Glaciers, 190 



gradually down the mountain side, following 

 the gulches and defiles that furnish a path of 

 least resistance to its flow. 



At the sides and bottom, where there is con- 

 tact with the earth, the movement is slower 

 than it is at the surface and in the middle of 

 the ice stream. If there were no curves in 

 the ravine or gulch through which it flows the 

 point of greatest movement would be confined 

 to the middle of its width. But in flowing 

 through a winding gulch the most rapid flow 

 follow the lines of greatest pressure, and this 

 line is deflected from side to side, so that the 

 line of greatest flow is more winding than is 

 the bottom of the valley through which it 

 flows. (The movement is called a " flow," but 

 it is very sluggish, only a few inches in a day, 

 as will appear later.) 



If the bottom and sides of the valley were 

 straight the surface of the ice would be com- 

 paratively even; I say comparatively, for as 

 compared with a smooth surface it would be 

 very rough; but there would be none of the 

 great crevasses or openings now to be found in 

 the ice, which sometimes are very large and 

 extend to a great depth. If in its downward 

 course the bottom of the ravine suddenly be- 

 comes steeper, the top of the ice is put under 

 a tensional strain which causes it to break, 

 thus forming the crevasses. 



If at the bottom of the descent the valley 



