the Hintereis Glacier. 173 



Since the tensile stress p — f sin 20, it is a maximum when 

 sin 20 = 1 or = 45°. 



It will be noticed that owing to the nature o£ the shear 

 movement o£ the glacier the inward end o£ the crevasse 

 moves more rapidly than the outward end, and so the 

 crevasse, after forming, rotates away from the bank ; 

 therefore such crevasses, except at the moment of formation 

 and before opening, would be inclined to the bank at a 

 greater angle than 45°. But although these peculiarities 

 follow from tbe theory, there is the fatal objection that the 

 actual stresses produced by viscous flow are not nearly 

 sufficient to rupture the ice ; indeed the conditions of stress 

 in a viscous substance flowing down a channel of uniform 

 section and slope are opposed to the formation of crevasses. 



That very many crevasses are the result of strains set up 

 in the ice byirregularities of the glacier-bed has been very 

 generally accepted, and it is very probable that all crevasses 

 are due to this cause. 



For the explanation of crevassing we must, therefore, look 

 to irregularities in the glacier-bed, and if it can be shown 

 that crevasses would be formed by changes of slope, whose 

 direction would be up the glacier and whose angle would be 

 about 45°, such an explanation would be more satisfactory. 



Referring to the map of the Hintereis Glacier, fig. 11, 

 it will be seen that on the northerly side there are three 

 groups of crevasses separated by clear spaces. The small 

 group on the 2750 contour-line occurs where there is a 

 change of slope. Between the 2750 contour and the 2710 

 the slope becomes less steep and there are no crevasses. 

 From the 2710 to the 2650 contour the slope again steepens, 

 the contours being crowded together, and the ice is much 

 broken. These crevasses extend to the middle of the glacier, 

 and their angle with the sides becomes less and less as the 

 centre of the glacier is approached. We have seen that in 

 the middle of the glacier there is very little shear and tensile 

 stress, the speed curve being very flat, and yet there are 

 crevasses. Here there is a marked increase in the slope of 

 the middle of the glacier from the 2750 contour to the 2690. 

 Fig. 12 shows a section along the line a, b, fig. 11. The 

 vertical lines show the positions of the crevasses and their 

 length, not their depth. From the 2650 to the 2590 contour 

 there is a decreasing gradient and there are no crevasses. 

 At the 2590 contour the slope again becomes steeper, and 

 crevasses are numerous down to the 2550 contour. 



On the southern side of the glacier the same features, but 

 in a less marked form, are evident. 



