A wave-cut platform is a flat expanse of rock surface found at the base of a steep sea cliff, most noticeable during low tide. This feature is a direct result of ongoing coastal erosion, representing the former base of a cliff that has since retreated inland. Geologically, these platforms offer a valuable record of past coastal dynamics, providing insights into historical sea levels. The creation of this landform is a physical process driven by the persistent energy of ocean waves acting upon the rock structure.
The Forces of Coastal Erosion
The process begins with the sustained action of two primary marine mechanisms: hydraulic action and abrasion. Hydraulic action involves the sheer force of water as waves crash against the cliff face, targeting weaknesses such as joints and bedding planes. This impact traps air within fissures, compressing it until the pressure dislodges small rock fragments, weakening the cliff structure from within.
Abrasion occurs simultaneously as the waves pick up and hurl loose sediment, such as pebbles and sand, against the rock. These particles act like a grinding tool, scraping and wearing away the cliff face. The effectiveness of both forces is concentrated in the intertidal zone, the area between the high and low water marks. This is where the waves repeatedly deliver their maximum energy.
Creating the Wave-Cut Notch
The erosion is not uniform across the entire cliff but is focused at its base, creating a specific indentation known as the wave-cut notch. This notch is a concave groove carved into the cliff face, typically extending only a few meters vertically and horizontally. The zone between the high and low tide levels receives the greatest concentration of wave energy, maximizing the impact of hydraulic action and abrasion.
As the waves repeatedly strike this narrow zone, the rock material is progressively removed, causing the notch to deepen and retreat into the cliff. This undercutting continues until the rock mass directly above the groove is left unsupported. The deepening of the wave-cut notch creates a structural imbalance, setting the stage for the next phase of development.
Cliff Retreat and Platform Widening
Once the wave-cut notch has sufficiently deepened, the substantial mass of rock above the undercut section is no longer capable of supporting its own weight. This overhanging rock mass, weakened by weathering processes, succumbs to the force of gravity. A sudden collapse, or rockfall, occurs, causing the unsupported upper cliff section to tumble down onto the shore below.
The collapse results in a temporary pile of debris at the cliff’s base, which is then broken down and removed by the continued action of the waves. The removal of this material exposes a new, near-vertical cliff face that has retreated inland. The flat, rocky surface left behind at the base of the newly positioned cliff is the nascent wave-cut platform.
The cyclical process then repeats: waves attack the base of the new cliff face, forming a new wave-cut notch, leading to collapse and further retreat. With each cycle, the cliff moves further inland, and the wave-cut platform expands seaward. This continuous sequence of notch formation, collapse, debris removal, and retreat is responsible for the platform’s widening over geological time.
Identifying the Final Landform
A fully developed wave-cut platform is recognized by several distinct physical characteristics, most notably its extremely low gradient. The platform surface slopes gently toward the sea, typically at an angle of less than five degrees, resulting in a broad, nearly horizontal shelf.
The width of the platform is directly limited by the tidal range and the wave power of the specific location. A wider platform causes waves to break sooner and dissipate energy before reaching the cliff base.
This exposed rock surface is often highly irregular, featuring numerous depressions and channels carved out by erosion and weathering. These indentations frequently retain water when the tide recedes, forming small, isolated rock pools that harbor marine life. The platform itself is generally smooth due to continuous abrasion from sediment. Its seaward edge marks the point where the rock is permanently submerged and protected from the most intense wave action.