140 



MORAINES AND ERRATIC BLOCKS. 



[Ch. XL 



half a mile in a single year. We also learn from M. Yenetz, that 

 whereas, between the eleventh and fifteenth centuries, all the Alpine 

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



Fiar. 128. 



Limestone polished, furrowed, and scratched by the glacier of Kosenlaui, in Switzerland. (Agassiz.) 



a a. White streaks or scratches, caused by small grains of flint frozen into the ice. 

 b &. Furrows. 



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 is the terminal moraine, which is a confused heap of un- 

 stratified rubbish, like the till before described ; all the mud, sand, 

 and pieces of rock, with which the glacier was loaded, having been 

 slowly deposited in the same spot where no running water interfered 

 to sort them, by carrying the smaller and lighter particles and stones 

 farther than the bigger and heavier ones. These terminal moraines 

 often cross the valley in the form of transverse mounds, more or less 

 divided into separate masses or hillocks by the action of the torrent 

 which flows out from the end of the glacier. Such transverse barriers 

 were formerly pointed out by Saussure, below the glacier of the Ehone, 

 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 ; there are also many boulders, of various sizes, 

 which have been rounded ; some, as before stated, by the power of 

 water beneath the glacier, others by the mechanical force of the ice 

 which has pushed them against each other, or against the rocks flank- 

 ing the valley. 



As the terminal moraines are the most prominent of all the monu- 



