Ancient Extent of the Glaciers of Chamonix, G7 



jcctcd to no violence. Atmospheric agents alone can affect 

 them; accordingly, the blocks composed of hard and resist- 

 ing rocks often preserve the colossal dimensions of which we 

 have spoken. 



Such is not the case with the fragments which do not form 

 part of the superficial moraines. The lateral walls of the 

 glacier are not in immediate contact with the sides of the 

 valley; there is almost always a small interval between 

 them. Numbers of blocks and debris are entangled between 

 this wall of ice, and the rocks which it polishes. Some of 

 them remain suspended in this space ; others, by degrees, 

 sink to the lower surface of the glacier, and form the deep 

 moraine. To these blocks are added, a part of those which 

 fall into the numerous crevasses and wells,* objects which are 

 so formidable to inexperienced travellers. All these debris, 

 enclosed between the rock and the glacier, pressed, bruised, 

 and broken by this great press continually in action, do not 

 preserve the dimensions they had when detached from the 

 mountains. The greater part of them are reduced to an im- 

 palpable mud, which, mingled with the water flowing from 

 the glacier, forms the bed of mud on which it rests. Others 

 preserve indelible traces of the pressure to which they have 

 been subjected. All their angles are blunted, their sharp 

 ridges effaced, and they assume the form of rounded pebbles, 

 or present unequal facettes, resulting from prolonged friction. 

 If the rock be soft like the limestones, then not only is the 

 pebble rounded, but it is covered with a multitude of striae 

 crossing each other in all directions. These striated pebbles 

 are of great importance in reference to the study of the an- 

 cient extension of glaciers ; they are medals, the presence 

 of which determines, almost with certainty, the former ex- 

 istence of a glacier, which has now disappeared. In fact, 

 a glacier alone has the power to shape, wear, and striate 

 these pebbles in the manner indicated. ^Yater polishes and 

 rounds them, but it never striates them. Nay, it effaces the 



* One of these wells, measured by MM. Dolfus, Otz, and myself, on the gla- 

 cier of the Aar, was 58 metres deep. On the glacier of the Finsteraar, M. Desov 

 sounded another, and found no bottom at 232 metres below tho surface. 



