326 A. p. COLEMAN THE LABRADOR ICE-SHEET 



morainic materials, its surface would have been 4,800 feet above sealevel 

 and 2,800 feet above the adjoining valleys, which are at about 2,000 feet. 

 Supposing that the ice filled the valleys to 4,800 feet, and that the moun- 

 tain masses occupied half the total volume below that level, the additional 

 load of ice would average 1,400 feet. One-third of this is about 470 feet, 

 while Fairchild^s map of isobases gives the old marine level in the Adiron- 

 dacks as about 600 feet. However, the group of highest peaks does not 

 occupy much space, and the ice over the rest of the region, with its much 

 lower mountains, would average thicker and thus supply the deficiency in 

 load equivalent to 130 feet of rock. 



I have had no opportunity to study the White Mountains with respect 

 to glaciation, but Goldthwait believes that the ice-sheet passed over them. 

 If so, it must have been far thicker than would be expected if the Shick- 

 shocks to the northeast and the Adirondacks to the west were not entirely 

 covered. Is it possible that local glaciers could account for the facts in 

 regard to the White Mountains ? 



Eelations to Isostasy 



It is of much interest to determine the limits within which inequalities 

 of load can be sustained by the earth's crust without adjustment by bend- 

 ing or faulting, and the Gaspe region may be considered from this point 

 of view. A belt of mountains 50 miles long by about 12 miles wide was 

 scarcely at all ice-covered and over double that lengih and breadth there 

 appear to have been only small local glaciers or ice-sheets, while in the 

 valleys to the south and north there was a thickness of from 1,250 to 

 2,300 feet. If there was perfect adjustment to the relief from load, the 

 borders of the peninsula should have risen from 400 to 800 feet while the 

 mountain axis remained at its old level. The uniformly graded chan- 

 nels of the rivers, which are undoubtedly preglacial, show that no such 

 differential adjustment occurred. The grade of Sainte Anne and Cap 

 Chat rivers to their forks, well within the mountains, is so uniform that 

 boats are poled all the way up with scarcely an interruption, their lower 

 course being almost as rapid as the upper part. This is even more mark- 

 edly the case on Cascapedia Eiver, flowing 45 miles south from the Shick- 

 shocks to the Bay of Chaleur. There is a fairly rapid current all the 

 way, but scows with several tons of freight are towed the whole way by 

 a team of horses. 



The excellent grading of the rivers and the almost complete absence of 

 lakes or floodplains where they enter the sea prove that the region rose 



