THE DATE OF THE GLACIAL PERIOD. 537 



the average rate. There is some evidence that a part of the 

 work above the Whirlpool had been done by a local stream 



Fig. 133.— Section of strata along the Niagara gorge from the falls to the lake. 1, 3, strata 

 of hard rock ; 2, 4, of soft rock. 



which formerly passed from the Whirlpool westward to St. 

 Davids, since there is no doubt of the existence of a filled-up 

 preglacial channel running from the Whirlpool to St. Dav- 

 ids. But the evidence that this channel extended above the 

 Whirlpool toward the present cataract is so imperfect that we 

 must leave it out of the question, and take the whole length 

 of the gorge from Queenston to Niagara as our dividend. 

 The problem remaining is to find the rate of recession, and 

 this will serve as a divisor. 



The comparative youth of the Niagara gorge is evident 

 from the present condition of its mouth at Queenston. This 

 is narrow, and its walls abrupt ; but it is well known that, by 

 the inevitable action of natural forces, the mouth of a river- 

 gorge must become, in process of time, very much enlarged, 

 since from the beginning its sides have been exposed to 

 the eroding action of the elements and to the undermin- 

 ing action of the river. In the unglaciated region the 

 months of such gorges are universally wide and Y-shaped, 

 and the banks much obscured at the bottom by the accumu- 

 lation of debris. In this respect the mouth of the present 

 gorge at Queenston is in striking contrast with that of the 

 old gorge which opened at St. Davids. As will be seen by 

 reference to the map, this, though narrower where it left 

 the Whirlpool than the present main gorge below the Whirl- 



