110 PHYSIOGRAPHY 



But in spite of all the facts and experiments which suggest the 

 fluidity of ice, it is very doubtful if its real motion is flowage. 



It has already been noted that a glacier often cracks when it 

 passes over irregularities of bed. The ice also cracks open when 

 the end of a glacier spreads (Fig. 2, PL XXXI, p. 108), and if the 

 spreading of the end shows fluidity, we must assume that the ice 

 flows until it cracks open. But fluids do not crack open. These 

 and many other considerations which need not be discussed here 

 have led to the view that the resemblance between glacier motion 

 and the motion of a stiff liquid is more seeming than real. 



It is probable that the melting and refreezing of its substance has 

 much to do with glacier motion. When water sinks into the glacier 

 and freezes again, it expands and crowds the ice all about it. The 

 force of the crowding is illustrated by the familiar fact, already 

 referred to, that strong vessels are broken when water freezes in 

 them. The freezing of the water which has sunk into the ice must 

 have the effect of moving the ice, and the movement must be chiefly 

 down the valley, for gravity helps motion in this direction, and 

 hinders it in all others. Furthermore, the water, before refreezing, 

 moves not only down toward the bottom of the ice, but often 

 toward the lower end of the valley as well. The flow of the water is 

 therefore a way of transferring the ice of the glacier down-valley. 

 More or less ice is melted from time to time within the glacier, and 

 this water on refreezing has the same effect as that which sinks in 

 from the surface. 



The ice of a glacier sometimes slides, for in some cases it may 

 be seen that portions of the ice have slidden or sheared over other 

 parts. This is best seen in the glaciers of high latitudes, where 

 the structure of the ice may be well seen in the vertical edges and 

 ends of the glaciers. Under some conditions, a glacier probably 

 slides over its bed, but such sliding is not believed to be a principal 

 element in its motion. 



Size. There are in the Alps nearly 2,000 glaciers, only one of 

 which has a length of ten miles. Less than 40 have a length of five 

 miles, while the great majority are less than one mile long. Some 

 of them are but a few hundred feet wide, and few of them are so 

 much as a mile wide. The thickness of ice is rarely known, but 



