Where the range of tide is considerable, the reefs are frequently broken by inlets. Through these the water of the mainland streams finds access to the sea. The shapes and depths of these inlets are in some cases so rapidly altered by the tidal currents that it is impossible to foretell for any length of time where vessels can find the best channel. Thus the inlets must be left uncharted and local pilots relied upon. Bars which cannot be crossed except at high tide often make moon time, not sun time, the determining factor in the sailing schedules of vessels leaving and entering port. The general set of the shore currents has an effect upon the position and shape of these inlets, deflecting the openings in the direction of their flow. They may also singularly modify the outlines of the reefs themselves as in the formation of the three much dreaded capes off the coast of North Carolina.
144.Depressed Coast.
Experiment 127. -Cover a small board with a piece of thin oilcloth which has been most irregularly crumpled. Take the board by one edge and inclining it slightly gradually submerge it in a dish of water. What kind of a line does the water form where it meets the oilcloth? In what way would this line change if the oilcloth were more crumpled? If it were less crumpled? If theboard is more submerged, does the position of the water line change? Why does its form materially alter?
Along a coast which has been depressed, the shore line has moved landward and a surface rendered irregular by erosion is lapped by the inflowing water. All the irregularities which lie below the water level are filled with water and the shore line bends seaward aroundPART OF THE COAST OF MAINE.
A fine example of depressed coast.
the projecting elevations and landward into the gullies and valleys. Isolated elevations surrounded by land low enough to be covered by the inflowing water have nowbecome islands.
The river valleys which crossed the region now submerged reveal themselves only to the sounding line, their landward extensions forming estuaries up which the tide sweeps far into the land. Theirunsubmerged portions contain A NORWAY FIORD.
fresh-water streams, the size of which seems insignificant when compared to the size of the estuary. Sheltered coves and harbors abound, affording protection to all kinds of crafts and fitting these coasts to be of great commercial importance.
The harvest of the sea replaces what might have been the harvest of the land. The distance along the coast between two points is much longer than the straight line distance over the sea. The boat, not the wagon, becomes the important vehicle of travel. Many submerged coasts, such as those of Maine, Alaska and Norway, have been modified by ice action. Their valleys have been smoothed and rounded.
A NORWAY FIORD.
Showing large vessels anchored near the shore.
A NORWAY VILLAGE.
At the head of a fiord.
In Norway the deep fiords conduct the sea from the island-studded coast far into the interior. Their sides rise steeply, sometimes for several thousand feet from the water"s edge and descend so steeply below it that large vessels can be moored close to the shore. Generally there is not sufficient level land along the sides of the fiord for building roads. The villages are usually situated where a side stream has built a little delta, or at the heads of the fiords where the unsubmerged portion of the valley begins.
These U-shaped valleys with their small streams extend back to the interior uplands, sometimes blocked toward their heads by descending glaciers. They often have hanging valleys which enter far up along their sides, the streams of which descend by abrupt falls and adorn the dark rock walls with bands of silver spray.
It was such a coast as this which bred the ancient Northmen, to whom the Sea of Darkness, as they called the Atlantic, was terrorless. While less favored and hardy sailors were dodging from bay to bay along the shore always in sight of land, they were pushing boldly west, guided only by the beacons of the sky, and discovering Iceland, Greenland and the American continent.
145.Harbors. -The importance to mankind of good harbors can-not be overestimated. No civilized country by its own products can fill all the wants of its inhabitants. Since earliest times man has been a barterer of goods. The sea offers him an unrestricted highway for his traffic. Harbors he must have to load and unload his wares safely.
Although many of the best harbors of the world are found along depressed coasts, such as the harbors of New York, San Francisco, London, Liverpool and Bergen, yet there are several other sorts of harbors. The delta of a great river may afford a good harbor, as those of New Orleans and Calcutta. Harbors may be formed by sand reefs and spits, like those of Galveston, Provincetown and San Diego. The atolls of the mid-Pacific and even the submerged craters of volcanic islands afford safe resting places where ships may ride out the storms.
146.The Safeguarding of Coasts. -As nations advance in civi-lization and their commerce develops, they realize the necessity of safeguarding in every way possible the ships bearing their citizens and their wealth. Thus a great system of weather signals, of lighthouses and of lifesaving stations has been established. From these mariners may know when it is safe to leave port, may be warned off from danger- ous shores, and, when in spite of all precaution their vessels become wrecked, may be rescued from a watery grave.
The lighthouses have lights of different kinds and colors, some fixed, some flashing, so that when unable to make out the coast itself, the mariner can still know his position by the kind of light seen. Indeed many wireless telegraph stations are being equipped to communicate with vessels at sea and to inform them of their position, the condition of the shore and the expected weather. Even the kinds of materialSAN FRANCISCO HARBOR.
A harbor due to depression of the coast.