402 GLACIAL GRAVELS OF MAINE. 



crevasses perhaps some hundred feet above the tunnels. Thus over all 

 those parts of the ice-sheet where the basal ice was submerged in the sea 

 but little of the water of local melting was available for enlarging the 

 main tunnels, but the melting was diffused, so to speak, over many times 

 as great an ice surface as where the water could pour down a crevasse and 

 escape at once along the tunnel, as happens in case of glaciers not flooded 

 by basal waters. The net result would be that in the parts of the ice-sheet 

 where the basal ice was submerged the subglacial tunnels were far less 

 enlarged than they would have been if the gently warmed surface waters 

 could have sunk at once into them. It is uncei'tain how far the outward 

 pressure of deep bodies of water cau overcome the inward flow of the 

 walls of the tunnels, but on the coast of Maine the depth of the perma- 

 nent water in the crevasses was at the most only about 200 feet, and it i& 

 improbable that such a pressure would perform any important work. But 

 irrespective of any possible partial collapse of tunnels formed on the land 

 as they were pushed beneath the cold sea, we can at least infer that the 

 tunnels were not enlarged commensurate with the supply of waters. For 

 there was as much water of surface melting here as elsewhere on the gla- 

 cier, but it could not help to enlarge the tunnels so much as that above 

 sea level. The tunnels naturally were inadequate to carry off" the water of 

 summer melting as fast as it fell down from above into the crevasses. 

 Each crevasse became filled with water above sea level and formed a stand- 

 pipe to the main tunnels. A great hydi-aulic "head" of water was the 

 result, but could never be greater than that which was due to the gradient 

 of the ice surface, since if the crevasses filled to the top the water would 

 then overflow on the ice. The result was a high velocity of the streams in 

 their narrow channels with consequent little sedimentation, and that only of 

 the coarsest matter and under the most favorable circumstances. The local 

 effect of basal waters would be felt by stream channels lying wholly in the 

 submerged area as well as by those originating above the sea and pushed 

 beneath it. 



In other words, the normal transfer of heat by surface waters to the 

 base of the ice, where it is the chief cause of the enlargement of the sub- 

 glacial tunnels, is in a large part arrested where the base of the glacier is 

 submerged to any considerable depth, and the heat is expended in melting 

 ice in the crevasses far above the tunnels. 



