THE WORK OF SNOW AND ICE. 261 



The average movement of the border of the inland ice of Green- 

 land is very small. Rink says that '^between 62° and 68° 30', the 

 edge of the inland ice is almost stationary for a remarkably long dis- 

 tance/' * The observations of the authors between 77° and 78° were 

 of like import. Probably the average movement of the border of the 

 Greenland ice-cap is less than one foot a week. 



Conditions affecting rate of movement. — The rate of glacier move- 

 ment appears to depend on (1) the depth of the moving ice; (2) the 

 slope of the surface over which it moves ; (3) the slope of the upper sur- 

 face of the ice; (4) the topography of the bed over which it passes; 

 (5) the temperature; and (6) the amount of water which falls upon 

 it or is carried to it by the drainage of its surroundings, in addition to 

 that produced by the melting of the glacier itself. Great thickness, 

 a steep slope, much water, smoothness of bed, and a high (for ice) tem- 

 perature favor rapid movement. Since some of these conditions, not- 

 ably temperature and amount of water, vary with the season, the rate 

 of movement for any given glacier is not constant throughout the year. 

 Other conditions, especially the first of those mentioned above, vary 

 through longer periods of time, and occasion periodic variations in the 

 rate of movement. 



Since the volume of ice concerned influences the rate of movement, 

 anything which changes the volume affects the rate. An excess of 

 snowfall with favorable conditions for its preservation for a period of 

 years, increases the volume of ice, and tends to accelerate its move- 

 ment. A deficiency in snowfall, or in its preservation, as from high 

 average temperature or from aridity, diminishes the quantity of ice, and 

 so retards the movement. An acceleration of velocity causes the ice to 

 move down the valley farther before being melted, that is, causes the 

 end of the glacier to advance, while a decrease of velocity produces 

 the opposite effect. As a matter of fact, the lower ends of glaciers 

 advance for a period of years and then retreat, to advance again at a 

 later time.^ Observation has shown that the periods of advance fol- 

 low 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 



^ Rink's Greenland. 



2Reid. 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, and X. p. 313. 



