74 



THE MUSEUM 



sheet of the glacial period was merely 

 compacted snow, and the climatic con- 

 ditions were such that each season 

 more snow fell than was melted, and 

 so the ice sheet grew. 



If the winter's snow fall is 60 inches 

 and the rummer is so cool that of this 

 snow but 50 inches melts, the ice upon 

 which the snow falls will receive an 

 annual increase of thickness to the 

 amount of 10 inches of uncompacted 

 snow, a very slight thickness. If the 

 ice sheet to-day does not cover any 

 considerable portion of the country in 

 which we live, it is because the sum- 

 mers are so long and warm that the 

 snow and ice melt fast, and that for 

 this reason the edge of the ice sheet 

 has crept further and further to the 

 north, so that now on the continent of 

 America it no longer exists, except that 

 on some high mountains and in the 

 north, fragments of it are left in the 

 glaciers found there and in ice masses 

 now covered with soil, which often 

 bear luxuriant vegetation. Such rem- 

 nants are found only in high northern 

 latitudes or on lofty mountains, where 

 the melting of the ice in summer does 

 not greatly exceed the snow fall of 

 winter. The grandest well-known 

 glaciers of the temperate zone are 

 those of Switzerland, but one must 

 travel to the Arctic to witness the 

 most stupendous exhibitions of their 

 work. 



Glaciers are simply rivers of ice, of 

 varying thickness and extent, having 

 their origin above the level of the per- 

 petual snows by which they are fed. 

 Though in its origin a dacier consists 

 merely of compacted snow, this snow, 

 as it advances down the mountain side, 

 is gradually changed by pressure into 

 an ice-like mass, and as it reaches the 

 point where there is alternate melting 

 and freezing, it becomes true ice. The 

 glacier tends constantly to move in the 

 direction of the least resistance, and 

 follows the inequalities of the ground, 

 thus moving in a b'd not unlike that 

 of a river. Yet as the momentum of 

 such an enormous mass is almost in- 



conceivably great it acts as an enor- 

 mous plow, which cuts a furrow both 

 wide and deep. In its course, it scrapes 

 away the surface soil and the loose 

 stones, and reaches down to the bed 

 rock, against which it continually grinds 

 and wears itself away. In its course 

 it picks up and carries away with it 

 gravel, pebbles, boulders and some- 

 times great masses of rock, and these, 

 whether torn away from the sides of 

 its bed or dropping on the ice from 

 overhanging cliffs, are lieely at last to 

 reach the bottom of the channel which 

 it has made. Here they are rolled 

 along, crushed beneath the mass of the 

 ice against or into the rock over which it 

 is passing, which is thus scratched and 

 scored or has its irregularities of sur- 

 face smoothed and planed off or some- 

 times is highly polished. In glacial 

 regions such surfaces are frequently 

 seen, as well as the smoothly rounded 

 knolls of rock called sheep backs or 

 roches moutonnees. Such surfaces 

 exist over much of northern North 

 America, though usually covered up 

 by earth and vegetation. 



The debris carried along in and 

 against the glacier is constantly being 

 ground up like the grain between two 

 mill stones, and the water of the stream 

 formed by the melting ice is charged 

 with this pulverized rock. Such 

 streams therefore are milky fn color, 

 and can often be recognized by this 

 character far away from their source. 

 On either side of the glacier and at its 

 lower end — if it does not reach the 

 sea — and often in the middle of it, are 

 great heaps or windrows of sand, 

 gravel, stone and great rocks, which 

 have been pushed before it or to one 

 side by the ice mass as it travels along 

 These accumulations of glacial debris 

 are called moraines, lateral, terminal 

 or medial, according to their positions. 



Although a glacier is, in fact, a river 

 of ice, it acts very differently from a 

 river of water. Thus the cross profile 

 of the stream valley is shaped like a 

 V, while in the glacier valley the same 

 profile is like a wide U. The curves 



