238 GEOLOGY 



used to mean merely the more solid portion outside (below) the 

 snow-field. 



Movement. The advance of a glacier is too slow, as a rule, to 

 be seen from day to day, and must be detected in other ways. If 

 its end advances, it may override or overturn objects in front of 

 it, or it may move out over ground previously unoccupied. But 

 even when the end of a glacier is not advancing, the movement of 

 the ice may be established by means of stakes or other marks set 

 on its surface. If the positions of these marks, relative to fixed 

 points on the sides of the valley, is determined, they are found 

 after a time to have moved down the valley. Rows of stakes or 

 lines of stones set across a glacier in its upper, middle and lower 

 portions have revealed many facts concerning the movement of 

 the ice. 



Generally speaking, the middle of a valley glacier moves more 

 rapidly than its sides, and its top more rapidly than its bottom. 

 In Switzerland, where the glaciers have been studied more care- 

 fully than elsewhere, the determined rates of movement range from 

 one or two inches to four feet or more per day. Some of the larger 

 glaciers in other regions move more rapidly, but it does not follow 

 that large glaciers always move faster than small ones. The Muir 

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

 and some of the glaciers of Greenland have been found to 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 SIM, 

 and then only in the summer. In the case of the glacier with the 

 highest recorded rate of summer movement, 100 feet per day, (lie 

 advance was only 34 feet at about the same place 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 appears to depend on (1) the depth of the moving i'-e. 

 (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) tem- 

 perature, and (6) the amount of water in the ice. Great thicki 



1 Reid. Natl. Geog. Mag., Vol. IV, p. 44. 



