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Acliffed coast, also called anabrasion coast, is a form ofcoast where the action of marine waves has formed steepcliffs that may or may not be precipitous. It contrasts with aflat or alluvial coast.
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In coastal areas in which the land surface dips at a relatively steep angle below the water table, the continuous action of marine waves on the coastline, known asabrasion, may create a steep declivity known as acliff, the slope angle of which depends on a variety of factors including the jointing, bedding and hardness of the materials making up the cliff as well as the erosional processes themselves.[1][2] The slope is constantly being eroded. The waves attacking the cliff-foot form awave-cut notch by constant abrasion action producing an overhang. This overhang grows in size as the cliff is undercut, until it collapses under its own weight. The loose debris that has broken off is gradually carried away from the area in front of the cliff by the action of the sea. As the coastal cliffs collapse, the shoreline recedes inland. The speed at which this happens depends, in particular, on the strength of the surf, the height of the cliff, the frequency ofstorm surges and the hardness of thebedrock. Thus, theMecklenburg coast inGermany recedes by about 25 centimetres per year, whereas the chalk cliffs of southern England retreat by just half a centimetre each year. A cliffed coast is made of a loose bedrock material, such as at theRed Cliff on the German island ofSylt, but can also occur in hard rock like the red sandstone cliffs onHeligoland. There are, however, differences between the former and the latter regarding some peculiarities of the coast line.
In the case of the large and widespreadcoastal cliffs of Atacama Desert the modern cliffs originated from a process ofscarp retreat of afault scarp, thus at present the cliffs do not follow anygeological fault.[3]
On a rocky cliffed coast made up of material which is relatively resistant to erosion such as sandstone, limestone or granite, a flat rockywave-cut platform or abrasion platform is formed in front of the cliff. It represents the foot of the cliff preserved at and below the level of water table. If there is atectonic uplift of the coast, these abrasion platforms can be raised to form coastal terraces, from which the amount of uplift can be calculated from their elevation relative to the sea level, taking into account anyeustatic sea level changes. On a cliffed coast made up of material which is only fairly or even hardly resistant to erosion no wave-cut platform but a beach is formed in front of the sea cliff.
If waves carve notches at a narrow point on both sides of a promontory on the rocky cliffed coast, anatural arch may be formed.[4] When the arch collapses as the coastline recedes further astack is left behind on the wave-cut platform. The best-known example in Germany is theLange Anna onHeligoland, while, in England, a prominent example areOld Harry Rocks inDorset.
Furthermore, on a rocky cliffed coast wave action is not the only driving force for coastline retreat. General weathering of the bedrock is almost equally important.[2]
"Living cliffs" are those on a coast that is still active, i.e. that is being eroded and is receding. A "dead cliff", by contrast, is only reached by very high marine waves and is therefore subjected to very little change. A clear indication of a lack of activity at a dead cliff is a covering of vegetation that appears on the cliff as wave action against it subsides.
Well-known coasts with living cliffs in Germany are the Red Cliff (Rote Kliff) inKampen on the island ofSylt or the chalk cliffs on theJasmund Peninsula. TheKönigsstuhl on the island of Rügen is a good example of a dead cliff. Others may be found in the regions of the present-dayWadden Sea coast of theNorth Sea a few kilometres inland. These show the former coastline from which the sea retreated as the level of water in the North Sea fell.
Steep sea cliffs can also be caused by catastrophicdebris avalanches. These have been common on the submerged flanks of ocean island volcanos such as theHawaiian Islands and theCape Verde Islands.[5][6]
