170 Trof. J. W. Spencer — Glacier-erosion in Norway. 



there M^as a differential movement of the ice and the block. What- 

 ever differential movement there is beyond that referred to, it must 

 be very inconsiderable even upon the inclined surfaces, and upon the 

 horizontal plains it must be indefinitely small, as here even the 

 movement of the ice is reduced to almost zero, as shown by the 

 measurements of Prof. Tyndall,^ upon the Morteracht, where the 

 velocity of the surface, some distance from its end, was 14. inches, 

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

 only two inches a day. 



11. The most favourable condition 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 the detached 

 blocks. However, when the whole mass of ice is charged with 

 sand and stones, there is no doubt that polishing and some 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 observation 

 shows to be soon reached. Consequently- we should not expect to 

 find great troughs or grooves scooped out 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 

 texture of the rock, although their surface may have been sub- 

 sequently polished. Generally speaking, as shown in the valley 

 behind theFondal Gaard, 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 moutonnees, with angles mostly removed, are not 

 smooth, but are as rough, and as much weather-worn as similar rocks in 

 warmer countries where no glaciers have been. Upon these surfaces 

 it is often difficult to discover any scratches, even where present, 

 for they are often so faint as to be only rendered apparent by 

 moistening the rock. Even the faces of the hummocks are commonly 

 unpolished. In other places, particularly at Tunsbergdalbree, 

 which contains much sand along the margin, the rocks are highly 

 i:)olished and but little scratched. One is everywhere surprised to 

 find beneath the glaciers the paucity of glaciated stones, and in many 

 terminal moraines they are scarcely if at all to be found. 



12. The insufficiency of glaciers to act as great erosive agents is 

 further shown at Fondal, where a mass of ice, 30 to 40 feet thick, 

 abuts against a somewhat steep ridge of rock, 10 feet or less in height. 

 In place of a stone-shod glacier sliding up and over the barrier, the 

 lower part of the ice appears stationary (or else is moving around 

 the barrier), whilst the upper strata bend and flow over the lower 

 layers of ice. 



13. When the barrier to the advance of a glacier is met with, 

 whether composed of hard rock, or of morainic matter, the ice, 



1 Tyndall's " Forms of Water." 



