304 H. L. FAmCHtLti I>08T-GLAC1AL SUBMERGENCE OF LONG ISLAND 



peculiarity of these troughs consists in the fact that though in form much 

 like ordinary stream beds they are not now and never have been the seats of 

 subaerial rivers. Their valleys, often several hundred yards in width, do not 

 present the smooth downward grade so characteristic of ordinary valleys; 

 their floors are generally more irregular than those of any ordinary stream 

 could be. Nor do they have the distinct banks common to all land streams. 

 The only explanation which can be given of these troughs is that they were 

 the channels through which the subglacial streams found their way sea- 

 ward" . . . (page 317). 



"It is nttt unlikely that when once instituted by the subglacial stream these 

 valleys would have been somewhat further developed by the to-aiid-fro move- 

 ment of the tides along shore. The outrunning streams from the glacial face 

 would have tended to determine the outflow of the tide into their channels, 

 thus increasing their scouring action"' (page 318). 



"In studying the form of these old channels, especially on Nantucket, where 

 they are best shown, we observe that they gradually widen as they depart 

 from the front of the moraine until at a distance of from three to six miles 

 from their origin thej' are from five to twenty times as wide as at their source. 

 We also note that the streams which form these channels apparently dimin- 

 ished their energy of movement in proportion to the distance traveled from 

 the glacier. This is shown bj^ the much finer sediment which they laid down 

 in their outer parts. Near their sources they were able to move considerable 

 pebbles, while at their southern extremities they were able to transport fine 

 sand alone. 



"I desire to say that while these peculiar channels may not in the end ap- 

 pear to be due to the causes above suggested, this cause is the only one out of 

 many considered which appears reconcilable with the facts. They are clearly 

 due to localized streams of some sort which originated at the ice-front. They 

 were evidentlj^ formed while the surface on which they lie was depressed 

 below the level of the sea, for their troughs do not have the shape of those 

 excavated by subaerial streams" (page 319). 



If we are "iinable to fully explain all the peculiarities of the plain, it is 

 because we are ignorant of the precise mechanical conditions and oper- 

 ations involved in the lifting of a gently sloping sand-plain out of tidal 

 waters. It would be unwise and unfair to let such uncertainty outweigh 

 the abundant, clear cut, positive proof of submergence. 



Explanation of Maps and Isobases 



In plate 11 the shaded areas indicate approximately the greatest sub- 

 mergence of the land and the broadest extent of the sealevel waters for 

 all localities, but not all at the same time. The northward-moving wave 

 of land uplift, following the receding glacier, was lifting the land on the 

 south and thereby diminishing the waters, while the land on the north 

 was at its lowest attitude. Perhaps Long Island had attained nearly its 

 present height before the Champlain district began to rise. 



