THE WORK OF SNOW AND ICE 275 



Much stratified drift (gravel, sand, and silt) deposited by glacial 

 streams' has no distinctive topographic form, and therefore no 

 special name. 



ICEBERGS 



When glaciers advance into water the depth of which approaches 

 their thickness, their ends are broken off (Fig. 232), and the de- 

 tached masses float away as icebergs (Fig. 233) . Many of the bergs 

 are overturned, or at least tilted, as they set sail. If this does not 

 happen at the outset, it is likely to occur later as the result of the 

 melting, wave-cutting, etc., which shift their centers of gravity. 

 The great majority of bergs do not float far before losing all trace 

 of stony and earthy debris; but the finding of glaciated pebbles 

 in dredgings far south of all glaciers shows that bergs occasionally 

 carry stones far from land. 



THE INTIMATE STRUCTURE OF GLACIER ICE 



The key to the study of the structure and motion of glacier ice 

 is based on the view that a glacier is a mass of crystalline rock 

 the purest and simplest type of crystalline rock known. It is made 

 of a single simple mineral, ice, which always has a crystalline 

 structure. 



The development of ice from snow. The fundamental concep- 

 tion of a glacier is best developed by tracing the growth of its 

 constituent crystals. When water solidifies from the vapor of the 

 atmosphere, it takes the form of separate crystals (Fig. 234). The 

 flakes are rarely perfect, but they are always crystals. Snow crystals 

 may continue to grow so long as they are in the atmosphere ; but if 

 they pass through a dry stratum of air, or a stratum whose temper- 

 ature is above 32 Fahr., they suffer from melting and evaporation. 

 When they reach the ground, the processes of growth and decadence 

 continue, and the crystals grow or shrink according to circumstances. 



A glacier is a colossal aggregation of crystals grown from snow- 

 flakes to granules of greater size and more compact form. The 

 microscopic study of snow reveals the mode of change from flakes 

 to granules. The slender points and angles of new-fallen flakes 

 melt and evaporate more than the central portions. The water 



