20 GLACIAL GRAVELS OF MAIiNE. 



the force of the current. But when water passes from beneath upward 

 through the gravel, the surface stones and grains are one by one hfted from 

 the others and the water bears them away as if they were a part of itself. 

 Thus the principal eroding and transporting work of subterranean waters is 

 done as they approach the surface as springs. There is an increased 

 velocity as the water nears the place of its release, and all loose matter 

 approaches the condition of quicksand. Clay and till are so compact that 

 they have suffered comparatively little in this way, but the quantity of 

 porous sand and gravel thus removed is surprising. 



TRANSPORTATION BY GLACIERS. 



For the purpose of this report it is not needful to discuss questions 

 relating to the structure or behavior of glaciers, except so far as pertains to 

 the geological work performed by them. We assume that snow which lasts 

 from year to year finally becomes consolidated into ice. Above the line of 

 perpetual snow the ice and semiconsolidated snow are known as the nevd, 

 or firn; below that line, as the glacier proper. Under favorable conditions 

 the ice slowly flows, at a rate varying according to the temperature, the 

 pressure from behind or the tension from before, the friction, the declivity 

 of the surface over which it moves, etc. Whether this is a true molecular 

 flow or only the apparent flow of a plastic body — of masses larger than 

 molecules — it is not necessary now to determine. Under suflicient tension, 

 or stretching force, the ice breaks, producing cracks called crevasses, which 

 are known as longitudinal or transverse accoi'ding- to their direction with 

 respect to the length of the glacier, or marginal when at the sides. When 

 fractured surfaces of moist ice are brought together, they at once cohere, 

 and surfaces of dry ice brought together under sufficient pressure also 

 cohere. Thus, no matter how often the glacier is rent and torn, it has the 

 power to heal its own wounds and to flow on, practically as solid as before. 



Glacial movement conforms to the general laws of flow of fluids. The 

 flow is from where there is greater pressure to where there is less, and it is 

 retarded by friction at the bottom and sides of the glacier. This friction is 

 but another name for the force which the glacier exerts in its efforts to push 

 along the rock and other substances in contact with it. 



When weathered rocks project above the glacier, more or less cliff 

 ddbris tumbles down upon the ice. This debris is known as moraine stuff, 



