388 GLACIi^L GRAVELS OF MAINE. 



matter it is thin aud covered by the marine clays. I fomid none exposed 

 in the banks of the Medomac River, though the discontinuous osar gravels 

 are easily traced. 



Frontal gravels ought especially to be abundant in the valleys of the 

 larger streams, such as the Penobscot and Kennebec, if the ice melted over 

 them before the rise of the sea, and we do not find them. On the contrary, 

 they do not either of them show even a marine delta near the rivers, though 

 there are a few situated a few miles back from the rivers. 



The conclusion follows that the ice had not melted over the coast 

 region previous to the rise of the sea over this area. The retreat of the ice 

 front was accompanied by the advance of the sea, if not in part caused by it. 



A related question refers to the earliest glacial sediments of the ice- 

 sheets. As before noted, the ice front during the time of tliickest ice must 

 have been far out in the present Gulf of Maine. While a part of the wast- 

 age then took the form of berg discharge, yet there must have been sub- 

 glacial rivers which deposited more or less glacial sediment near the ice 

 front. We do not know what development these gravels took, or how far 

 they were incorporated with moraines and berg droppings, nor do we know 

 the extreme depth beneath the sea to which a subglacial stream can pene- 

 trate and retain sufficient velocity to transport sediment. Omitting details, 

 we can at least affirm that the earliest glacial gravels are now under the ocean. 



What is the date of the earliest gravels now exposed on the land? As 

 elsewhere stated more fully, in the coast region there are several places 

 where the glacial gravel systems follow the scratches last made when the 

 ice was deflected by hills and therefore much reduced in thickness from the 

 maximum. These reach as far southward as any of the systems except 

 the great ones that come to Portland, Jonesport, and Columbia, and appear 

 to be as old as any, with perhaps these exceptions. This gives field support 

 to what we should expect from general considerations — that the osars were 

 not deposited till the later days of the ice-sheet. 



In comparing the quantity of the coast gravels with those of the inte- 

 rior, we have to consider the effect of the position of the n6vi line at various 

 periods of the ice-sheet's history. I conceive that only under extraordinary 

 conditions is the ndv^ line stationary during periods of the advance and 

 the retreat of the ice front. It is perhaps possible that there can be such a 

 balance of circumstances — such as length of the glacier, surface gradients. 



