248 DYNAMICAL GEOLOGY. 



the undermining of bluffs, and consequently large falls of rock and other 

 debris, (b) It Avorks with greater results through impact or forward thrust 

 of its bottom, sides, and front ; for it thus tears off angular stones, slate, and 

 great rocks, from rifted, laminated, and jointed terranes that are alongside 

 or extend up in ledges into the glacier, and takes them into its mass ; it also 

 plows into weakly consolidated deposits, such as fragile sandstones, and 

 gathers other supplies, though not able deeply to abrade the harder rocks ; 

 and in its movement up the under-glacier slope of a ridge or peak, it bears 

 along stones and other materials from low levels to high, (c) Further, it 

 works by corrosion, in its ever-shifting and crevassing movements, grinding 

 stone against stone or grain against grain, rounding angles and making the 

 finest of earth called rock-flour, which may become clayey by partial decom- 

 position of the feldspar present. 



The material gathered by the ice is called moraine material. The larger 

 part in ordinary glaciers lies along or near the borders and constitutes the 

 lateral moraine ; that occurring along the bottom, in the glacier and that 

 pushed along by it, is the ground moraine; and the deposit accumulated 

 at the extremity of the glacier, the melting place, is the terminal moraine. 

 The moraine material thus deposited is not stratified ; but it has a linear 

 order ; for it lies in lines which point upward to the summits from which its 

 materials were gathered. The terminal moraine is a low ridge, belt, or 

 mound of stones and earth transverse to the valley. Agassiz observes (1840) 

 that on the retreat of a glacier, a new moraine may form each year. He also 

 mentions the fact that the stones over the surface of a glacier outside of the 

 lateral moraine gradually move obliquely toward the latter, owing to the 

 greater velocity at the center. 



When two glaciers join, the lateral moraines of the two uniting sides 

 become one medial moraine. The number of moraines on a glacier, therefore, 

 can never exceed the number of coalesced glaciers by more than one. An 

 isolated peak rising above a glacier will send off its stones and earth all in a 

 single line or moraine. In the view of the Gorner Glacier on page 237, the 

 nearest moraine is that of the Biffelhorn ; the second is a union of moraines 

 of the Gornerhorn and Porte Blanche ; the third, a union of two moraines 

 from two Monte Rosa glaciers ; the fourth, the great moraine of the Breithorn, 

 the summit in the middle of the view. Other moraines may be seen in the 

 distant part of the glacier. Fig. 209 shows the moraines of the Mer de 

 Glace and of the glaciers above it. 



The transported masses of rock sometimes have great magnitude. One 

 among those of the Alps contained 200,000 cubic feet. In the lower part of 

 the Glacier of the Aar, after the junction of the great glaciers of the Fins- 

 teraar and Lauteraar, the medial moraine is 100 to 250 yards vv^ide and has a 

 height of 100 to 140 feet above the general surface of the ice either side. 

 The wasting of the ice of a glacier by melting often leaves the broader stones 

 perched up on ice-columns (like the perched stones in Figs. 158, 159), the 

 stones having protected the ice beneath it from the sun. 



