HISTORY OF THE COASTAL GRAVELS. 403 



In a minor and more indirect way the noncontinuoiis gravels appear 

 to owe their pecuhar development to the presence of the sea in front of 

 the ice. Under any admissible surface gradient of the ice the presence of 

 200 or more feet of frontal water rising above the base of the ice would 

 arrest the flow of such of the subglacial streams as did not have their 

 sources more than 3 to 5 miles back from the front, so as to have sufficient 

 head to drive them after the rise of the sea. Several of the shorter dis- 

 continuous systems do not exceed this length. In such cases the rising 

 of the sea would cause the development of the gravels to cease, and we 

 would now find tliem in that stage in which they happened to be when 

 their streams were arrested. If we grant that the sea had no direct, only 

 a modifying, eff'ect in causing noncontiuuous sedimentation, still it would 

 be a not unimportant r6le to fossilize, so to speak, the work of the shorter 

 glacial rivers at a particular period of their history and preserve it for our 

 inspection. 



Having thus set forth what appear to be the more important agencies 

 in producing noncontinuous sedimentation, it remains to examine them in 

 their mutual relations and thus obtain a more general view of the subject. 



EESUMlfi: HISTORY OF THE COASTAL GRAVELS. 



As ah-eady repeatedly noted, the three distinguishing features of the 

 discontinuous coastal gravels are their rapid decrease in size toward the 

 coast, their occurrence at longer and longer intervals, and their termination 

 a short distance north of the shore and at only a few feet above tide water. 

 The three phenomena are so widely associated that they would appear to 

 have had, in respect to their principal causes, a common origin. 



The history and causation of the coastal glacial sediments, so far as 

 now appears, were probably about as follows, assuming that in the coastal 

 region of Maine most of the glacial sediments were deposited by subglacial 

 streams : 



If these changes were observed in case of only a few of the gravel 

 systems that reach nearest the coast, one here and there, the facts would 

 seem to indicate local causes. But when gravel systems are found every 

 few miles along 200 miles of coast, all of them exhibiting the first two of 

 the above-named characteristics, and all but five the third, w^e are forced to 



