434 GLACIAL GRAVELS OP MAINE. 



But then tlie inward flow of the walls is antagonized by the outward 

 pressure of the contained water. Also in Maine no glacial rivers are 

 known to have flowed over hills higher than 200 to 250 feet, except in one 

 extreme case of 400 feet, measured above the valleys lying to the north of 

 them. This represents only one-fifteenth to one-twentieth of the maximum 

 depth of ice. If we assume so great plasticity of the ice that so small a 

 difl^erence in thickness could make much difference in the sizes of the sub- 

 glacial tunnels in the two situations, it seems difficult to account for very 

 deep crevasses or subglacial channels. On the whole, it seems improbable 

 that so small differences in thickness of ice would much restrict the enlarge- 

 ment of the subglacial tunnels in the valleys ; yet it might to some extent. 



Second. It will be seen that the basal ice north of the hill is perma- 

 nently bathed in cold waters, and that the crevasses also are filled to the 

 same height as the top of the hill. All the waters of local melting that 

 pour into the crevasses in this part of the tunnel must more or less become 

 mixed with the cold waters of the crevasses, and their heat will largely be 

 expended in melting the walls of the crevasses, not in expanding the tun- 

 nels, just as has been pointed out in the case of a glacier flowing down into 

 the sea. Now some of these dams or permanent bodies of subglacial water 

 must have been several, perhaps many, miles in length. Thus the subglacial 

 dam north of Springfield, in the course of the Seboois-Kingman-Columbia 

 osar river, was at least 15 miles long, and that of the Portland system north 

 of North Woodstock extended as far as Andover, a distance of 20 miles. 

 Because of the greater subsidence toward the north, the length would at 

 that time be somewhat greater than now. The ice would be many years 

 in passing over such distances, and the cumulative effects of such dissipa- 

 tion of the energy that otherwise would help to enlarge the tunnels must 

 have been considerable wherever the courses of the glacial rivers were so 

 nearly parallel to the ice flow that the same body of basal ice in its progress 

 was thus continuously modified for a term of years. 



We are therefore justified in assuming that in the longer valleys situated 

 north of hills crossed by the glacial rivers the subglacial tunnels would 

 be small relatively to the supply of water, and the velocities would be 

 rather high during all the earlier stages of ice-channel sedimentation. Ridges 

 deposited at this time would be rather narrow and composed of coarse 

 material. Indeed, the sedimentation might often be of the discontinuous 



