Moreover the rivers, in the very act of wearing down the land and with it filling up the sea, help keep the land from being flooded, as it would be if something were not done. For, as we learned in the story of why the mountains border the sea the sediment poured in by the rivers helps raise the mountains and the land along the sea border. It is during the downward movement of the continental margins that most sediment is spread from the inpouring rivers because the dip of the land is greater and the swifter current not only cuts down the land faster, but carries the sediment farther out from shore. Here the new rock is made from old worn-out soil, and, since these new rocks when brought to the surface will in time decay, fresh soil is thus prepared for future generations. More immediate benefits of this sinking of shores and advance of waters are the harbors which have made great cities like New York and London, on or near the seacoast. These harbors are all the results of "transgressions," combined with the digging action of wave and tide. [Illustration: _Copyright by Underwood & Underwood_ STONE TERRACES FOR THE GANNETS This picture shows what the rising of the land and the architectural engineering of the sea did for the gannets on the coast of Canada. ] TAKING A HINT FROM THE SEA'S SHORE ENGINEERING But the sea builds shores as well as eats them. Its chief work in this line is the widening of the continental shelf by building it up with rock made of the sea's own grist from its shores, and the sediment poured in by the rivers. This work is not "delivered," so to speak, for millions of years, when the sinking shores begin to rise again, but the sea, in its wave work, does shore building of another kind that shows above the waters in the generation in which it is done. On wide, shallow beaches, storm waves break some distance from the shore, and, so losing their force, drop the sediment which they have stirred up, after carrying it forward only a little way. As a result of this repeated dumping, an embankment forms, broadening seaward in the middle and bending shoreward at the ends. A portion of the sea itself is finally cut out and enclosed by this embankment, thus forming a lagoon. Finally this lagoon is filled with material, washed from the land and by sediment brought in from the sea at high tide. Human engineers, taking the hint, now put the sea to work on similar undertakings of their own. An embankment is built enclosing an area of the sea; then the tides and the land wash do the rest. [Illustration: THE DROWNED RIVERS THAT HELPED MAKE ENGLAND GREAT Her fine harbors have helped to make England the great commercial nation that she is. Notice here the relation of her largest cities to the bay-like mouths of the drowned rivers and to the drowned valley north of the Isle of Wight. ] [Illustration: HOW THE SEA TAUGHT SHORE ENGINEERING TO MEN