A wave-cut platform, also called a shore platform or abrasion platform, is a smooth, nearly flat expanse of rock found horizontally at the foot of a steep sea cliff. This coastal feature is created by the sustained erosional power of ocean waves. While often hidden beneath the water, the platform becomes visible as a broad, rocky ledge during low tide. It represents the surface upon which the cliff once stood, marking where the sea’s energy is most concentrated.
The Mechanics of Formation
The creation of a wave-cut platform begins with the formation of a deep undercut, known as a wave-cut notch, at the base of the cliff face, specifically between the high and low tide marks. This localized erosion is a result of mechanical and chemical processes working together to attack weaknesses in the rock, such as joints and bedding planes.
One primary mechanical process is hydraulic action, where the sheer force of waves striking the cliff compresses air within rock fissures. As the wave retreats, this compressed air rapidly expands, exerting pressure that weakens the rock structure and causes fragments to break away. Abrasion is equally important, involving the grinding action of sediment—such as sand, pebbles, and cobbles—carried by the waves and hurled against the cliff face.
The debris broken off by hydraulic action and abrasion is then subjected to attrition, a process where the rock fragments repeatedly collide with one another and with the platform surface. This action wears down the collapsed material, making it smaller, smoother, and more easily transported away by the waves and currents. Simultaneously, processes like chemical weathering and bioerosion by marine organisms further destabilize the rock.
As the wave-cut notch deepens, the overhanging rock becomes unstable and eventually collapses. This collapse causes the cliff line to retreat inland, leaving the newly exposed, flat rock surface at the base as the embryonic wave-cut platform. The repetition of notch formation, collapse, and debris removal systematically widens the platform over time.
Distinctive Physical Characteristics
The platform is rarely perfectly horizontal; instead, it exhibits a gentle seaward slope, commonly ranging from zero to about four degrees. This slight inclination allows the backwash of the waves to effectively carry eroded debris away from the cliff base and into deeper water.
The width of these platforms can vary significantly, but it is ultimately limited by the wave energy available for erosion. As the platform lengthens, it absorbs more of the incoming wave energy, causing the waves to break farther from the cliff. This reduction in energy means the rate of erosion at the cliff base slows down, effectively putting a brake on the platform’s widening process.
The surface of the platform is often sculpted with numerous smaller erosional features. These include depressions that collect water at low tide to form tidal pools, or rock pools. Linear grooves known as gutters may also be present, which channel water flow across the platform. The vertical extent of the platform is defined by the tidal range, with its upper limit near the high-tide mark and its lower limit extending down to or below the low-tide mark.
Relict Platforms and Geological Significance
Wave-cut platforms serve as valuable records of past environmental conditions. In certain regions, platforms are found significantly above the reach of modern waves, known as a relict or raised platform. These elevated platforms indicate that the relative sea level was once higher at that location.
The emergence of these relict platforms occurs either through a global drop in sea level, known as glacio-eustatic change, or through the tectonic uplift of the landmass itself. Some raised platforms exist many meters above the current mean sea level, often dating back to high sea-level stands during interglacial periods. The study of these features allows geologists to reconstruct the history of coastal evolution and the long-term patterns of sea-level fluctuations.
The presence of relict landforms provides a direct physical link to former climates and geological dynamics. By analyzing the elevation and age of these abandoned platforms, scientists can gain insights into the rates of tectonic movement and the complex relationship between land and sea over thousands of years.