CHAPTER 9
COAST LINES
138.Coast Lines. -On examining a map or chart of an extended coast one cannot fail to be impressed with its irregularity. Although it may extend for long distances in an almost unbroken straight line， as along the southeastern part of the Bay of Biscay， yet if we follow evenPOSITANO.
A precipitous and densely populated coast.
this coast far enough in either direction， it becomes markedly irregu- lar. Islands composed of the same material as the coast itself are often found near， separated from it only by shallow arms of the sea. These are evidently a part of the adjacent mainland with a submerged portion between.
Sometimes in warm latitudes where conditions are suitable， the coast is bordered for a long distance by coral reefs. The northeast coast of Australia affords a marked example. Here a barrier reef extends for a thousand miles along the coast， protecting it and leaving a smooth water channel 10 to 30 miles broad for coastwise navigation.
NORTH CAPE.
A famous promontory which defies the waves
of the North Atlantic.
A coast in warm regions may also be protected by the growth of dense thickets of mangroves. This tree grows in shallow water and sends down from its branches roots upon which crabs and oysters live and among which mud and débris accumulate. Thus low shores are slowly being built out into the sea. Shore borders of this kind are found extending many miles along the coasts of southern Florida and Texas and in many other places.
FINGAL"S CAVE.
In extreme northern and southern latitudes the coast m a y b e f r i n g e d f o r l o n g distances with ice. In middle latitudes， however， the coast is generally composed of rock， sand or mud and is sometimes protected by a thick seaweed mantle. Here， besides the usual forces oferosion and deposition found active on the land， are the forces of the waves and currents.
139.Wave Cutting. -Wherever the waves strike on an unprotect-ed shore， they wear it away. The rapidity of the cutting and the forms carved depend upon the strength of the waves and the kind of shore. Wherever there is a point of weakness along the shore， there the wavescut back more rapidly. The harder parts stand out sharply as points and promontories. In some cases the waves cut back so rapidly on lofty coasts that high cliffs are formed.
If the material of the coast does not readily break off when undercut by the waves， a sea cave may be formed. Such is the well-knownFingal"s cave on an island off the coast of Scotland where the structure of one of the igneous rock layers allows the waves to quarry it comparatively easily. Spouting holes and caves are usually due to an easilyAN ELEVATED ROCK BENCH.
eroded place in the shore
where the waves are able to cut back a somewhat horizontal tunnel and by their impact upon the end of the excavation form an opening to the surface through which spray is ejected. The hole may be at some little distance from the shore.
Since waves have the power of cutting only to a small depth， it may happen that an exposed rocky coast will have a bench cut along it， under the surface， backed by a sea cliff against which the waves are still cutting. If such a coast becomes elevated， the rock bench will appear with a cliff terminating it on the landward side. If a coast stays at the same elevation long enough， or if its material is easily eroded， large areas of what was formerly dry land may be cut away and brought under the sea.
In 1399 Henry of Lancaster， afterward Henry IV of England， returned from his exile and landed at Ravenspur， an important town in Yorkshire， to begin his fight for the crown. A person disembarking at the same place to-day would be so far from shore that he would need to be a sturdy swimmer to reach the beach. The entire area of the ancient town has been cut away by the waves and now lies under the sea. This is an example of what has occurred in many sea coast regions.
140.Beaches and Bars. -Unless the material pillaged from the land by the waves falls into too deep water， it is buffeted about by them and broken and worn into small pieces. These are then borne along by the shore currents until they find lodgment in some protected place where they can accu- mulate. When sufficient material has been accumulated， the storm waves and the wind sweep some of it above sea level and fringe the water"s edge with a border of waterworn sand and pebbles. These accumulations of shore drift are called beaches.
The incoming waves are constantly
A LAKE BEACH FORMED
BY STREAM AND WAVE ACTION.
A year after this picture was taken a landslide formed a wave which swept away the entire beach and village.
sweeping in material from the shallow bottom against which they strike， and the returning undertow bears its load seaward. Except in time of great storms the accumulation of material along a beach is at least equal to the wearing away.
NAHANT， MASSACHUSETTS.
A land-tied island.
The currents moving the loose material with them som eti m es for m i t i n t o bars which inclose or tie islands to the mainland or extend into the sea free ends， forming what are called spits. A famous example of a land-tied island is that of the great English fortress at Gibraltar； although now a promontory， it was once an island detached from the coast of Spain. Shifting sand bars， especially if coveredwith water， are exceedingly dangerous to vessels， and coasts where these are abundant need especial protection by lighthouses and life- saving stations. The greatest Mediterranean port of France during the thirteenth century， Aigues-Mortes， has been closed in by sand bars so that there is no longer access to the sea and only the relics of the former great city now exist. Thus have the moving sea sands overthrown the plans of men.
A SAND SPIT.
Formed by the waves and currents.