Cliffed coast

A cliffed coast, also known as an abrasion coast, is a type of shoreline where the constant pounding of marine waves carves out steep cliffs, which may or may not be vertical. This stands in contrast to flat or alluvial coasts. The process begins with waves relentlessly attacking the base of the cliff, cutting a notch through abrasion. As this notch deepens, the rock above becomes an overhang that eventually collapses under its own weight. The resulting debris is then carried away by the sea, causing the coastline to gradually retreat inland.

The speed of this retreat varies widely. It depends on factors like the power of the surf, the cliff’s height, how often storm surges occur, and the hardness of the bedrock. For instance, the Mecklenburg coast in Germany loses about 25 centimeters per year, while the chalk cliffs of southern England erode by only half a centimeter annually. A cliffed coast can form in loose material, such as the Red Cliff on Germany’s island of Sylt, or in hard rock like the red sandstone cliffs on Heligoland, though these types show some differences in coastal features. In the Atacama Desert, the extensive coastal cliffs originated from the retreat of a fault scarp, meaning they no longer follow any geological fault line.

On rocky cliffed coasts made of resistant stone like sandstone, limestone, or granite, a flat wave-cut platform (or abrasion platform) forms at the cliff’s base, preserved at or below the water table. If the coast is tectonically uplifted, these platforms become raised coastal terraces, allowing scientists to calculate the uplift by measuring their elevation relative to sea level, while accounting for eustatic changes. On coasts made of less resistant material, a beach forms instead of a wave-cut platform. When waves carve notches on both sides of a narrow promontory, a natural arch may develop. If the arch collapses as the coastline recedes, a stack remains on the wave-cut platform. A famous German example is the Lange Anna on Heligoland; in England, Old Harry Rocks in Dorset is a notable one. On rocky cliffed coasts, general weathering of the bedrock is almost as important as wave action in driving retreat.

Coasts are classified as "living" or "dead." Living cliffs are still actively eroding and receding. Dead cliffs are only reached by very high waves and change very little; a clear sign of inactivity is vegetation covering the cliff face as wave action diminishes. In Germany, living cliffs include the Red Cliff on Sylt and the chalk cliffs of the Jasmund Peninsula. The Königsstuhl on Rügen is a good example of a dead cliff. Other dead cliffs can be found a few kilometers inland along the present-day Wadden Sea coast, marking an old shoreline left behind when the North Sea’s water level fell.

Steep sea cliffs can also result from catastrophic debris avalanches, which have been common on the submerged flanks of ocean island volcanoes, such as those in the Hawaiian Islands and the Cape Verde Islands.