VALLEY DEIFT OF FLUVIATILE ORIGIN. 473 



finer composition. No reasons for greater fluviatile erosion in one class of 

 Yalle3's than in the otlier, other things being equal, have as yet suggested 

 themselves. 



The quantity of the valley drift in valleys is very greatly dependent 

 on the positions of the glacial rivers, and is to some extent independent of 

 the drainage surface. 



While, then, we must assume a certain amount of rain wash and erosion 

 of till by streams as having helped to bring down sediment that is now in the 

 valleys, this process can account for only a small part of the valley di-ift. 



Was the valley drift deposited in the sea? If so, it might be under 

 the following conditions: 



1. The valley drift was deposited, in part, by glacial streams pouring 

 into the sea. It is plainly a different formation from the marine glacial 

 delta as ordinarily developed. It is possible that in narrow valleys the 

 structui'e would be modified by tidal wash and scour, yet I see no way to 

 account for the total absence of the reticulated ridges formed at the land- 

 ward ends of the deltas. 



2. We may attribute the alluvium to erosion by the sea waves. If so, 

 the residual beach gravels left after so much of the finer matter was washed 

 away ought to be recognized, and such are not found. Even in the most 

 exposed coasts the till was not all washed away. Still less can we postulate 

 in the interior valleys, which the rise of the sea would change into land- 

 locked fiords, any such erosion as the valley drift calls for. 



3. Sheets of valley drift comparable in most or all respects to the 

 valley drift of the higher parts of Maine are found in the vicinity of the 

 Green and the White mountains, and thence extend south through northei'n 

 New England far above any admissible or alleged former level of the sea. 

 Even if we admit that a part of the valley drift is marine, it is certain that 

 the larger part was deposited above the sea. 



4. The valley drift Avas deposited in the sea by ordinary rivers. This, 

 I think, is true for a portion of the valleys, but only below the former level 

 of the sea, say 450 or possibly 500 feet in the interior valleys. This 

 structure will be referred to hereafter, and the limits wherein found. 



I conclude, as the result of this discussion, that the valley drift extends 

 above the former level of the sea. It is a subaerial formation, as a whole, 

 though it locally passes into fluviatile deltas deposited by the ordinary 

 rivers in the sea. 



