Along rocky coastlines, the wave-cut platform and the marine terrace offer clear evidence of the geological work performed by the ocean. While these two distinct features appear at different elevations today, they are sequentially linked. They represent two phases of a single, long-term process that records the history of sea level and land movement. The formation of one is a prerequisite for the existence of the other, which is fundamental to understanding coastal evolution.
The Wave Cut Platform
A wave-cut platform is a smooth, gently sloping erosional surface found at the base of a sea cliff, typically within the intertidal zone. This flat bench is actively formed by the constant erosive power of waves through hydraulic action and abrasion. Hydraulic action uses the force of water and compressed air to exploit cracks, while abrasion occurs as waves hurl sediment against the cliff face.
The wave action concentrates at the cliff base, carving out a hollow known as a wave-cut notch. As the notch deepens, the rock above becomes unsupported and collapses due to gravity. The waves then break down and carry away the fallen debris, causing the cliff line to retreat inland.
The resulting feature is the wave-cut platform, a wide, flat surface that extends seaward from the cliff base. It maintains a slight angle of descent, usually only a few degrees. The platform’s width is proportional to the duration of time the sea level has remained stable, allowing the cliff to retreat. This platform is constantly being smoothed and widened by wave action at or near the present-day sea level.
The Marine Terrace
A marine terrace is an elevated, ancient coastal surface that sits well above the reach of modern wave action. It is essentially a preserved wave-cut platform, lifted out of the current shoreline’s dynamic zone. A marine terrace consists of a flat or gently sloped bench—the remnant of the former platform—and an ancient sea cliff forming the landward boundary.
This elevated bench may still display the bedrock structure of the original platform. It is sometimes covered by a thin layer of marine sediment, such as rounded beach cobbles and sand, known as the terrace deposit. The elevation of the inner edge of the bench, where it meets the relic cliff, marks the highest local sea level reached during the terrace’s formation.
The existence of a marine terrace signifies a change in the relative positions of land and sea. Since these features are no longer subjected to daily wave erosion, they are preserved. They offer geologists a tangible marker of past sea-level stands, acting as a relict landform from a previous geological era.
The Process of Transformation
The relationship between these two landforms is sequential: a marine terrace is simply an uplifted and preserved wave-cut platform. The shift from an active platform to a relict terrace is triggered by a change in relative sea level. This means the sea level changes in relation to the landmass, occurring through one or a combination of two primary geological mechanisms.
Tectonic Uplift
Tectonic Uplift occurs when the landmass itself is slowly raised relative to the global sea level. This process is common in tectonically active regions, such as along the Pacific coast of North and South America, where crustal plates converge. As the land is pushed upward, the wave-cut platform is hoisted above the tidal zone. This effectively removes the platform from wave erosion and preserves it as a marine terrace.
Eustatic Sea Level Fall
Eustatic Sea Level Fall involves a global decrease in the volume of seawater, typically linked to glacial cycles. During ice ages, massive amounts of water are locked up in continental ice sheets, causing global sea level to drop significantly. This global drop exposes the former wave-cut platform, leaving it high and dry above the newly lowered sea level.
When a coastline is both tectonically active and subject to fluctuating sea levels, a sequence of multiple, parallel marine terraces can form. This pattern is often called a “flight” of terraces. Each distinct terrace represents a period of stable sea level when a wave-cut platform was carved, followed by uplift or sea-level drop that preserved it.