CHAP. XV. MORAINES AND GLACIAL FURROWS. 293 



rounded or ground down into sand, or even the finest mud, 

 of which the moraine is lai'gely constituted. 



As the terminal moraines are the most prominent of all 

 the monuments left by a receding glacier, so are they the 

 most liable to obliteration; for violent floods or debacles are 

 sometimes occasioned in the Alps by the sudden bursting of 

 glacier-lakes, or those temporary sheets of water before al- 

 luded to, which are caused by the damming up of a river by 

 a glacier, which has increased during a succession of cold 

 seasons, and, descending from a tributary into the main 

 valley, has crossed it from side to side. On the failure of 

 this icy barrier, the accumulated waters, being let loose, 

 sweep away and level many a transverse mound of gravel 

 and loose boulders below, and spread their materials in con- 

 fused and irregular beds over the river-plain. 



Another mark of the former action of glaciers, in situa- 

 tions where they exist no longer, is the polished, striated, and 

 grooved surfaces of rocks before described. Stones which lie 

 underneath the glacier and are pushed along by it some- 

 times adhere to the ice, and as the mass glides slowly along 

 at the rate of a few inches, or at the utmost two or three feet, 

 per day, abrade, groove, and polish the rock, and the larger 

 blocks are reciprocal!}^ grooved and polished by the rock on 

 their lower sides. As the forces both of pressure and propul- 

 sion are enormous, the sand, acting like emery, j)olishes the 

 surface; the pebbles, like coarse gravers, scratch and furrow 

 it; and the large stones scoop out grooves in it. Lastly, pro- 

 jecting eminences of rock, called "roches moutonnees" (see 

 above, p. 269), are smoothed and worn into the shape of flat- 

 tened domes where the glaciers have passed over them. 



Although the surface of almost every kind of rock, when 

 exposed to the open air, wastes away by decomposition, yet 

 some retain for ages their polished and furrowed exterior : 

 and, if they are well protected by a covering of clay or turf, 



