412 GLACIAL GRAVELS OP MAINE. 



mixed with them, and thus a large portion of the heat is expended in 

 melting ice within the crevasses and not within the tmmels. 



Other things being equal, surface melting is independent of the basal 

 condition of the ice, i. e., Avhether the ice is submerged or not. In other 

 words, the flowing of a glacier down into a body of water prevents the 

 enlargement of the subglacial tunnels to the full sizes they would have 

 had but for the ^jresence of the water, while the supply of surface waters 

 under like conditions is not diminished. 



An increased supply of water with a corresponding enlargement of the 

 outlets implies an increase in the velocity of flow, hence increased trans- 

 portation and diminished sedimentation. 



A sudden and marked decrease in sedimentation at or near a certain 

 contour along 200 miles of coast implies some agency acting horizontally 

 over a wide area to produce an increase in power of transportation (with 

 decrease of sedimentation) below that level. 



In Maine we have such a transition at the southern ends of such of 

 the gravel systems as reach nearest the coast, and thence extending for a 

 few miles northward. In general there are topographical conditions favor- 

 able to a somewhat more rapid rate of ice flow south of this line, but on a 

 somewhat hilly and uneven coast this cause ought to result in differences 

 in the elevations of the southern ends of the gravel systems. Hence, 

 while it is probable that the rate of ice flow was accelerated south of the 

 northern ends of the fiords (fiord line), it could have been only a contribu- 

 tory rather than a controlling cause of the relatively small enlargement of 

 the subglacial tunnels south of the fiord line. 



The ending of the gravels at nearly the same elevation can best be 

 accounted for by supposing the basal ice to have been submerged in the 

 sea to an unknown depth not exceeding, along the outer coast line, about 

 200 feet below the highest level attained by the sea. 



The highest beaches along the outer coast have nearly the same eleva- 

 tion above the present sea level. This is independent evidence that the 

 surface of the sea, measured northeast and southwest, at the time of its 

 greatest elevation was approximately parallel to its present shore, with per- 

 haps a little local warping in the Penobscot Valley and in a few other 

 places. If the petering out of the gravels near the fiord line was largely 

 the result of basal submergence of the ice-sheet in the sea, the termination 



