THE WORK OF SNOW AND ICE 239 



steep slopes, smoothness of bed, a high (for ice) temperature, and 

 abundance of water favor rapid movement. Since some of these 

 conditions, notably temperature and amount of water, vary with 

 the season, the rate of movement of a glacier varies during the year. 

 Other conditions, especially the first of those mentioned above, 

 vary through longer periods of time, and cause corresponding 

 variations in the rate of movement. 



A declining upper surface is essential to glacier motion. There 

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

 occasional places where the upper surface slopes backward, as 

 where the ice is pushed up over a swell in its bed; but such cases 

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

 of the 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 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 re- 

 cent geologic times, the ice frequently moved up slopes for scores, 

 and even hundreds of miles; but in all such cases, the prevailing 

 slope of the upper surface must have been down in the direction of 

 movement. 



Fluctuations of glaciers. Observation has shown that the lower 

 ends of glaciers advance and retreat at intervals, 1 and that the 

 periods of advance follow a succession of years when the snowfall 

 has been heavy and the temperature low, while the periods of 

 retreat follow a succession of years when the snowfall has been 

 light and the temperature above the average. The periods of 

 advance and retreat lag behind the periods of heavy and light 

 snowfall respectively, by some years, and a long glacier responds 

 less promptly than a short one. 



Likenesses and unlikenesses of glaciers and rivers. Slope, 

 roughness of bed, and volume affect the movement of glaciers some- 

 what as they affect the movement of rivers. The temperature of 



'Reid. Variations of Glaciers. Jour, of Geol., Vols. Ill, p. 278: V, p. 

 378; VI, p. 473; VII, p. 217; VIII, p. 154; IX, p. 250; X, p. 313; XI, p. 285; 

 XIII, p. 313; XIV, p 402; and XV, pp. 46 and 664. 



