316 PHYSICAL FEATURES OF THE UNITED STATES 



through which ships may enter. The waves, buffeting the coast 

 and drifting sand and gravel to and fro, endeavor to clog the 

 riverway with submerged bars, making the water too shoal for 

 shipping. Over small rivers the waves are victorious, and 

 unless engineers cooperated with the rivers the entrance-ways 

 are sealed. Large rivers overpower the waves and clear their 

 channels faster than the waves can clog them. Only one of our 

 rivers, the Mississippi, has proved competent to maintain its 

 channel to the sea, but .that affords a harbor of peculiar value, in 

 that it is connected with a system of inland navigation hundreds 

 of miles in extent. 



The fiord harbors associated with prehistoric ice-fields are an 

 important group. The ice descended to the shores of both oceans, 

 and by its remodeling of the surface left steep slopes with a tor- 

 tuous contour, creating a great abundance of deep harbors. New 

 England at the east and Washington at the west are thus en- 

 dowed, and their maritime commerce requires neither piers nor 

 dredges to maintain its natural channels. 



Natural harbors of a third class are connected with vertical 

 movements of the land. When the margin of the continent is ; 

 lifted the coast line, following a slope new-risen from the sea, is 

 a simple contour on an even plain, and there are no harbors ; 

 but when the land is deprest the sea-water enters each valley of 

 the coastal plain, making a bay. Then the waves, driving sand 

 and other land waste along the coast, build a spit across the 

 mouth of each bay, converting it into a sheltered harbor, whose 

 entrance is scoured four times a day by the incoming and out- 

 going tide. Into the estuaries thus formed the streams build 

 deltas, gradually filling and obliterating them ; but so long as 

 subsidence continues they remain open and available for com- 

 merce. It is our good fortune that nearly the whole of our coast, 

 both Atlantic and Pacific, is now subsiding,* so that estuaries 

 are numerous and the maintenance of serviceable harbors re- 

 quires onl}' - moderate aid from the engineer. The bays and 

 sounds of San Francisco, Galveston, Mobile, Tampa, Savannah, 

 Charleston, Wilmington, Pamlico, Chesapeake, and Delaware are 

 of this type; and the Hudson estuary, which is also a fiord, 

 carries tidewater one hundred and fifty miles from the coast. 



Climatically the United States lies within the zone of variable 

 winds. Instead of being swept by continuous trade winds or 



* Strictly speaking, the determined fact is that the relation of land to sea is chang- 

 ing, and we do not know which one actually moves. 



