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Translations of Encyclopedia about Geology

 

Coasts and Shores

Coasts are ever-changing boundaries between the ocean and the land. With a length of almost half-a-million kilometres, the coastline is a very important area for people. More than a half of the world population lives within a few kilometres of a coast.

Coasts change relatively quickly as far as their shape and boundaries are concerned. This phenomenon is due mainly to the fluctuation of the sea water level and to the geological changes of the land. In addition, wind and waves, together with the related erosion and sedimentation, as well as currents and the tide, all join forces in shaping the shores. If we are to classify shores, we may divide them into primary and secondary coasts.

Primary shores are those that are not exposed to a major marine impact. One of them is ria (narrow, long inlets, more shallow than fjords), fjords, landslides, as well as tectonic and volcanic phenomena. Secondary shores include coastal areas reshaped by marine sediments into the form of a coastal scythe, or shores created by the corals or mangrove vegetation. In general, however, the destructive activity of the sea is the one that affects the coastal areas most. In some areas, human efforts may prevail and reclaim a portion of the land.

Strong storms may change the landscape of a beach very quickly, but a much longer-lasting effect is that caused by endless waves. Here, the direction, length, and height of the waves play an important role. Waves are generated by the wind. Waves travel across the sea as surf, gradually losing their strength, until they reach the shore and crash into it. The profile of the shore determines the impact of the waves.

If waves of long wavelength (wave length = horizontal distance between to wave crests or wave troughs) crash into a gently sloping shore, they gradually lose their energy and are absorbed by the sandy material. The churning that results carries away from the beach only a small amount of material and, on the contrary, helps enlarge its surface.

Shorter wavelength waves occur near steeper shores. They cause heavy erosion and since the resulting churning is stronger, it washes away more material. Shores shaped by short wavelength waves are generally steep, while long wavelength waves create more gently sloping shores.

The motion on steep shores (coastal cliffs) is in the direction of the land. When surf crashes against the cliffs, it loosens sand and small rock fragments, which are thrown back on the shore by the next wave. The so-called "hollow troughs," which can be observed in Helgoland, form in the company of cracks produced by freezing and by impact of chemical processes.

Less solid material settles as till at the foot of the cliff. Fragments breaking off of the cliff often fall down and are washed away by the surf. When the rocky shore is so heavily eroded by the surf that it is accessible only during high tide, it becomes a complete cliff, steep dead shore.

On gently sloping shores, the impact of the sea is not so destructive. However, if due to tectonic processes such as when the shore is lowered by sinking, storm surges may flood or tear away large areas of the shore. This is how a major part of German and Netherlands shores were formed. People also contributed to the shaping of these shores, by overexploiting the peat deposits found in the coastal region, which led to the lowering of the ground level.

There are various measures for the preservation of coastal regions. On high shores, where strong surf crashes directly onto the cliffs, the protection is very difficult. People used to drive iron posts into the shallow seafloor to decrease the strength of the surf. The effect of this measure, however, was relatively short-lived due to the impact of the salt and fine coastal sand. Today, people built in the sea stone dykes in parallel to the coastal line. These dykes slow down the coastal currents, help create rock sediments, and prevent the washout of rock fragments falling down from the cliffs. Another way to protect the coastline is to fill plastic sacks with sand and deposit them in the sea, thus stopping the sand from being washed away. The plastic is later protected by quickly-reproducing marine algae.

The ocean forces on one hand, and the tectonic structure and the material and movement of the land on the other, shape different types of shores.

Steep and gently sloping shores are classified by their profile. In areas of steep or cliff shores, the land descends steeply into the sea. Heavy surf breakers impact the shore directly. As a result, the shore gradually retreats. Crags, caves, and coves form. The so-called abrasion platform gradually emerges. It gently rises towards the land. Steep coastlines often form mountains.

Low shorelines descend gradually into the sea. The waves slide slowly towards the coastal zone. They bring and deposit gravel and sand, creating a sandy beach. When the churning force of the retreating seawater is not strong enough to wash away all the sand, it leaves behind small sandy barriers. When the deposited sand dries, the wind carries it, creating coastal dunes.

When the prevailing direction of the wind is transversal to the coast, that is the way the waves roll towards the coast. Water always looks for the shortest way to retreat back into the deep waters, thus it flows back in perpendicular direction to the coast. The following wave again crashes on the shore at an angle as the previous one, which creates a zigzag motion of the water. Washed away sand particles are also often carried some kilometres alongside the shore. This movement is called shoreline drift. The retreating shore gradually allows the formation of a bay. More and more sand is deposited alongside the old line of the shore, forming a sandbar that extends into the sea. If it reaches the opposite shore, this bay or sound is separated from the sea. This is a shore or beach lake. Newly-created shoreline is called the "balancing" shoreline.

If a river empties into such a bay, the opening towards the sea remains open as a result of its current. The sandbar in this case is called scythe, and the bay becomes an inlet or a lagoon. An example of this form is found in the Gdansk Inlet or in the southern Baltic Sea, near Rujana.

Fjord coastlines formed on seacoasts when massive ice streams travelled on the land. Glaciers dug deep valleys, which were later flooded by sea when the land sank. Fjord coastlines are found, among other places, in Norway, Scotland, and Alaska.

Rocky coastlines were also formed by glaciers, however, they are low rocky shores in front of which there are small circular islands called "sher." Sher coastlines are found between Finland and Sweden. Many shers are immediately underneath the water and represent a great danger for the ships.

Ria-type coastlines are narrow sea bays, which were formed when the land submerged. These coastlines form where the mountain ranges and their dividing valleys are situated at right angle to the coast, and where the sea penetrates far into these valleys. An example of this type of mountain ranges is found in Brittany and Ireland.

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