CHAP. xv. MOEAINES AND GLACIAL FURROWS. 293 



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

 of which the moraine is largely 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 some 

 times 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 confused 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, sometimes 

 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 reciprocally 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, polishes 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 

 flattened 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, 



