146 NEW YORK STATE MUSEUM 



No doubt the region was very elevated immediately preceding and 

 during a part of glacial time. But beyond the fact that the river 

 gorge is several hundred feet deeper than the present water level, no 

 direct evidence on that point is obtainable. The region has since 

 been depressed, however, somewhat lower than this former level. 

 This is indicated by the fact that the river is drowned. 



Postglacial changes. Evidence that the river level has changed 

 since the time of the withdrawal of the ice is found in the occurrence 

 of terraces of sand, gt"avel and associated delta and modified drift 

 deposits at the mouths of certain streams emptying into the Hudson, 

 such as Peekskill creek. The state camp at that point is located on 

 one of these terraces which is just above the 100 foot contour. This 

 agrees fairly well with evidence elsewhere that the difference of 

 level since glacial time is something like 100 or 125 feet for this 

 quadrangle. In the absence of ether conclusive evidence to the con- 

 trary, it may be assumed that it means re-elevation of the land to 

 that amount, the river representing sea level in each case. It is evi- 

 dent, from the behavior of the streams and their accumulations in 

 these terrace materials, that the deposits must have been made dur- 

 ing the withdrawal of the ice and while it was melting. In other 

 words, they were furnished by the thawing ice. Therefore, they 

 mark the immediate close of the ice occupation for this locality. 



This may be taken as the starting point for postglacial history. 

 Such erosion effects as have been accomplished on the drift are 

 chiefly on the lighter m.odified drift types and especially on the river 

 deposits which at one time may have largely filled the channel of 

 the river at certain points. Reelevation rejuvenated the streams 

 somewhat and they cut down through these silts and sands leaving 

 the remnants as terraces. Farther back from the river and on more 

 substantial types of drift and on bedrock few changes have been 

 accomplished in this territory. The streams are all small and the 

 modifications are limited to trenching soft deposits and cutting out- 

 lets a little deeper. 



Some of the lakes of that time have been drained. This amount 

 of elevation also emphasizes the islands in the river, some of 

 which would be entirely beneath the water level if change of level 

 had not set up new conditions. Even Constitution island would 

 hardly be seen or little Stony Point or lona island if it were not for 

 postglacial reelevation and consequent readjustment of river level. 

 None of the others would be seen at all. 



