684 DYNAMICAL GEOLOGY. 



raines from two Mt. Rosa Glaciers ; the fourth, the great moraine of 

 the Breithorn, the summit in the middle of the view. Other moraines 

 may be seen on the distant part of the glacier. In Fig. 1101, on 

 page 676, representing a section of the Bois Glacier near Trelaporte, 

 there are six distinct moraines. 



Toward the lower extremity of a glacier, the several moraines 

 usually lose their distinctness, through the melting of the ice ; for this 

 brings the stones and earth that were distributed at different depths to 

 one level, and thus produces a coalescence of the whole over the sur- 

 face. 



The stones are both angular and rounded ; the former are the more 

 abundant in the Alps, and the latter about the much larger Greenland 

 glaciers. Many are of great magnitude. One is mentioned, contain- 

 ing over 200,000 cubic feet, or equal in size to a building one hun- 

 dred feet long, fifty wide, and forty high. As the large masses shade 

 the ice below from the sun, and so protect it from melting, they are 

 often left capping a column of ice. 



At the glacier of the Aar, the central moraine is raised 100 to 140 

 feet above the general surface either side ; but this is partly owing to 

 the pressing up of the ice itself, by the mutual pushing of the two 

 combined glaciers of which it is made. The breadth where narrowest 

 is 250 feet ; and from this it increases to 750 feet, half-way to the 

 termination of the glacier, and to treble this below. 



The final melting of a glacier leaves vast piles of unstratified stones 

 and earth, or moraines, along its sides, toward and about its lower ex- 

 tremity. The stream which proceeds from the glacier works over all 

 that comes within its reach, carrying it onward down the valley, and 

 making deposits on its banks which are usually more or less perfectly 

 stratified. 



2. Erosion. — (1.) The movement of a glacier is attended with so 

 much wrenching of the ice, that the blocks have their angles more or 

 less blunted or rounded by mutual attrition. 



(2.) As the glacier has its sides and bottom here and there set with 

 stones of large and small size, it is a tool of vast power as well as 

 magnitude, scratching, ploughing, and planing the rocks against or 

 over which it moves. Besides this, it pushes along gravel and stones, 

 between itself and the rocks, with the same kind of effect. The rocky 

 cliffs and ledges in the vicinity of the glaciers are in many places fur- 

 rowed, planed, and rounded, over their whole exposed surfaces. 



The rounded knolls of rock along the track of a glacier have been 

 called sheep-backs (roclies moutonnees) in allusion to their forms. They 

 are a prominent feature of all glacier regions ; and those of the 

 Glacial period (p. 531), when they were formed over a vast extent of 



