292 OSCILLATIONS OF ALPINE GLACIERS. CHAP. xv. 



found at the lower end of the ice being called terminal 

 moraines. Such heaps of earth and boulders every glacier 

 pushes before it when advancing, and leaves behind it when 

 retreating. When the Alpine glacier reaches a lower and 

 a warmer situation, about 3,000 or 4,000 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 glacier leaves behind it as it retrogrades ; and among 

 these the most prominent, as before stated, are the terminal 

 moraines, or mounds of unstratified 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 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 



