8 



THE ICE AGE IN NORTH AMERICA. 



An active glacier is also characterized by fissures. When- 

 ever the ice-stream reaches a point where its slope is increased 

 even by a very small amonnt (a change in inclination of two 

 degrees being sufficient), the ice instead of moving in a con- 

 tinuous stream, forms crevasses across the current, which 

 gradually enlarge at the top, until they present a series of 

 long chasms, very difficult for the explorer to traverse. 

 AVhere there is considerable irregularity in the bottom, and 

 the increased slope extends for some distance, these crevasses 

 become very complicated, and the surface presents an ex- 

 panse of towers and domes and pinnacles of ice, often of fan- 

 tastic appearance; 

 c but at the bottom 



these masses are 

 still joined, and on 

 coming down to. a 

 gentler slope they 

 close up again at 



Fig. 5.— c, c, show fissures and seracs where the glacier moves the SHrfaCe for 

 down the steeper portion of its incline ; s, s, show the 



vertical structure produced by pressure on the gentler their Onward 



slopes. 



march. 

 In addition to the crevasses or fissures, produced by the 

 tension where the ice-stream passes over a steeper incline, a 

 set of marginal iissures extend from the sides of the glacier 

 toward the center, bnt pointing upward at an angle of about 

 forty-five degrees. These, 

 too, appear to be the result 

 of tension. The motion of 

 the ice in the center, being 

 more rapid than that to- 

 ward the sides, produces a 

 line of tension, or strain, ex- 

 tending from the center di- 

 agonally downward toward figs. 6, 

 the sides at an angle of 

 forty-five degrees. The pressure upon these masses of ice. 

 whose central point is being wheeled downward by the differ 



t 



Fig. 



Fig. 7. 



Illustrate the formation of margin- 

 al fissures and veins. 



