THE WORK OF SNOW AND ICE 109 



and (2) that familiar objects near the ends of glaciers were some- 

 times overturned or pushed forward by the ice. 



Rate of movement. After the fact of movement was known, 

 means were devised for measuring its rate. Rows of stakes were 

 set across a glacier in a straight line (a, Fig. 112). 

 After weeks or months they were found to have 

 moved down the valley, and in most cases those in 

 the central part of the glacier were found to have 

 moved farther than the others, as shown in the 

 figure. 



The rate of movement of numerous glaciers has Fig. 112. 



been measured in this way or in some other. The Diagram show- 



, , ., . , , . ing how a 



rates range trom one so slow that it is hard to row of stakes 



measure, up to several feet a day. Of those whose set 



rate of advance has been measured, few move more cieTbec* mes 



than two feet a dav. and very few as much as seven. curved as 

 " ' .. theicemoves 



Conditions affecting rate of movement. The forward, 

 rate of movement appears to depend chiefly on 

 (1) 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, and 

 (6) the amount of water in the ice. Great thickness, a steep 

 slope, a smooth bed, a high (for ice) temperature, and much 

 water favor rapid movement. Since temperature and amount of 

 water vary much from season to season, the rate of movement for 

 any given glacier varies much during the year, and is greater in 

 summer than in winter. 



Nature of glacier movement. It was formerly thought that gla- 

 cier ice flowed somewhat as a stiff liquid flows, and this view is, per- 

 haps, the one most widely held. It seemed at first to be supported 

 by the fact that the movement was most rapid at the top and in 

 the center, as in the case of a river. The spread of the end of a 

 glacier, too (Fig. 1, PI. XXIX, p. 100), as it moves out from its moun- 

 tain valley to the plain beyond, was thought to suggest flowage. 

 Furthermore, various experiments have been performed with ice 

 showing that a bar of it may be bent or moulded into almost any 

 shape, if it be pressed slowly enough through long periods of time. 



