472 J. W. SPENCER — PLEISTOCENE SUBMERGENCE. 



years ago that the least troublesome hypothesis of the origin of the Great 

 Lake basins was by their excavation by glaciers ; but the writer, going into 

 a field of investigation almost sealed by prejudgment, has shown that gla- 

 ciers did not scoop out the basins, and has otherwise found satisfactory ex- 

 planation of their origin* without invoking the necessity of ice being con- 

 verted into rock-diggers. So, also, the evidence of glacial dams has not been 

 found, so far as my investigations have extended. 



Let us examine how the glacial-dam theory applies to the shore-lines 

 already described. 



The physical features of the Outario basin are the most favorable for the 

 constructions of a great lake retained by glacial dams. As proved by its 

 deformation, the Iroquois beach was formed at sea-level. If this proof of 

 the altitude of its birth-place did not exist, the evidence of its elevation 

 would be obtained from a consideration of the ability of glaciers to close the 

 St. Lawrence valley to the northeast. Such a barrier would have been from 

 60 to 100 miles wide and from 800 to 1,300 feet deep (below surface of 

 water), according to location. Yet the drainage of the then expanded lake, 

 over 300 miles long (so far as surveyed) and 100 miles or more in width, 

 was against, into, or under the supposed glaciers, except to a limited extent 

 in its earliest stages, when a partial overflow was by the Mohawk valley. 

 Had the lake been above sea-level, a river as large as the St. Lawrence 

 would soon have eaten its way through the ice and lowered the lake, for in 

 that direction alone it had to flow; consequently, it appears that the great 

 cut terraces and beaches, requiring centuries or millenniums of time, could 

 not have been formed except at sea-level. 



If the Algonquin beach of the upper lakes were formed in a glacial lake, 

 then the ice barrier in the region of Lake Nipissing would have reached 600 

 or 700 feet beneath the surface of the water. The drainage must have been 

 under the ice, and have amounted to a discharge equal to that of the modern 

 Detroit river, as the discharge of Lake Superior, Lake Michigan and Lake 

 Huron basins would have been thus borne seaward, descending 300 feet to 

 the level of the Iroquois water. Under such conditions, the question may 

 be asked. How could the lake surface be retained long enough at any level 

 to carve out the deeply graven water lines and terrace plains of the Algon- 

 quin beach, in place of the discharging waters melting away the icy barriers, 

 which were supposed to have been the means of retaining the lake 300 feet 

 above the level of the Iroquois waters ? 



We now rise to the shores which bounded the Warren water. These I 

 have explored from Lake Michigan to New York, and to northeast of To- 

 ronto, upon the Ontario peninsula. Upon the glacial-dam theory, this sheet 



•"Origin of the Basins of the Great Lakes of America," by J. W. Spencer: Quart. Journ. Geol. 

 Soc, vol. XLVL1890, p. 523. 



