100 BELT ON THE GLACIAL PERIOD IN NORTH AMERICA. 



surface sufficient to carry it off bodily, as a glacier it would accumu- 

 late and be piled up until the higher portions slipped over the 

 lower. 



It has been suggested above that one of the most powerful causes 

 that brought on the glacial period, was the shutting off the gulf stream 

 and other warm currents from the arctic area, Avhere at the present 

 time they are ceaselessly employed in ameliorating the climate and 

 melting wp the ice at its source. They now eat into the very vitals 

 of the icy foe, which attacked and routed in the rear draws in its 

 forces to its inmost citadel. In the glacial period the breaches that 

 now let in the invading waters from the south, were closed, and the 

 icy hosts gathering in the north, pushed out their legions south- 

 wards, and drew their very sustenance from the forces that now beat 

 them back to their arctic fortress. 



Piled up then in the north the ice and snow would spread south- 

 wards. Where it encountered a hi^h. rans^e running transverselv 

 to its flow, it Avould at first be diverted from its course, but it would 

 gradually accumulate behmd the obstacle until it overtopped it, at 

 fij'st flowing through passes in the range and ultimately overwhelm- 

 ing the whole ridge. In advance of the great mass, streams of ice 

 would flow down pre-existing valleys and through passes in opposing 

 ranges, deepening, widening and glaciating them, like pioneers cut- 

 ting out roads for the main body. In some cases, passes through 

 chains of hills would determine the erosion of valleys in front of 

 them, by the ice that poured through from behind. 



This moving margin of the advancing ice would be the effective 

 tool in glaciating the country. To its action every portion of the 

 surface would be exposed, whether its slope conformed to the course 

 of the ice flow or was opposed to it ; just as every part of a coast 

 between tide marks is washed by the rising flood. And as the 

 waves run forward on the shore and retire, to again advance, and 

 again retreat, although the whole body of Avater is steadily rising, so 

 we may suppose that the ice margin might greatly advance during 

 a series of cold seasons, and retreat during warmer ones, to be again 

 thrust forward and a^'ain drawn back, althoui>'h during a yreat num- 

 ber of years the advance of the main body of ice A\"()uld can-y the 

 fluctuating margin flu- forward and subject a new zone to its action. 

 Thus the A\'hole continent from the arctic cij'clc to as far south as 



