262 GEOLOGY. 



average. The periods of advance and retreat lag behind the periods 

 of heavy and hght snowfall respectively, by some years, and a long 

 glacier responds less promptly than a short one. Present knowledge 

 seems to point to a period of 35 to 40 years as the time within which a 

 cycle of fluctuation, that is, an advance and a retreat, takes place. 



A declining upper surface is essential to glacier motion. There 

 are short stretches where this is not the case, and indeed there are par- 

 ticular places where the upper surface slopes backward.* This may 

 occur where the ice is pushed up over a swell in its bed, or is crowded 

 up against any considerable obstacle ; but such cases are no more than 

 local exceptions, and do not militate against the truth of the general 

 statement that the upper surface of a glacier declines in the direction 

 of motion. A declining lower surface is less necessary. In the case of 

 valley glaciers, the bed does, as a rule, decline in the direction of motion, 

 but that there are local exceptions is shown by the deep basins in rock 

 which such glaciers often leave behind them when they retreat. In the 

 great continental glaciers of recent geologic times, the ice frequently 

 moved up slopes for scores, and even hundreds of miles; but in all such 

 cases, the upper surface must have declined in the direction of move- 

 ment. With a given thickness of ice, the greater the decline of its 

 lower surface in the direction of motion, the more rapid its progress. 

 A rough bed, or a crooked course retards the motion of a glacier, 

 while a smooth bottom and a straight course facilitate it. 



Slope, roughness of bed, and volume affect the movement of glaciers 

 somewhat as they affect the movement of rivers. The temperature 

 of the water, on the other hand, has little effect on the flow of a river 

 so long as it remains unfrozen; but the effect of temperature on the 

 motion of ice is most important. In many cases, indeed, the temper- 

 ature, together with the water that is incidental to it, seems to be the 

 chief factor in determining the rate of movement. The way in which 

 its effects are felt will be discussed later. 



Likenesses and unlikenesses of glaciers and rivers. — Many of the 

 characteristics of a valley glacier may be understood from the study of 

 the accompanying figure (Fig. 237) of the White (Alaska) glacier. 

 From this figure it will be seen that the glacier is an elongate river- 

 like body, following the curves of the valley in stream-like fashion. It 



* For example, in the Middle Blase Dale glacier, Island of Disco, Jour, of Geo!., 

 Vol. II, p. 784, and in the Bowdoin glacier (Fig. 242). 



