ICE AND GLACIERS 229 



ing to the levels and measurements taken by Forbes, with 

 the view of the right bank of the glacier. The letters stand 

 for the same objects as in FIG. 105; p is the Aiguille de 

 Lechaud, q the Aiguille Noire, r the Mont Tacul, f is the 

 Col du Geant, the lowest point in the high wall of rock that 



10000* 



FIG. 1 06 



surrounds the upper end of the snow-fields which feed the 

 Mer de Glace. The base line corresponds to a length of a 

 little more than nine miles: on the right the heights above 

 the sea are given in feet. The drawing shows very dis- 

 tinctly how small in most places is the fall of the glacier. 

 Only an approximate estimate could be made of the depth, 

 for hitherto nothing certain has been made out in reference 

 to it. But that it is very deep is obvious from the following 

 individual and accidental observations. 



At the end of a vertical rock wall of the Tacul, the edge 

 of the Glacier du Geant is pushed forth, forming an ice 

 wall 140 feet in height. This would give the depth of one 

 of the upper arms of the glacier at the edge. In the middle 

 and after the union of the three glaciers the depth must be 

 far greater. Somewhat below the junction Tyndall and 

 Hirst sounded a moulin, that is, a cavity through which the 

 surface glacier waters escape, to a depth of 160 feet; the 

 guides alleged that they had sounded a similar aperture to a 

 depth of 350 feet, and had found no bottom. From the 

 usually deep trough-shaped or gorge-like form of the bottom 

 of the valleys, which is constructed solely of rock walls, it 

 seems improbable that for a breadth of 3,000 feet the mean 

 depth should only be 350 feet; moreover, from the manner 

 in which ice moves, there must necessarily be a very thick 

 coherent mass beneath the crevassed part. 



To render these magnitudes more intelligible by reference 

 to more familiar objects, imagine the valley of Heidelberg 



