536 THE ICE AGE W NORTH AMERICA. 



confidently look for light : 1. The amount of erosion and 

 disintegration which has occurred since the Glacial period, 

 2. The extent to which lakes and kettle-holes have been filled 

 with sediment. 3. The apparent freshness of organic re- 

 mains in glacial deposits. 



Beginning with the extent of erosion which has taken 

 place since the withdrawal of the ice, our attention is, natu- 

 rally enough, directed first to the gorge below the Falls of 

 Niagara. How this comes to be a glacial chronometer has 

 already been seen.* The old outlet of Lake Erie, which 

 must have existed as the result of preglacial erosion, was 

 filled up during the great Ice age, and the Niagara River is 

 the outlet of the pond thus created. Originally the water 

 plunged over the escarpment at Queenston, about seven 

 miles below the present cataract. This escarpment is formed 

 by an outcrop of a thick deposit of Niagara limestone, the 

 summit of which is about three hundred feet above the level 

 of Lake Ontario. This is underlaid by a softer rock, which 

 is more rapidly disintegrated than the upper strata ; hence 

 the upper strata always project beyond the lower, and every- 

 thing favors the continuance of the cataract at the head 

 of the gorge as it wears back. The problem is to de- 

 termine how long the Niagara River has been in wearing 

 out the gorge between Queenston and the present cata- 

 ract. The solution of that problem will furnish an answer 

 to the other question, How long has it been since the ice- 

 barrier across the valley of the Mohawk was removed, so 

 that the dammed-up waters of Lake Ontario and Lake 

 Erie would subside sufficiently to permit the formation of 

 the cataract at Queenston and the commencement of erosion 

 in the present gorge ? The problem is comparatively simple, 

 and its bearing upon the date of the Glacial period is clear. 



The Falls of Niagara have receded about seven miles. 

 The conditions have been from the first so nearly uniform 

 that the present rate of erosion can not differ largely from 



* See chapter xii, p. 303 et seq. 



