INTRODUCTORY : ICE NOT AX EROSIVE AGENT. 9 



a peculiar erosive power, the bottom and the sides of the valleys through 

 which former glaciers have passed would exhibit some peculiarities of form 

 iudicatinu; that the work done below the level of the surface of the ice was 

 of a different character from that done above that level. But, in point of 

 fiict, on examining any valley once partly filled with ice, it will be found 

 that there is no such essential diffei'euce. No change of form can be ob- 

 served at the former line of the ice. Aside from the morainic accumulations, 

 there is nothing to prove the former existence of the glacier except the 

 smoothed, polished, or roimded surfaces of the rocks, which have no more to 

 do witli the general outline of the cross-section of the valley than the marks 

 of the cabinet-maker's sand-paper have to do with the shape and size of the 

 article of furniture whose surface he has gone over with that material. 



Admirable instances of the difference between the erosive action of water 

 and that of ice may be seen in several of the valleys of the Bernina group, 

 in the Upper Engadine. Near Pontresina, for instance, there is a rise in the 

 bottom of the valley of between one and two hundred feet over which the 

 Morteratsch glacier once passed without eroding it away. Since the glacier 

 disappeared, the stream issuing from the ice, the end of which is now almost 

 three miles above Pontresina, has cut a gorge through the elevation in 

 question, which gives the stream a channel with nearly continuous grade, 

 and hemmed in by almost vertical walls. The same thing may be seen in 

 the Val de Fex, which leads down from the glacier of that name to tlie 

 village of Sils, a few miles soutliwest of Pontresina. It is manifest, in both 

 these cases, that the ice for a long time rose over an obstacle which was in 

 its path, without the power of removing it ; but that the water has quickly 

 sawn itself a passage through the same, aided by the peculiar lithological 

 character of the rock. 



The most characteristic if not the most important work done by ice is that 

 which it effects on its rocky bottom by means of the detrital material which 

 has been carried down by water from tlie surflice of the glacier. The icy 

 mass moving over its bed smooths the edges of its inequalities by contiiuially 

 forcing fragments and particles of rock over them. Such smoothed surfaces 

 arc often found to be covered with fine lines or grooves, running, on the 

 whole, parallel with the course of the glacier. These striated surfaces, which 

 are so well known that they need not be described, are a peculiar result of 

 the movement of the glacier, and their existence is in most instances accepted 

 as positive proof of the former presence of ice. 



