THE DATE OF THE GLACIAL PERIOD. 



133 



amount of sedimentation which has taken place in lakes 

 and kettle-holes. We will consider first the evidence 

 from erosion. 



The gorge below Niagara Falls affords an important 

 chronometer for measuring the time which has elapsed 



100 '50 



THOUSAND YEARS AGO FROM 



Fig. 103.— Diagram of eccentricity and precession : Absciss represents time and 

 ordinates, degrees of eccentricity and also of cold. The dark and light 

 shades show^the warmer and colder winters, and therefore indicate each 

 10,500 years, the whole representing a period of 300,000 years. 



since a certain stage in the recession of the great North 

 American ice-sheet. As already shown, the present Niag- 

 ara River is purely a post-glacial line of drainage ; * the 

 preglacial outlet to Lake Erie having been filled up by 

 glacial deposits, so that, on the recession of the ice, the 

 lowest level between Lake Erie and Lake Ontario was in 

 the line of the trough of the present outlet. But, from 

 w T hat has already been said, it also appears that the Niagara 

 River did not begin to flow until considerably after the 

 ice-front had withdrawn from the escarpment at Queens- 

 ton, where- the river now emerges from its canon to the 

 low shelf which borders Lake Ontario. For a considerable 

 period afterwards the ice continued to block up the east- 

 erly and northerly outlets through the valleys of the 



23 



* See above, p. 200 et seq. 



