TIME ESTIMATES AND CLIMATE 365 



The usual estimates of the time since the retreat of the ice allowed 

 Niagara to begin its work, 7,000 to 35,000 years, indicate very different 

 rates of deformation. If we allow one-half the time since the beginning 

 of Niagara for the lifetime of lake Iroquois, and this is clearly too high 

 an estimate, the tilting during the 3,500 years would progress at the rate 

 of 1 foot in 14 or at most 25 years. Taking the other extreme of 17,500 

 years, the rate of elevation would be from 1 foot in 70 to 1 foot in 125 

 years. For the minimum total deformation of 500 feet the time allowed 

 per foot would be from 14 to 70 years. Whether such a rapid rate of 

 uplift as this should be assumed or whether the usual time limits for the 

 work of Niagara falls should be extended is a question into which we 

 need not enter here, but the lower limit of 7,000 years has always seemed 

 to me quite insufficient for the formation of the Iroquois beach, of the 

 thick marine post-Glacial deposits of eastern Ontario, and of the present 

 mature shore forms of lake Ontario. 



It is now many thousand years since the Labradorean ice-sheet began 

 to be removed from this region, and it is a considerable number of thou- 

 sands of years since it totally disappeared. If the removal of the load 

 of ice occasioned the elevation of the region, which is highly probable, 

 we ma}' suppose that the rate of elevation was formerly more rapid and 

 is now slowing down before ceasing altogether when equilibrium is 

 reached. During Iroquois times, although the load had vanished from 

 the southwestern part of the region, some hundreds of feet of ice still 

 blocked the Saint Lawrence, but thousands of feet must have been re- 

 moved already from the region to the northeast, giving a sufficient cause 

 for rapid elevation in that direction. 



Climate and glacial Relationships 



All the conclusions reached in this paper point toward the presence 

 of a dam of ice across the northeastern end of the Iroquois basin from 

 the neighborhood of Havelock to a point on the Adirondacks somewhat 

 northeast of Watertown, New York, the shore of ice having a length of 

 more than 100 miles and a thickness of not less than 500 feet where the 

 Saint Lawrence now passes out of lake Ontario. The exact position of 

 this ice barrier is uncertain, but the finding of the lowest, and therefore 

 latest, beach near Trent Bridge, southwest of Havelock, and not the 

 higher, earlier ones, suggests that the ice occupied that point in the 

 earlier stages of lake Iroquois, but had retired from it before the end. 

 Probably careful search may yet disclose glacial deposits in connection 

 with one or other of the beaches into which the Iroquois is split up in 

 that region, but this has not yet happened, so that we are reduced to 

 inference in settling the position of the ice-front. 



