268 Dr. J. W. Spencer — Subsidence versus Glacial Dams. 



Eussel makes similar reports. The out-flowing waters enlarge the 

 tunnels, thereby draining lakes ; but the ice-roofs fall in, and by 

 the accumulation of ice blocks the tunnel becomes temporarily 

 obstructed, causing the water of the lakes to rise. In the very 

 nature of the case, large lakes could not be expected, for the 

 conditions which would permit their formation would cause the 

 glaciers to recede. Especially would this be the case if the glaciers 

 were hundreds of feet above the sea, with rivers draining beneath 

 or through them. It would be difficult to perceive how any water- 

 level could be maintained long enough to permit the waves to carve 

 out terraces and sea-cliffs. With glaciers coming down into the sea, 

 it is easy to understand how bays and inlets could be obstructed by 

 the ice so as to allow the water to be freshened. In such lakes the 

 water-level would be maintained long enough to leave inscriptions 

 in the form of terraces and beaches. 



Such is a brief account of the natural history of glacial dams. 

 It has been said that the easiest explanation of the theory of our 

 great lakes is by regarding them as formerly great glacial dams. 

 So it was thought ten 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 pre-judgment, has shown that glaciers did not scoop out 

 the basins, and has otherwise found satisfactory explanation of their 

 origin,' without invoking the necessity of ice being converted into 

 rock diggers. So also the evidence of glacial dams has not been 

 found as far as my observations have been extended. 



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

 lines already described. 



The physical featui-es of the Ontario basin are the most favour- 

 able for the construction 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 birthplace 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 

 north-east. Such a barrier would have been from 80 to 100 miles 

 wide, and from 800 to 1300 feet deep (below surface of water) 

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

 over 300 miles long (as 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, 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 



1 "Origin of the Basin of the Great Lakes of America," by J. W. Spencer, 

 Quart. Journ. Geol. See. Lend. vol. xlvi. p. 523. 



