A sea cliff is a steep, vertical slope of rock or earth found at the edge of the ocean. These dramatic coastal features result from the continuous interaction between the landmass and the powerful forces of the sea. Their shape is governed by the underlying properties of the rock and the erosive work performed by ocean waves. The formation of a sea cliff is an ongoing process of destruction and retreat that reshapes coastlines over geological time.
Geological Conditions Required for Formation
The presence of a sea cliff depends on specific geological conditions within the landmass. A primary requirement is that the coastal material must be relatively resistant to erosion, such as hard sedimentary rock, granite, or volcanic basalt. These rock types erode slowly, allowing the steep profile to persist against the constant attack of the waves. Softer materials, like clay or unconsolidated sediment, tend to form gentler slopes or bluffs rather than true vertical cliffs.
The land structure must also provide significant vertical relief near the shoreline. This height is often established by tectonic processes, such as the uplift of continental plates. Within the rock itself, pre-existing structural weaknesses, including joints, bedding planes, and faults, are a major factor in cliff development. These inherent fissures are preferentially targeted by erosional forces, allowing the sea to exploit the rock’s vulnerability.
The Active Forces Shaping Sea Cliffs
The actual sculpting of the cliff face is carried out by three primary active processes concentrated at the cliff’s base. The most direct mechanism is hydraulic action, which involves the sheer power of moving water. As a wave crashes against the rock, it forces water and air into existing cracks and crevices.
The trapped air is instantly compressed by the wave’s force, exerting immense pressure on the rock walls. When the wave retreats, the compressed air rapidly expands, loosening and dislodging rock fragments. This continuous cycle gradually widens the weaknesses, destabilizing the cliff face from within.
A second mechanism is abrasion, sometimes referred to as corrasion, which acts like natural sandpaper. Waves pick up and carry sediment, such as sand, pebbles, and larger rocks, and hurl them against the cliff base. The grinding and scraping action of this debris physically chips and wears away the solid rock. This process is highly effective because the abrasive tools are constantly renewed and propelled by wave energy.
The third active force is solution, or corrosion, a chemical process that affects specific rock types. Seawater contains weak acids that can chemically dissolve soluble rocks, such as limestone or chalk. This chemical breakdown steadily weakens the rock structure, particularly where the rock is frequently saturated. The combination of these three processes focuses wave energy at the cliff’s base, leading to concentrated erosion.
Features Created by Cliff Erosion and Retreat
The combined effect of hydraulic action, abrasion, and solution forms a wave-cut notch at the base of the cliff. This indentation forms near the high-water mark, where erosional forces are most concentrated. As the notch deepens, the mass of rock above it becomes unsupported and unstable.
Eventually, the overhanging rock collapses under gravity, and the debris falls to the base. This material is carried away by the backwash, exposing the cliff base to renewed erosion and causing the cliff line to retreat inland. The continuous repetition of this notching and collapse sequence results in a gently sloping, rocky surface extending seaward, known as a wave-cut platform.
The relentless erosion of headlands creates a sequence of other dramatic features. Erosion along lines of weakness can excavate a sea cave at the base of the cliff. If this cave erodes completely through a narrow headland, it forms a sea arch. Continued erosion of the arch will eventually cause the top to collapse, leaving an isolated column of rock standing offshore called a sea stack.