There are many places in the world where the sea has cast up sand-bars and beaches, and has changed the whole face of the country. These travelling beaches and growing sand-bars sometimes close up the rivers, and sometimes turn bays into lakes, and these lakes in time turn into dry land. The great South Bay, on Long Island, is one of these places where great changes are going on; the meadow back of Chelsea Beach, near Boston, is another.
When a beach makes trouble for a river, the river behaves very strangely. At first it is quiet, and does not say much. It rests awhile, as if to gain strength, and then some day it makes a grand rush, and tries to break down the barrier the beach has thrown across its mouth. If it fails, it turns aside and goes out another way; but it soon settles down into a kind of sullen silence. It seems to be discouraged, and instead of a swift and pleasant river, it turns into a sluggish stream that does not seem to care for anything except to creep along in a lazy fashion.
Now a great and wonderful change begins. Before, it was swift and muddy. Now, the dull water begins to grow clearer, and the mud and fine sand in the water sink softly down to the bottom. The water spreads wider and wider on each side, and instead of a river running into the sea, there is a broad pool or lagoon behind the beach. Then month by month, year after year, the river brings down the mud and sand from the country and drops them far and wide over the broad salt-water lake.
Perhaps the beach in cutting off the river shut in a part of the sea, so that there are fish and oysters, sea-mosses and crabs, shut in behind the beach. They do not seem to care. They grow all the better in the still water, safe from those terrible waves that used to tear them from the sand in storms. The oysters find the quiet water a good home, and they grow there by millions on millions. As the old fellows die or are killed by the star-fish, the young oysters build their homes on top of the shells of their fathers. Millions of other fish, hermit-crabs, lobsters, and clams, live and die there, and they too cover the bottom of the lagoon with their dead shells. Thus it happens that even the fishes begin to fill up the place by covering the bottom with their empty houses.