GLACIERS. 699 



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, or 

 imperfectly stratified, stones and earth toward and about its lower ex- 

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

 that comes within its reach, abrading it and carrying it onward down 

 the valley, and making deposits on its banks which are more or less 

 perfectly stratified. 



2. Erosion. — A glacier depends for its power of erosion on the 

 pressure of the mass, its rate of motion, and the stones it carries in its 

 bottom and sides. The pressure per 100 feet of thickness for glacier 

 ice is about 40 lbs. to the square inch, or, for a thickness of 500 feet, 

 200 lbs. It also erodes indirectly through its sub-glacial stream. 



(1.) Through the wrenching the ice undergoes, transported rocks or 

 stones have their angles more or less blunted or rounded by mutual 

 attrition, and a general grinding of the earthy material takes place. 



(2.) The stones, large and small, in the bottom and sides of a glacier 

 make it a tool of vast power as well as magnitude, for scratching, 

 ploughing, and planing the rocks against or over which it moves. Be- 

 sides 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 furrowed, planed, and 

 rounded, over their whole exposed surfaces ; and the abrading stones 

 are also smoothed and scratched. 



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

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

 are a prominent feature of all glacial regions ; and those of the Glacial 

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

 are sometimes preserved to the present time in great perfection. The 

 view on the following page (Fig. 1107), from a photograph obtained 

 by the expedition under Dr. Hayden in 1873, represents a portion 

 of a great crouching flock of them, extending for 2,000 feet along 



