosion period 
(already discussed) from the Paleozoic to the Cenozoic, no lakes, 
except possibly a few very small ones due to landslides, beaver 
dams, etc., could have existed. Compared with such an immense 
length of time lakes are, at most, only ephemeral features of the 
earth’s surface because they are soon destroyed either by being 
filled with sediments, or by having their outlets cut down, or both. 
Since the Great Lakes are of postglacial origin it is, then, proper to 
ask how they came into existence. During preglacial time, as we 
have learned, broad valleys were cut out along belts of weak rock 
in the Great Lakes region, and these old valleys, to a considerable 
extent at least, account for the present depressions, but not for 
the closed lake basins. This idea of preglacial stream valleys is 
not at all opposed by the fact that some of the lake bottoms are 
now well below sea level because there has been a notable sub- 
sidence of the region since preglacial time. The surface of Lake 
Erie is 573 feet and its deepest point 360 feet = abeveusem 
level, while the surface of -Lake Ontario is 247 eceeaneme 
and its deepest point is 4091 feet below) sea leyelummmeae 
greatest depth (738 feet) of Lake. Ontario is well toward the 
east end and not far from the south shore, and if we consider this 
deep place as due to preglacial erosion, we ought to find an outlet 
channel. But no such outlet channel exists because the whole 
eastern end, at least, of the lake is certainly rock-rimmed. As 
Tarr has said: “There could hardly be a valley over 700 feet ~ 
deep and broad enough to form:the continuation of the preglacial 
Ontario valley, which is so completely obscured by drift that not 
the least trace of it has been found on the surface.’* To assume 
that this deep part of the basin was produced by warping of the 
land is not borne out by examining the exposed strata on all sides. 
It therefore seems quite certain that the preglacial Ontario depres- 
sion was here considerably deepened by ice erosion. The conditions 
were very favorable for such erosion because the rocks were chiefly 
soft Ordovicic shales; because the ice flowed through a deep pre- 
glacial valley; and because there was unusual crowding of ice into 
this valley due to the pronouiiced deflection of a great ice current 
around the Adirondacks on the west side. Strong arguments might 
be adduced to show that by ice erosion, portions, at least, of all the 
1 Tarr’s Physical Geography of New York State, p. 235. 
