Birth of Lake Ontario. 451 



at the narrowest point, it would need to be 50 or 60 miles wide. 

 If it bad no greater depth than the water north of Fine used to 

 have, the ice would need to be thick enough to fill a channel 

 of 800 feet. But as the differential uplift probably continues 

 throughout the Adirondack region, we would need to be pre- 

 pared to accept a clam of at least 1300 feet in thickness, and a 

 hundred miles across. Apparent beaches in Vermont at 2100 

 feet above the sea (Hitchcock),* and the Post-Pleistocene emer- 

 gence of Mt. Desert, observed in the coastal markings to its 

 summit of 1500 feet (Shaler),f increase the probability of our 

 regional uplift continuing throughout the Adirondacks. 



Any water proof dam in front of the Iroquois Beach would 

 have had to endure throughout the long period of its forma- 

 tion. But all known glacial dams are small and evanescent. 

 Yet the one suggested as closing up the Ontario's basin would 

 have had to restrain a greater sheet of open water than that of 

 modern Lake Ontario, receiving not merely the waters of the 

 then upper lakes, but also those of the melting of the hypothe- 

 cated glacial dam. It is questionable what thickness of ice 

 would hold in the waters, for the modern glacial dams of Mt. 

 St. Elias discharge beneath 500 feet of ice for a distance of 

 eight miles. £ As soon as the waters fell below the Mohawk 

 outlet, the discharge of the glacial lake ought to have melted 

 and lowered the ice on the one side and carved out terraces on 

 the other, unless the river were 5U to 100 miles wide. And there 

 are terraces upon the northern side of the Ottawa valley, as 

 well as upon the flanks of the Adirondacks. 



There seem to me to be no phenomena in the later lake 

 history of Ontario necessitating the existence of a dam across 

 the St. Lawrence valley. In short, the Iroquois water was a 

 gulf. The Adirondacks and New England formed great 

 islands. The Iroquois episode commenced almost synchronous 

 with the birth of the Niagara Falls. And the history of Lake 

 Ontario records interesting and great changes which now form 

 a simple story. 



* Geology of Vermont. 



f Geology of Mt. Desert. Eighth Annual Report of U. S. Geol. Survey. 



% Harold Topham in Proc. Roy. Geog. Soc, 1889, p. 424. 



