DEPTH OF THE ICE DURING THE GLACIAL PERIOD. 199 



miles. The difference in the elevation of the glacier could 

 not have exceeded a thousand feet. In that direction the 

 slope was less than on a meridian line from the Catskills 

 southward." 



Professor Dana estimates that the height of the ice 

 " above the region of Xew Haven, in southern Connecticut, 

 may have exceeded two thousand feet, and could hardly have 

 been less than fifteen hundred." 



So far the evidence is direct and positive, because the 

 glacial marks are left upon the mountain-summits mentioned. 

 How far still above these summits it rose is not so easily 

 determined. From this amount of direct evidence it may 

 also reasonably be inferred that the depth of the ice over the 

 lake and prairie region of the West was equally great. If 

 our interpretation of the facts implying the presence of an 

 ice-sheet in Xorth America is correct, we have also positive 

 evidence of a great depth of ice over the central portion of 

 British America between Hudson Bay and the Rocky Mount- 

 ains. Here we find, according to Dawson, that bowlders 

 from the Lam-entian axis of the continent, which stretches 

 from Lake Superior northward to the west of Hudson Bay, 

 have been transported westward a distance of seven hundred 

 miles, and left upon the flanks of the Rocky Mountains at 

 an elevation of something over four thousand feet* But 

 nowhere does the Lauren tian axis reach two thousand feet, 

 its average elevation, according to Sir Wilham Logan, being 

 from fifteen to sixteen hundred feet. If these bowlders 

 were, as we suppose, transported by glacial ice, then the ice 

 must have accumulated over the Laurentian axis to a deptli 

 of 2,400 or 2,500 feet, and must have been several hundred 

 feet deeper in the central part of the Red River Valley. 

 Mr. R. G. McConnell. of the Canadian Survey, also, from 

 more direct evidence, estimates f that, in the plains surround- 

 ing the Cypress Hills (in the upper valley of the South 

 Saskatchewan, in latitude 49° 30', longitude 110°, and not 



* See p. 214. f See his " Report on the Cypress Hill? Wood Mountain.'' 



