486 GLACIAL GRAVELS OF MAINE. 



THE COARSER UPPER STRATUM. 



1. The fact that the till was only partially eroded from the outer islands 

 proves that the retreat of the sea was geologically rapid, especially if, as is 

 probable, the surf beat against the ice all the time of the retreat to the sea 

 margin and only once on the land situated beneath the sea at its highest 

 level, and that during the time of elevation of the land. 



2. While we do not know the amount of early glacial subsidence, we 

 do know approximately the amount of postglacial elevation. I assume that 

 this elevation has been about three times as great in northwestern Maine as 

 at the outer coast line. The moment this differential elevation began, the 

 gradients of the valleys leading southward became steeper, and grew more 

 and more so during all the time the land was rising (the apparent retreat of 

 sea) to its present position. 



3. Marine glacial deltas are formed at the ice front. The presence of 

 such deltas in the interior of the State within 100 feet or less below the 

 highest admissible level of the sea in their respective localities, and that, 

 too, at elevations of about 100 feet above the highest marine glacial deltas 

 that lie nearest the coast, proves that the ice still covered all the northern 

 part of the State at the time the sea had reached its highest elevation, or 

 nearly. Indirectly they furnish proof that the greater subsidence to the 

 north had at this time been already accomplished. 



4. The inference follows that at the time the sea reached its highest 

 level (i. e., when the subsidence of the land was arrested) glacial sediments 

 were still being poured down the valleys in front of the retreating ice. 

 Above the sea of that time these glacial sediments formed valley drift; 

 below that level, fluviatile marine deltas. During the differential elevation 

 of the northern lands this delta would recede southward with the shore of 

 the sea. The steeper gradients would now enable the coarser glacial sedi- 

 ments to be transported to longer distances from the ice, where they would 

 be deposited over the beds of finer sediments already spread over the 

 bottoms of the valleys. Moreover, there would be more or less erosion of 

 the coarse sediments previously deposited farther up the valleys than the 

 basal clays extend, and the eroded matter would be transported nearer to 

 the sea and often might reach it and help form a fluviatile delta where the 

 rivers flowed into the sea. 



