THE DATE OF THE GLACIAL PERIOD. 549 



the height of the cliff at this point. If the subaerial erosion 

 of the face of the gorge proceeded at the rate of one-quarter 

 of an inch per annum, the material would have been removed 

 in less than 10,000 years. That this rate is not excessive 

 was shown both from the vast amount of debris that is now 

 annually precipitated upon the railroad track, and from actual 

 measurements of the extent to which the hard strata of lime- 

 stone had been undermined since the track was laid, in 1854, 

 when it was found that the underlying Clinton and Niagara 

 shales had worn away more than three inches a year, leaving 

 the harder strata to project from thirteen to fourteen feet. 

 As illustrating the rapidity of erosion from the sides of the 

 gorge it is in point to remark that in 189S there fell off at one 

 time from the face of the cliff on the east side of the Whirlpool, 

 100,000 tons of rock, whereas the amount which we have 

 supposed to fall away annually from the one mile and a half 

 measured by us is only 1,237 tons. From these facts it is 

 evident at once that the erosive agencies tending to give a 

 V-shape to the mouth of the gorge could not have been in 

 operation much more than 10,000 years. To suppose they 

 had been at work for 30,000 or 40,000 years, as many still 

 try to do, involves an absurdly low rate of activity on the part 

 of the forces which have been constantly at work. 



Something more also needs to be said about the significance 

 of the preglacial channel leading from the Whirlpool to St. 

 Davids. In the first place it should be noted that the mouth 

 of this gorge is very wide, being in fact nearly a mile in width, 

 thus indicating great age. In the second place, the depth of 

 the Whirlpool (150 feet), and the width of the head of the St. 

 Davids gorge (fully twice that of the Niagara gorge immedi- 

 ately above and below), point to an extreme age. It seems 

 altogether probable, indeed almost demonstrable, that the 

 St .Davids' gorge had been worn back by a small stream formed 

 by the junction of two streams, one coming along the line of 

 the present gorge through the Whirlpool Rapids, and the 



