92 



J. W. SPENCER ON 



The movement or flow of the ice about detached stones, resting upon rocks, has been 

 observed by Prof. Sexe beneath the Buarbrae, and by Prof. J. W. Niles beneath the Aletsch 

 glacier. ^ Prof. Sexe illustrates the moulding of the ice about a loose stone, which was 

 held beneath the glacier by a projection of the rock. My observations were upon stones, 

 not held up by rocky projections, but upon surfaces often sloping downward. Although 

 Prof. Niles did not record observations shewing that there was definite movement of the 

 stone, yet he conchided that there was a differential movement of the ice and the block. 

 WhatcA'er differential movement there is, it must be very inconsiderable, not only upon 

 horizontal plains, but upon inclined siTrfaces. In the former case the movement of the 

 ice is reduced almost to zero, as shewn by the measurements of Prof. Tyndall, upon the 

 Morteracht, where the A^elocity of the surface, some distance from its end, was fourteen 

 inches, whilst that of the tongue of the glacier, as it reached the plain, was only two 

 inches a day. " 



The most important condition favorable for holding stones in ice as graving tools is 

 low temperature, which impedes its progress ; but this condition beneath the glacier does 

 not generally exist. At higher temperatures, the velocity of the glacier is not great 

 enough to overcome its plastic movement and to drag along detached blocks. How- 

 ever, when the whole mass of ice is charged with sand and stones, there is no doubt that 

 polishing and scratching are effected ; but when there are only occasional fragments in 

 the bottom of the ice, as is commonly the case, the erosion from the sliding ceases as soon 

 as the resistance due to friction between the stones and the rock equals that due to 

 viscosity, which, as observations shew, is soon reached. Consequently, we should not 

 expect to find great troughs or grooves scooped out of solid rock by the actual glacier. 

 These I have not seen about the existing glaciers of Norway, which are not dependent 

 upon atmospheric and aqueous erosion and the text^^re of the rock, although their surfaces 

 may have been subsequently polished. Generally speaking — as seen in the valley behind 

 Fondalen Graard, where the glacier is nearly free from sand, and contains comparatively 

 few stones, as well as at many other places — the surfaces of the subjacent crystalline 

 rocks, although of the form of roches moutonnées, with angles mostly removed, are not 



mm- 



*yr>'' 



Fig. 4. — Section at Fondalsbrseen, hh, zone along which ice (6) is flowing upon its lower layers. 



' American Journal of Science, Nov., 1878. 



- Tvndall's Forms of Water. 



