410 GLACIAL GEAVELS OF MAINE. 



time the ice had retreated back to these deltas — how much more we do 

 not know. We thus reach the conclusion that the sea was somewhat above 

 its present level at the time the coastal gravels were deposited, but how 

 much is not yet determined by the gravels themselves in their development 

 as deltas. 



During the thinning of the ice the subglacial streams were extended 

 farther north into regions before drained by superficial streams which were 

 situated far up on the glacier and extended into the slush zone of snow. 

 None of the basal debris could get up so high above the ground as this, 

 and only Mount Katahdin has been supposed to have been above the ice 

 surface. This class of superglacial streams could not have deposited the 

 coastal or, unless rarely, any other glacial gravels. The class of superficial 

 streams that form near the ice front may have assisted in the formation of 

 the coastal gravels, as at the marine deltas, the glacial lakes, aud by collect- 

 ing sediment, which they poured into subglacial tunnels. No matter where 

 the ndv^ line had been at the time of deepest ice, it certainl}^ was far north of 

 the shore at the time the coastal gravels were deposited, for this was well 

 on in the period of retreat. As the n^v^ line retreated northward and the 

 subglacial di^ainage was correspondingly extended, the time came when 

 that portion of the ice-sheet drained by subglacial rivers was at a maxi- 

 mum over the State. Obviously the longer a glacial river is, the greater 

 will be the enlargement of its channel, other things being equal. The 

 amount of water passing southward at the shore would increase so long as 

 the length of the subglacial streams north of the shore was increasing, up 

 to the time of the retreat of the ice to that line, if the sea did not interfere 

 with the development. The time of maximum subglacial drainage surface 

 probably was near the time when the coastal gravels were deposited, or 

 somewhat later. This would cause a large flow of water, but not a large 

 sedimentation, except where tliere was a corresponding enlargement of the 

 subglacial channels. For a time the base of the ice in the coastal regions 

 was flooded with cold waters because of the subsidence beneath the sea, 

 and the flow of the ice was probably more rapid south of the fiord line. 

 These and other physical causes so far prevented enlargement of the sub- 

 glacial tunnels in the coast region that sedimentation became more scanty 

 and at longer intervals southward and finally ceased near the fiord line. 

 ■South of this line, in all except a few instances, the glacial rivers were so 



