152 



PHYSICAL GEOLOGY 



the walls and bed of the valley through which the glacier flows. It 

 follows from the above that the rate of movement will be reduced 

 if the bed of the glacier is rough, and that a smooth bed will favor 

 rapid motion. 



It has been found that the line of swiftest motion is not always in 

 the middle of glaciers, but that as in the case of rivers, although to a 

 lesser degree, it is deflected from side to side, being nearer the outside 

 of a curve. 



Factors Influencing the Rate of Movement. — The rate of move- 

 ment of a glacier increases with (i) the slope of the bed upon which it 

 rests, but depends even more upon (2) the slope of the upper surface 

 of the ice and upon (3) its thickness. A general inclination of the 

 upper surface of a glacier is necessary for glacial movement, although 

 for short stretches the surface of the ice may even have a backward 

 slope. The beds of valley glaciers slope in the general direction of the 

 movement of the ice, but there are many local exceptions, as is shown 

 by the deep basins in valleys formerly occupied by glaciers. The great 

 ice sheets of North America moved to the south over a land surface 

 which for many miles sloped towards the north, i.e., in the direction 

 opposite to that of the movement of the ice. In all such cases the 

 upper surface of the ice must have sloped in the direction of the 

 glacial movement. 



The velocity of a glacier is greater in summer than in winter, and 

 at midday than at night; that is, when the glacier is melting more 

 rapidly and is most thoroughly saturated with water. The Mer de 

 Glace, France, moves in summer at an average rate of 27 inches a 

 day in the middle and 13 to 19 inches a day near the sides; in winter 

 the rate is about half as much. Other factors influencing the rate 

 of motion of a glacier, besides the slope of the ground, the slope of its 

 upper surface, and the quantity of water with which the ice is sat- 

 urated, are the amount of load in its basal portions, which tends to 

 retard the rate, and the straightness of its course and the smoothness 

 of its bed, which tend to increase it. 



Lower Limit of Glaciers. — Glaciers move down their valleys until 

 they reach a point where the melting (ablation) equals the forward 

 movement (Fig. 137). When the melting exceeds the forward move- 

 ment, the glacier is said to retreat; when it is less the glacier advances. 

 It is evident that the lower limit of a glacier will not be fixed (except 

 when it reaches the sea) unless the conditions of temperature and 

 mowfall remain constant. Since both temperature and snowfa" 



