352 



W. UPHAM — GLACIAL ACCUMULATION AND INVASION. 



of general glacial recession are conspicuously hilly on their outer side, 

 they record some readvance, rather than a mere halt or a slackening^ in 

 the rate of retreat. 



The origin of continental ice-slieets by gradual accumulation from 

 snowfall upon nearly their entire areas, without extensive bodily ad- 

 vances of their borders, can be best referred, as the writer believes, to 

 high epeirogenic uplifting of the regions which became thus ice-clad. 

 These uplifts probably also affected, in a less degree, large areas outside 

 the limits of the ice-sheets and drift. The central parts of the glaciated 

 area of North America were apparently raised higher than its borders, 

 and in some instances streams formerly flowing northward may have 

 been then turned to the south, as Carll, Spencer, Chamberlin and Lev- 

 erett have shown for tributaries of the Allegheny and Ohio rivers. Dur- 

 ing the epoch of high elevation and the Ice age in which it culminated, 

 these streams and all others flowing from the uplifted area were cutting 

 down their channels at a much faster rate than during the Tertiary era 

 of lower altitude and less slope. As the ice-sheet grew in only compara- 

 tively slight degree by invasion, it was not probably an efficient agency 

 for the formation of glacial lakes and deflection of rivers from their pres- 

 ent courses during the oncoming of the Glacial period. With the first 

 envelopment of the country by snow and ice, a continuall}^ cold climate 

 appears to have begun and thence to have lasted through the time of the 

 ice accumulation, excepting marginal fluctuations of probably not very 

 great extent, either in area or time, as compared with the ice-covered 

 area and the duration of the Ice age. 



