920 HARRY FIELDING REID 



snow into ice. During the first part of its course the angle 

 which it makes with the bed of the glacier steadily increases ; 

 but later diminishes until probably it at last becomes nearly 

 parallel with the glacier's bed. 



It is to be noticed that the largest accumulation occurs under 

 the mountain cliffs where the glacier receives, in the form of 

 avalanches, much of the snow that falls on the slopes above. It 

 is here then that the vertical component of motion is greatest, 

 and it is also in this neighborhood that we find the bergschrund ; 

 which marks the line where the more rapidly moving ice below 

 pulls away from the more slowly moving ice above. Reverend 

 Coutts Trotter^ has made the important suggestion that the 

 bergschrund may mark the line above which the mean annual 

 temperature is below freezing, and therefore the ice above it is 

 frozen to the mountain, whereas, the ice below slides on its bed 

 in addition to its shearing motion. Unfortunately, there is, so 

 far as I know, no experimental evidence bearing on this point. 

 Without disputing Mr. Trotter's idea, we see that the general 

 theory leads us to expect a considerable change in motion in this 

 part of the glacier; and when we recall that the surface slope 

 diminishes rapidly below the bergschrund, we are driven to infer 

 a rapid increase in the thickness of the glacier, and at least a 

 large factor in the formation of the bergschrund becomes clear ; 

 for above it there is a thin coating of ice on the mountain slope, 

 and below a much thicker mass of ice resting on an almost 

 equally steep bed ; and this would certainly result in a marked 

 difference in velocity of the two parts. 



Accurate observations on the direction of motion and on the 

 stratification are few. Desor found at the side of the Unteraar 

 glacier a motion of the ice having components down the valley, 

 towards the side, and upwards.^ Agassiz found by a line of 

 stakes across the same glacier, whose positions and levels had 

 been carefully determined, that during the winter the ice near 



'Proc. Roy. Soc, 1885, Vol. XXXVIII, pp. 92-108. In this article Mr. Trotter 

 describes some very instructive experiments showing that glacier ice will shear under 

 very small forces if sufficient time is allowed. 



^ Agassiz, Systeme Glaciare, pp. 499-504. 



