312 GLACIAL GRAVELS OF MAINE. 



process, since water is a poor conductor of molecular heat, while the absorp- 

 tion of radiant energy is practicallj^ instantaneous. Volumes are as the 

 cubes of the diameters, and surfaces as the squares of the diameters; hence 

 the larger superficial streams contain a larger proportion of water Avarmed 

 above 32° as they pour into the ice. This heat melts the ice of the 

 crevasse as it descends and enlarges the passage into a shaft, and continues 

 the work after it is beneath the ice in the enlargement of even the narrow- 

 est crevasses into tunnels. Water at 32° would find its way through the 

 crevasses as do the subterranean waters tlu-ough the joints of the insoluble 

 rocks, without enlarging the natural joints except to a limited extent by 

 mechanical erosion. Surface waters of the ice never become heated very 

 much above 32°, and their melting power is much more feeble than waters 

 warmed on the land. 



Crevasses not opening into established subglacial or englacial channels 

 may become filled with water in which convective currents soon begin to 

 carry heat to the bottom, since water at 39.1° sinks and forces that of 

 32° to rise. But crevasses are so deep in proportion to their Avidth that 

 only a sluggish circulation can be kept up in them, and rarely, vmless 

 in exceptional cases, Avill stationary water be able to melt for itself a sub- 

 glacial outlet. The flow of a surface stream over the mouth of the crevasse 

 aids the melting by fm-nishing a constant supply of warmed water. 



When a supei-ficial stream pours down a crevasse, an enlargement of 

 the base of its shaft is formed, where the Avater, falling at a high velocity 

 to the ground, rebounds outward in all directions. A new creA^asse soon 

 opens at a short distance aboA'e the last one, and in the course of time the 

 stream opens a new shaft in this and abandons the old one. As the ice 

 flows past the place where the crevasses fonn, each part is in succession 

 hollowed out at the base of the waterfall, and thus a large continuous 

 tunnel is prolonged by the forward movement of the glacier. It is not 

 meant to imply that the water acts only at the base of the waterfall, but it 

 acts there most energetically. Given, then, a waterfall or any other condi- 

 tions whereby warmed waters can melt a passage underneath each suc- 

 cessive block between the crevasses, and the glacier itself will prolong a 

 tunnel distally. 



Let lis take the case of a moulin supposed to be formed at the proximal 

 end of a subglacial stream — the successive transverse crevasses not opening 



