Cliffed coast

A cliffed coast, often called an abrasion coast, is a coastal landscape defined by steep walls.

These cliffs may be very steep or slightly less precipitous. They form in areas where the land surface dips at a steep angle below the water table. The shape of the coast depends on many factors. These include the hardness of the material and how the rock is layered. The specific processes of erosion also change the final appearance of the coastline.

The formation of these cliffs involves a specific sequence of events driven by marine waves. This process is known as abrasion, which is the continuous action of waves on the coastline. The waves attack the foot of the cliff constantly. This constant energy carves out a feature called a wave-cut notch. As the notch grows deeper, it creates an overhang of rock above the water. Eventually, this overhang becomes too heavy and collapses under its own weight. The sea then carries the loose debris away from the cliff face.

As these collapses happen, the shoreline undergoes a process called retreat. This means the coastline moves further inland over time. The speed of this retreat varies greatly depending on the environment. Factors like the height of the cliff and the frequency of storm surges matter. The strength of the surf and the hardness of the bedrock are also key. For example, the Mecklenburg coast in Germany recedes by about 25 centimetres per year. In contrast, the chalk cliffs of southern England retreat by only half a centimetre each year.

The type of rock present determines the specific features found at the coast. On rocky coasts made of resistant materials like granite, limestone, or sandstone, an abrasion platform forms. This is a flat, rocky wave-cut platform located at the foot of the cliff. It stays at or below the water table level. If the coast experiences tectonic uplift, these platforms can be raised into coastal terraces. On coasts made of less resistant material, a beach forms instead of a platform.

Wave action can also create dramatic shapes like arches and stacks. If waves carve notches into both sides of a narrow promontory, a natural arch is formed. When that arch eventually collapses, a tall pillar called a stack is left behind on the platform. Notable examples of stacks include the Lange Anna on Heligoland in Germany. In England, the Old Harry Rocks in Dorset are prominent examples. These features show how the coastline changes as it recedes.

Geologists also categorize cliffs as either "living" or "dead." A living cliff is an active coast that is still being eroded and receding. The Red Cliff on the German island of Sylt is a living cliff. A dead cliff is one that is only reached by very high waves. Because they are not constantly hit, they undergo very little change. A sign of a dead cliff is the presence of vegetation growing on it. The Königsstuhl on the island of Rügen is a well-known dead cliff.

While waves are a primary force, other processes can create steep sea cliffs. General weathering of the bedrock is almost as important as wave action. Some cliffs are even formed by catastrophic debris avalanches. These massive landslides are common on the submerged flanks of ocean island volcanoes. Examples of such volcanic islands include the Hawaiian Islands and the Cape Verde Islands. These events show that cliffs can form through sudden, large-scale movements as well as slow erosion.