THE WORK OF SNOW AND ICE 



231 



time. 1 These changes may be brought about without visible frac- 

 ture, and have been thought to point to a viscous condition of the 

 ice. There is much reason, however, to question this interpretation. 

 Whatever the real nature of the movement, the aggregate result of 

 the movement in a field of ice is comparable, in a superficial way at 

 least, to that which would be brought about 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. 



H A 



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 cover- 

 ing the larger part of Greenland 

 (Fig. 186) is a good example. 

 The glaciers on some of the flat- 

 topped peninsular promontories 

 of the same island are good 

 examples of small ice-caps (Fig. 

 187). If ice-caps cover a large 

 part of a continent, as some of 

 those of the past have done, 

 they are called continental gla- 

 ciers. 



Where ice-caps lie on pla- 



Fig. 187. Ice-caps of small size. The 

 figure also shows some valley gla- 

 ciers extending out from the main 

 ice-sheet and from the local ice- 

 caps. A portion of the North 

 Greenland coast, north of Ingle- 

 field Gulf. Lat. about 78. 



teaus whose borders are trenched 



by valleys, ice-tongues from 



the edge of the ice-cap may 



extend down the valleys, and constitute one type of valley glacier. 



A second and more familiar type of valley glacier occupies mountain 



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

 Am. Jour. Sci., Vols. V, 1873, p. 305, and XXXIV, 1887, p. 149, and Nature, 

 Vol. XXXIX, p. 203. 



