314 GLACIAL GEAVELS OF MAINE. 



the concltision that the motion of the ice not only indirectly establishes the 

 subglacial drainage by furnishing the necessary crevasses, but also directly 

 aids in the formation of the channels in the direction of motion. This it 

 does because the modifications of the base of the ice that are made as the 

 ice passes a given point are carried forward by the motion. As one of the 

 possible combinations, let us postulate a new crevasse appearing far from . 

 any others, opening into the basal cavity formed in the lee of the obstruc- 

 tion causing the crevasse, and where no previous water channel exists. 

 When the crevasse fills with water, its outward pressure, owing to the 

 higher specific gravity of water, somewhat exceeds the pressure due to 

 the weight of the superincumbent ice. The inward flow of the ice to 

 fill the cavity in lee of the obstruction is resisted by the viscosity of the 

 ice and the antagonistic pressure of the water. In the absence of specially 

 great pressure, such as would be caused by converging ice flow owing to 

 lateral pressure of obstructions, it might happen that the pressure of the 

 water filling the crevasse and basal cavity could resist the collapse of the 

 latter, or make it very slow. If so, as the lower ice moved forward, the 

 water would fill the lengthening furrow which would extend from the base 

 of the crevasse backward to the obstruction. When a new crevasse was 

 formed at the proximal end of the same cavity, the water pressure would 

 still be maintained, and would continue while the base of that crevasse was 

 in turn piished forward. Under favorable conditions the basal furrow 

 might thus be prevented from collapsing until its forward end had been 

 brought to where it opens into other basal cavities or into a crevasse. In 

 this analysis we have avoided a comparison of the pressure exerted by the 

 inward flow with that due to the weight of the ice. Without insisting on 

 details where so much remains unknown, we may in a general way safely 

 affirm that the motion of the ice greatly assists in the formation of sub- 

 glacial tunnels in other ways than simply by the formation of crevasses. 

 Probably a mass of motionless ice would have only a surface drainage. 



How are the channels of subglacial streams maintained transversely to 

 the movement? Where transverse crevasses form a part of such a channel 

 it is easy to account for the maintenance of the channel. As new crevasses 

 opened, the stream would occupy them in turn for a time, and then abandon 

 them, as it did old moulin shafts. This implies that the stream is pushed 

 onward, whatever distance intervenes between the successive crevasses, and 



