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NEW YORK STATE MUSEUM 



Originally the two localities were at the same sea level. If the 

 tilting took place at the rate of .42 of a foot in 100 miles, in 26 

 miles it would take place at the rate of .1092 feet in 100 years, 

 and as the Montreal station is 100 feet above that at Covey hill 

 we obtain the ratios (neglecting the third and fourth decimals) : 

 as .1 foot is to 100 years, so is 100 feet to the time required to 

 elevate the Mt Royal station 100 feet above the Covey hill station. 

 Solving this simple proportion we obtain 100,000 years as the time 

 required. At this rate the land must have risen at the rate of .55 

 feet (6-i fi - inches) a century at Montreal and .45 feet (5^ inches) 

 a century at the international boundary on the north and south 

 line passing through the Champlain and Hudson valleys. 



This estimate of 100,000 years is for the time since the highest 

 marine beach was level. This highest beach marks approximately 

 the time of disappearance of the ice sheet from the St Lawrence 

 valley so as to permit the free incursion of the sea. If the assump- 

 tion used in this calculation were right it would follow that the 

 Laurentide ice sheet disappeared from the St Lawrence valley as 

 long as 100,000 years ago. 



It may be, though, that the rate of tilting in the northsouth 

 direction through the Champlain-Hudson valleys is now and has 

 been at a more rapid rate than that ascertained for the Great Lake 

 district on a south-southwest line. If we assume the rise at Mon- 

 treal has been at a rate five times as fast as that inferred above 

 or 2.75 feet a century then it has required 20,000 years to effect 

 the change. It does not seem possible that the rate of uplift at 

 Mt Royal could have been on the average more rapid than 2.75 

 feet in 100 years and there is no reason to assume that it was 

 slower than the rate of tilting now observable in the Great Lakes 

 district. The disappearance of the ice sheet from the low grounds 

 about the northern open mouth of the Lake Champlain valley may 

 be said therefore with some probability to have taken place not 

 less than 20,000 years ago and not longer than 100,000 years ago. 

 That it was somewhere between these limits is more probable than 

 that it was either 20,000 years or else 100,000 years ago. 



There is yet another method which though equally interesting 

 is not more trustworthy perhaps. By reference to the diagram, 

 plate 28, it will be noted that on the assumption of essential 



