Page:Popular Science Monthly Volume 13.djvu/678

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of the land or elevation of the water began in this region. Gradually the sea flowed in over its shores, crept up the valleys of the streams, checking their flow and converting them into tideways, until it washed the base of the highlands. Up to this time the surface of the littoral plain in its gradual submergence formed a broad expanse of shallow

PSM V13 D678 New york harbor in preglacial times.jpg
Fig. 6.—New York Harbor in Pre-glacial Times, from South End of New York Island.

water bounded by a monotonous line of beach, with no good harbors—a shifting, dangerous shore, such as is most dreaded by mariners. By further subsidence, however, the water flowed up into the valleys among the New York hills and into the deeper river-channels, making of the first safe, landlocked harbors, of the second navigable inlets or tide-ways. In this manner were produced the magnificent harbor and the system of natural canals connected with it, which determined the position and created the subsequent prosperity of the commercial emporium of the New World.

The subsidence which resulted in the formation of New York Harbor and its connections seems also to have affected all the coast, and the influx of the sea-water filled the valleys of the rivers which drained the Atlantic slope south of New York, and gave it the fringed and irregular outline which constitutes its most striking characteristic. James River, York River, the Potomac, the Susquehanna, and the Delaware, are, like the Hudson, half-drowned rivers—if we may use the expression—for all the lower portions of their valleys are estuaries, in which the tide sets up to the base of the highlands. But that portion of the littoral plain which separates these estuaries is too low, too much cut up with water-ways, and its harbors are too shallow and ill-