342 HISTORICAL GEOLOGY 



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 from the Paleozoic 

 to the Cenozoic, no lakes of any consequence could have persisted. 

 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 post- 

 Glacial origin it is, then, proper to ask how they came into existence. 

 During pre-Glacial time 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 pre-Glacial 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 

 notable subsidence of the region since pre-Glacial time. The 

 surface of Lake Erie is 573 feet, and its deepest point 369 feet, 

 above sea level, while the surface of Lake Ontario is 247 feet above, 

 and its deepest point is 491 feet below, sea level. The greatest 

 depth (738 feet) of Lake Ontario is well toward the east end not 

 far from the south shore, and if we consider this deep place as due 

 to pre-Glacial erosion, we ought to find an outlet channel. But 

 no such outlet channel exists because the whole east end, at least, 

 of the lake is 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 pre-Glacial Ontario Valley, which is so com- 

 pletely obscured by drift that not the least trace of it has been 

 found on the surface." x To assume that this deep part of the 

 basin was formed by warping of the land is not borne out by exam- 

 ining the exposed strata on all sides. It therefore seems quite 

 certain that the pre-Glacial Ontario depression was considerably 

 deepened by ice erosion. The conditions were very favorable for 

 such erosion because the rocks were chiefly soft Ordovician shales; 

 because the ice flowed through a deep pre-Glacial valley; and 

 because there was an unusual crowding of the ice into this valley 

 due to pronounced deflection of a great ice current around the 

 Adirondacks on the west side. Strong arguments might be adduced 

 1 R. S. Tarr: Physical Geography of New York State, p. 235. 



