GLACIERS 



127 



thought (n point to a viscous condition of the ice. There is much 

 reason, however, to question this interpretation. Whatever the 

 real nature of the movement, its aggregate result in a field of ice is 

 comparable, in a superficial way at least, to that which would occur 

 if the ice were capable of moving like a viscous liquid, the motion 

 taking place with extreme slowness. This slow motion of ice in 

 an ice-field is glacier motion, and ice thus moving is glacier ice. 

 The cause of movement is gravity, which tends to bring the ice to 

 lower levels, just as it tends to bring water, in similar positions, to 

 lower levels. 



GLACIERS 



Types. The different shapes of glaciers have given rise to differ- 

 ent names. If the surface on which the ice-sheet develops is plane, 

 the ice will move outward in all directions, and ice spreading in 

 all directions from a center is an ice-cap. The glacier covering the 

 larger part of Greenland (Fig. 128) is a good example. The glaciers 

 on some of the flat-topped 

 peninsular promontories of the 

 same island are examples of 

 small ice-caps (Fig. 129). If 

 ice-caps cover a large part of a 

 continent, as some of those of 

 the past have done, they are 

 called continental glaciers. 



Where ice-caps lie on pla- 

 teaus whose borders are dis- 

 sected by valleys, tongues of 

 ice from the ice-cap may ex- 

 tend down the valleys. They 

 constitute one type of valley 

 glacier. A second and more 

 familiar type of valley glacier 

 occupies mountain valleys, and 

 is the offspring of mountain snow-fields. The former type, confined 

 chiefly to high latitudes, are polar or high-latitude glaciers (Fig. 130); 

 the latter are <///>/;/<: glaciers (Figs. 131, 132). The distinctive feat- 

 ure of high-latitude glaciers is their steep slopes at sides and ends. 



1 For an account of experiments illustrating the mobility of ice see Aitkin, 

 Am. Jour. Sri., Vols. V, 1873, P- 3S and XXXIV, 1887, p. 149, and Nature, Vol. 

 X XXIX, p. 203. 



Fig. 1 29. Ice-caps of small size. The 

 figure also shows some valley glaciers 

 i-xu-nding out from the main ice-sheet 

 and from the local ice-caps. A portion 

 of the North Greenland coast, north of 

 Inglcfield Gulf. Lat. about 78. 



