Cn. XII.] MOKAHSTES OF GLACIEES. 147 



boulders every glacier pushes before it wben advancing, and leaves 

 behind it when retreating. When the Alpine glacier reaches a lower 

 and warmer situation, about 3000 or 4000 feet above the sea, it melts 

 so rapidly that, in spite of the downward movement of the mass, it can 

 advance no farther. Its precise limits are variable from year to year, 

 and still more so from century to century ; one example being on record 

 of a recession of half a mile in a single year. We also learn from M. 

 Venetz, that whereas, between the eleventh and fifteenth centuries, all 

 the Alpine glaciers were less advanced than now, they began in the 

 seventeenth and eighteenth centuries to push forward so as to cover 

 roads formerly open, and to overwhelm forests of ancient growth. 



These oscillations enable the geologist to note the marks which a gla- 

 cier leaves behind it as it retrogrades, and among these the most promi- 

 nent, as before stated, are the terminal moraines, or mounds of unstrati- 

 fied earth and stones, often divided by subsequent floods into hillocks, 

 which cross the valley like ancient earth-works, or embankments made 

 to dam up a river. Some of these transverse barriers were formerly 

 pointed out by Saussure below the glacier of the Rhone, as proving how 

 far it had once transgressed its present boundaries. On these moraines we 

 see many large angular fragments, which, having been carried along on the 

 surface of the ice, have not had their, edges worn off by friction ; but the 

 greater number of the boulders, even those of large size, have been well 

 rounded, not by the power of water, but by the mechanical force of the 

 ice, which has pushed them against each other, or against the rocks 

 flanking the valley. Others have fallen down the numerous fissures 

 which intersect the glacier, where, being subject to the pressure of the 

 whole mass of ice, they have been forced along, and either well 

 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 monu- 

 ments left by a receding glacier, so are they the most liable to oblitera- 

 tion ; for violent floods or debacles are often occasioned in the Alps by 

 the sudden bursting of what are called glacier-lakes. These temporary 

 sheets of water 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 are let loose, 

 which 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 situations where 

 they exist no longer, is the polished, striated, and grooved surfaces of 

 rocks already alluded to. Stones which he 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 



