250 



GEOLOGY. 



changes may be brought about without visible fracture, and have been 

 thought to point to a viscous condition of the ice. There is much rea- 

 son, however, as wih be seen later, to question this interpretation of 

 the ultimate nature of the movement. Whatever this may be, the 

 mass result of the movement in a field of ice is comparable, in a super- 

 ficial 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. 



Fig. 226. — A glacial lobe, midway between an ice-cap and a valley glacier. A pro- 

 trusion from a local ice-cap east of Cape York, Greenland. 



If both the surface on which the ice-sheet develops and its surround- 

 ings be essentially plane, as may happen in high latitudes, and if the 

 snow- and ice-field be symmetrical in shape, the outward movement 

 will be approximately equal in all directions, and the area covered by 

 the spreading ice-field will remain more or less circular. If the ice- 

 field rests on a steeply inchned surface, hke a mountain slope, the move- 

 ment becomes one-sided in conformity to the slope. If the surface, 

 otherwise plane, be affected by valleys parallel to the direction of move- 

 ment, the ice in the valleys will be deeper than that on the divides be- 

 tween them, and its movement stronger. In the valleys, therefore, 

 th^ ice will advance farther than elsewhere before being melted, and 

 the outhne of the ice will become lobate, the lobes occupying the depres- 

 .sions. These general relations are shown in Figs. 224 and 225. If the 

 depressions be wide and shallow, the lobes will be broad and short 



