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.
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.
HOW THE SEA TAUGHT SHORE ENGINEERING TO MEN
This is a salt marsh at mid-tide. How the sea itself adds such regions to the dominion of the land, and how human engineers, taking the hint, have put the sea to work, you will learn in this chapter.
The sea also works with the busy little corals in building reefs and islands. Corals can only live and build where the water is kept in constant and vigorous motion by current and wave. From the air imprisoned in the bubbles by the stirring and turmoil of the waves and particularly from the air in the white foam of the crests these little people get their oxygen. At the same time they absorb out of the water the food on which they grow. The sea not only feeds these little wards of its bounty during their busy lives, but extends their usefulness after death, either by cementing to the reef the coral, ground up by the waves, or in storms scattering it over wide areas, to be made later into the finest of limestone; and still later into the best of soils.
FATHER NEPTUNE FEEDING THE CORAL PEOPLE