GLACIAL AND POST-GLACIAL HISTORY 643 



to 40 feet above tide. The Narrows gap has steep slopes, which indicate 

 that it may' have been cut down from an elevation of 60 feet above 

 tide. These estimates are not so reliable as they would be if the gaps 

 were cut in a plain, because the moraine surface rises and falls, and 

 a depression at a lower level than that indicated by the top of the 

 steep valley side may have existed where these gaps now are.' How- 

 ever, it does not affect the results greatly whether the height of the 

 barriers was a few feet more or less than the above estimates, but it is 

 a matter of considerable importance to know whether the sea had 

 access to the Hudson without an altitude of the land lower than the 

 present, as the ice was retreating from the Brooklyn-Perth Amboy 

 moraine. While it cannot be said to be demonstrable, the weight of 

 the evidence seems to indicate that it did not. 



According to the hypothesis that the Hudson water body was 

 an estuary, these gaps must have been first scoured out when the land 

 was enough lower to permit the sea to enter and the tide to scour. 

 This requires a depression somewhat less, possibly, than 60 feet for 

 the Narrows gap and 25-40 feet for the Arthur Kill gap. According 

 to this hypothesis, tidal scour must have lowered these gaps to such 

 an amount that, on the subsequent uplift which permitted the sub- 

 merged channels to be carved out, either there was free passage for 

 the streams that flowed through them, or they were scoured to a 

 level lower than that of any other part of the barrier, and thus took 

 off the drainage which finished the work of cutting away the barrier. 



According to the hypothesis that the Hudson water body was a lake, 

 these gaps were made by the outflow of fresh waters and not by tidal 

 scour. This does not refer to the gaps at their present level, which 

 may in part be due to tidal scour since the recent depression, but to 

 their depth before the recent depression. If these gaps were cut by 

 outflow of fresh waters, their relations are such as to require first a 

 cutting as outlets of independent lakes, and later, when the ice had 

 retired far enough to permit these independent water bodies to 



I Professor Salisbury has suggested that these steep slopes may be due to recent 

 wave-action. If this be the correct explanation, it makes the amount of cutting of 

 the gap through the moraine even less certain. If, however, the overwash plain 

 fronting the moraine was once continuous across the Narrows, as seems likely, the 

 altitude of its inner margin (20-40 feet A. T.) marks the level from which the gap 

 has been cut here. • 



