MOVEMENT OF GLACIERS 133 



in other regions move more rapidly, hut it does not follow that 

 lar-je uluriers always move faster than small ones. The Muir 

 ^larier of Alaska has been found to move some seven feet per day, 1 

 and some of the glaciers of Greenland move, in the summer time, 

 50 or 60 feet per day; but these rates have been observed only where 

 the ice of a large inland area crowds down into a comparatively 

 narrow fiord, and debouches into the sea, and there only in the 

 summer. In the case of the glacier with the highest recorded 

 summer rate of movement (100 feet per day), the advance was 

 only ,^4 feet a day in April. The average movement of the border 

 of the inland ice of Greenland is very small, probably less than a 

 foot a week. 



Conditions affecting rate of movement. The rate of glacier 

 movement depends on (i) the depth of the moving ice, (2) the slope 

 of the surface over which it moves, (3) the slope of the upper surface 

 of the ice, (4) the topography of its bed, (5) the temperature of the 

 ice, and (6) the amount of water it contains. Great thickness, steep 

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

 ance of water, favor rapid movement. Since some of these condi- 

 tions, notably temperature and amount of water, vary with the 

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

 Other conditions vary through longer periods of time, and cause 

 corresponding variations in the rate of movement. 



A sloping upper surface is essential to glacier motion, and the 

 motion is down-slope. There are short stretches where this is not 

 the case; indeed there are places where the upper surface declines 

 away from the direction of motion, as where the ice pushes 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 a valley glacier, the 

 bed does, as a rule, decline in the direction of motion ; but the deep 

 basins in rock which many such glaciers leave behind them when 

 they retreat, show that the bottom of a valley glacier does not slope 

 downward at all points. In the great continental glaciers of recent 

 geologic times, the ice moved up slopes for scores, and even hundreds 

 of miles; but in all such cases, the prevailing slope of the upper sur- 

 face was down in the direction of movement. 



Fluctuations of glaciers. The lower ends of glaciers advance 



1 Reid. Xatl. ( ;..>. Mai:.. Vol. IV, p. 44. 



