V ALL FA' HARBORS 1 67 



commerce of the stream. Tluis these modern imjjrovements 

 in the way of towboats and jetties promise to avoid all reall)- 

 serious inconveniences connected v\ith delta harbors. 



Ports of the class which we are now to consider resemble 

 those just described, owing their origin to the work done by 

 considerable rivers. In every other [)articular, however, their 

 features are peculiar. The havens belonging in this second 

 group are formed where a river valley was carved in the land 

 at a time when the surface of the country in which it lies was 

 at a considerably higher level than at present. The depth 

 and form of the gorge so excavated by the flowing stream 

 may have varied greatly. In most cases we may assume that 

 there was a delta at its mouth like those formed at the mouths 

 of all great rivers along shores which have not recently sunk 

 down. When tlie coast line subsided, say to the depth of 

 one or two hundred feet, the sea was permitted to enter 

 the valley for a distance dependent upon the degree of its 

 slope and the amount of the downsinking of the land. Under 

 these conditions the path of the river next the sea is con- 

 siderably shortened, and at the head of the ba\- thus formed 

 the stream begins to make a new delta, which in time may 

 completely close the reentrant formed by the subsidence. 



At first sight it may seem as if this were an improbable 

 means whereby harbors could be formed, but the fact is that 

 a large number of the great commercial ports, both in this 

 country and in Europe, owe their origin and much of their 

 value to such vertical movements of the land. The most im- 

 portant havens on the Atlantic coast from New York to the 

 Rio Grande, and probabl\- man\- of those on the Pacific shore 

 of the United States, doubtless owe their formation to this 

 subsidence of the shore lands. Delaware and Chesapeake 



