of the Hudson River Valley. 465 



nels through them below present tide-level. The mouths of 

 the tributary streams are now generally silted up and the pro- 

 cess of filling seems to be going on at present. It seems 

 indisputable that the brick clay deposits once filled the entire 

 valley up to a certain level, aud that the present depth of the 

 channel of the Hudson is due to the erosion of the still water 

 deposits by a river current. It is also probable that in the 

 narrow gorge of the Highlands some of the deltas filled the 

 valley, but this point has not been fully determined. 



Between Poughkeepsie and Albany at many points near the 

 water's edge are steep, unglaciated rock surfaces much fresher 

 in appearance than the glaciated surfaces upon which the 

 Champlain deposits rest. These may be the result of river 

 erosion subsequent to the formation of the terraces. 



The evidences of fiuviatile erosion enumerated suggest a 

 rapid flow of water down the Hudson valley in the late 

 Quaternary. Such a flow doubtless began when the valley rose 

 from its submergence. With these evidences of erosion may 

 be correlated the gorge of the Narrows at the entrance of New 

 York harbor. This is a gap in the terminal moraine about 240 

 feet deep and one mile wide at tide-level and there is no 

 evidence that it could have resulted from non-deposition of the 

 drift. The bottom of the present channel has a maximum 

 depth of 100 feet below tide-level. 



It seems highly improbable that the present navigable 

 channel of the Hudson could have been excavated to its 

 present depth in the Champlain deposits by any agency except 

 that of a river current,* and taking the maximum depth of the 

 channel in the Narrows as an example of this erosion we have 

 the amount of post-Champlain subsidence suggested as about 

 100 feet in the vicinity of New York. 



Observations on the coast of New Jersey and Long Island 

 have well established the fact of recent subsidence which can 

 be measured to the extent of 20 feet, by submerged tree 

 stumps. The evidences of fiuviatile erosion in the Hudson 

 valley suggest that this may be not more than one-fifth of the 

 total amount. 



From the evidence quoted it may be stated provisionally 

 that after the retreat of the continental glacier from the Hud- 

 son River valley, the land stood for a long time at a lower 

 level than at present. What the maximum of depression 

 amounted to is not known but in the vicinity of Albany the 

 minimum depression amounted to about 340 feet and at New 

 York to about 80 feet. Next occurred a gradual elevation of 

 the land amounting to about 180 feet at New York and at 

 Albany to an amount undetermined, but probably not less than 



* See J. D. Dana, this Journal, vol. xl, p. 435. 



