LATE GLACIAL HISTORY OF THE COASTAL REGION. 409 



ceased to flow because of the rise of the sea before the retreat of the ice as 

 far back as the southern terminations of the gravel systems — that is, their 

 work ceased while they were yet in the early or discontinuous stage of ice- 

 channel sedimentation. It is uncertain how far this remark applies to the 

 longer rivers that formed no marine deltas. 



As previously pointed out, the subsidence of the ice-covered land 

 beneath sea level would cause the tunnels and lower part of the crevasses 

 to become permanently filled with water at 32°. The manner in which 

 these basal waters tend to restrict the enlargement of the subglacial tunnels 

 has been already described at some length. Of all the agencies known to 

 me for the production of the coastal gravels and their limitations, this 

 appears to have been the most efficient. 



LATE GLACIAL HISTOKY OF THE COASTAL REGIOjV. 



The history of the coastal region appears to have been about as follows: 

 Without assuming any definite positions for the southern border of the 

 area of accumulation at particular periods of the life of the ice-sheet, we 

 may confidently affirm that during the time of maximum glaciation a large 

 part of the zone of waste was south of the present shore and that the eai'lier 

 kames and overwash gravels are now beneath the ocean. At the time when 

 the coastal gravels were being deposited the higher hills of that region were 

 able to deflect the ice from its earlier direction of movement. The height 

 of these hills limits the tliickness of the ice of this period to not much if 

 any more than 1,000 feet, and it may have been considerably less. On the 

 other hand, the flow of the osar rivers that deposited the Medomac Valley 

 system of discontinuous gravels had ceased at the time the Waldoboro 

 moraine was being formed — that is, before the ice had become less than 

 100 to 200 feet in depth. The coastal gravels date, then, from the time just 

 preceding the retreat of the ice to the present shore, or perhaps to the north 

 ends of the fiords. The absence of frontal gravels from the coastal region 

 except in the form of marine deltas proves that the sea beat against the 

 front of the ice, or at least against its base, during all the time of the 

 retreat up to the highest beach. Some of the marine deltas were formed 

 not more than 100 feet above the present sea level and only 2 to 5 miles 

 north from the southern ends of the gravels of the same systems. We 

 infer that the sea had reached. at least one-half of its final elevation by the- 



