492 A. Lindenkohl — Post-glacial Subsidence 



Delaware, Susquehanna, and Potomac, which all follow a 

 southern and anticlinal course before they take up the straight 

 road to sea. This uniformity in behavior of these four rivers 

 points to a common cause, and a slight tilting of the Atlantic 

 plain in a north and south direction suggests itself as the 

 readiest way to account for the southern deflection of the 

 rivers. 



Effects of existing currents on the sub-marine channel. — The 

 following table, giving the mean maximum strength of both 

 sets of tidal currents in nautical miles, has been compiled from 

 the latest observations for the purpose of testing the ability of 

 those currents to create channels outside of the Sandy Hook 

 bars. 



Tabid of Tidal Currents in the lower New York Bay. 



Mean Maximum Ebb. Mean Maximum Flood. 



face. Bottom. 



Surface. Bottom. Bur 



Narrows 19 0*9 1 



Fourteen Foot Channel. 1-9 0-9 1 



East Channel 2 2 0-9 1 



Swash Channel 2-1 0-9 1 



Main Ship Channel 2-3 0-8 1 



Outer East Channel .. . 2-1 1-2 1 



Gedney's Channel 2-3 9 1 



South Channel 2-2 0-2 1 



3 1-0 

 6 1-0 

 6 0-8 

 8 1-2 



8 1-2 



6 0-9 



8 1-0 



4 0-7 



It will be seen from this table that the ebb current is the 

 strongest surface current and maintains its velocity (1*9 to 2*8 

 knots) until the outer bars have been passed. . But its strength 

 at the bottom of the channels is less than half that of the sur- 

 face currents, and less than that of the flood current. The 

 flood current is essentially a deep current and retains at the 

 bottom, in spite of friction, nearly two-thirds of its surface 

 velocity ; it is the flood current's speciality to attend to the 

 scouring business. But we cannot realize that a current which 

 has but 1*2 nautical miles velocity at the places where it must 

 be supposed to exert its greatest strength, can have the power 

 to scoop out a channel forty-five feet deep and only a mile 

 wide at a distance of fifty miles from the coast. At the same 

 time the opinion is well-founded that the submarine channel is 

 the principal passage way for the tide to and from K"ew York 

 Bay, and that this almost ceaseless flow tends to keep the chan- 

 nel clear from encroachments, especially by that formidable 

 bank, the Cholera Bank on the New York side of the channel. 

 Often the tides may be reinforced by high seas produced by 

 continuous easterly winds off New York ; and although such 

 high tides are known to be very destructive along the whole 

 coast from Atlantic City to Fire Island, we have no reason to 

 believe their effect to extend to greater depths than fifteen 

 fathoms or to the depth of our channel. 



