141.Final Effect of Wave and Current Action. -Whether the coast is shallow so that the storm waves spend their force off shore, or deep so that they batter the shore with their full strength, the tendency is to straighten the coast line. In the former case sand reefs with gently flowing outlines are built, and in the latter case the headlands are cut away by the waves and the material moved along by the coast currents to fill the protected bays and coves. As a?rial erosion is constantly tend- ing to smooth off the irregularities of the land surface, so the waves and currents of the ocean are constantly straightening out the water margins.
Uniformity seems to be the goal for the erosive forces. But when the results of surface erosion are brought to the sea by the rivers, deltas are formed which interfere with the straightening of the coast. If the material brought by the rivers is sufficient, a delta is built out into the sea in spite of the action of the waves and currentsA BEACH AT CATALINA ISLAND.
Notice how the water is smoothing out the irregularities of the land.
and the coast line becomes increasingly irregular. Lake shores are acted upon in a similar way and with similar results, only the forces here are less powerful than those that act upon the sea coasts.
142.Instability of Sea Coasts. -It often happens that in makingexcavations at a considerable distance from the sea, shells are dug up very similar to those now found on the shore. Some inland strips of land are found composed of sand and pebbles like a beach and having the beach slope. Sometimes tree trunks standing with their roots in the ground just as they do on the dry land are seen at a considerable depthINLAND SEA CAVE AND BEACH.
This coast has been recently elevated.
in the sea. It can be proved that an old temple near Naples, Italy, has stood above and then in the sea more than once since it was built.
Facts like these show that the sea coast is not stable, but subject to upward and downward movements, some of which are slight and some great. If the land near the sea rises, the coast line is moved into the area which was formerly covered by the water and if the land sinks, a new coast line is formed where before the hills and valleys of the land appeared. Changes of this kind have a marked effect upon the outline of the coast and upon the industries of its future inhabitants. TheTEMPLE OF JUPITER NEAR NAPLES.
Although it can be proved that this coast has been elevated and depressed several times, so gradual has been the movement that the pillars have not been overturned.
coastal plain which borders a large part of our Atlantic coast shows the results of an upward movement. The fiorded coast of Alaska affords an example of a downward movement.
143.Elevated Coast.
Experiment 126. -Tack enough sheet lead to a very rough board so that it will remain submerged when placed in water. Place the board in a shallowdish of water, lead side down. Taking the board by one edge, gradually lift this edge above the water surface. What kind of a line does the water form where it meets the board? In what way would this line be changed if the board were smoother? If it were rougher? If the edge of the board is lifted higher, does the position of the water line change? Does its form materially alter?
The main characteristics of a coast which has been elevated, that is, of one in which the shore line has moved seaward, will readily suggest themselves to any one who considers what has taken place. Soundings show that the continental shelf has a comparatively smooth surface. Thus the water will meet the land in an almost straight line and the deepening of the water off shore will be gradual. The material forming the shore both belowand above the water line will be easily eroded, since it has been recently deposited and has not had time to consolidate.
Waves rolling in upon the shore will strike the bottom at aconsiderable distance off shore.
SAND DUNES FORMED UPON A SAND BAR.
Thus the water rapidly loses its velocity and its power to carry eroded material shoreward, so it builds at a distance from the shore a sand reef inclosing a lagoon. The currents caused by the prevailing winds and the tide flowing along the outside of this barrier give it for long distances a smooth outline, sometimes almost straight and sometimes gently curving. Dunes are formed upon these bars. In time landward-blown sand, together with the silt brought by the streams from the mainland, may fill up the lagoon.
The filling of these lagoons, both naturally and artificially, has considerably increased the habitable land of the earth. The inclosed waterway back of the sand reefs has in some places rendered coastwise traffic safe and easy. It is proposed artificially to extend and develop certain of these inclosed water areas along the eastern coast of the United States so as to form a protected waterway from New Englandto the southern ports. At present the low, almost featureless shore of this region, with its shifting sand bars and capes, makes coastwise navigation dangerous, although it is protected by many lighthouses and life-saving stations.
The sand reefs along the southern Atlantic and Gulf coasts have in some places attained sufficient width and height for considerable settlements. The tidal inlet, the sea-beach resort, and the commercial city with reef-protected harbor are natural results of receding shore lines. In time the waves, by their own erosive action, will deepen the bottom off shore from the reef enough to enable them to attack its front. Thus they will drive it back over the inclosed lagoon, destroying their own work and attacking the shore, which for a time they had shielded against their own rapacity.
