Marine terraces are flat, stair-step landforms along coastlines created by a combination of wave erosion and land uplift. Each “step” represents an ancient shoreline that was carved by the ocean and then lifted above sea level, preserving it like a fossil beach. The process takes thousands to hundreds of thousands of years, and some coastlines display multiple terraces stacked one above another, each recording a different moment in Earth’s history.
How Waves Carve the Platform
The foundation of every marine terrace is a wave-cut platform, a flat rocky surface carved at sea level by the constant action of the ocean. Three main processes do the work. First, the sheer hydraulic force of waves slamming into rock creates pressure that pries apart joints and fractures, plucking blocks of stone from cliff faces. This quarrying action is most effective on rock that already has natural cracks or bedding planes. Second, sand and pebbles tumbled by the surf act like natural sandpaper, grinding the rock surface through abrasion. This is probably the single most important erosional force shaping coastlines. Third, repeated cycles of wetting and drying, along with chemical and biological weathering from marine organisms, weaken rock over time and can sometimes contribute as much erosion as wave energy itself.
Together, these forces carve a notch at the base of a sea cliff. As the notch deepens, the overhanging rock above it becomes unstable and collapses. The fallen debris gets broken down and swept away by waves, exposing fresh rock to start the cycle again. Over time, the cliff retreats inland, leaving behind a gently sloping rock platform at the waterline. Beaches actually slow this process down: where sand covers the shore, it absorbs wave energy and shields the bedrock from abrasion, preventing platforms from forming.
Uplift Turns a Platform Into a Terrace
A wave-cut platform only becomes a marine terrace when it gets lifted above the reach of the ocean. This usually happens through tectonic uplift, the slow rise of the Earth’s crust driven by forces deep underground. Along many coastlines, this uplift occurs gradually at rates of fractions of a millimeter per year. In other places, it happens in sudden jumps during earthquakes.
Research on New Zealand’s Mahia Peninsula has documented exactly this earthquake-driven process. Each uplift event raised the active shoreline above sea level, removing it from marine influence. The former wave-cut platform became a dry, flat terrace mantled by old beach deposits and eventually covered by soil and vegetation. Meanwhile, the ocean immediately began cutting a new platform at the newly established waterline. When another earthquake struck years or centuries later, that platform was lifted too, creating a second step below the first. Repeat this over many cycles and you get a “flight” of terraces, a staircase of ancient shorelines climbing up from the coast.
The Role of Sea Level Changes
Tectonic uplift isn’t the only force at play. Global sea level itself rises and falls dramatically over time, driven by glacial cycles. During ice ages, enormous volumes of ocean water get locked up in continental ice sheets, dropping sea level by over 100 meters. During warm interglacial periods, that ice melts and sea level rises again. These swings have repeated roughly every 100,000 years throughout the last million years of Earth’s history, with smaller fluctuations occurring on shorter timescales.
Each time sea level reaches a high point during a warm period, the ocean carves a new platform into the coast. If the land is also rising tectonically, previous high-stand platforms get carried progressively higher, creating terraces at different elevations that correspond to different interglacial periods. The interplay between the rate of tectonic uplift and the timing of sea level highs determines how many terraces form, how far apart they’re spaced vertically, and whether individual terraces are preserved or eroded away. Along California’s Santa Cruz coast, where uplift runs about 0.3 millimeters per year, terraces dating back 500,000 years are still visible in the landscape.
Anatomy of a Marine Terrace
When geologists describe a terrace, they break it into a few key parts. The flat surface you’d walk across is the “tread,” which is the old wave-cut platform now covered with a thin layer of ancient beach sand, gravel, and marine fossils. The steep slope rising from one tread to the next is the “riser,” which is the remnant of an old sea cliff. At the back of each tread, where it meets the base of the riser above, sits the “shoreline angle.” This is the most important point for scientists because it marks the exact position of the ancient shoreline and, by extension, the sea level at the time the terrace was carved.
Over time, erosion softens these features. Rain, streams, and landslides wear down the risers and deposit sediment across the treads. The older and higher a terrace is, the more degraded it becomes. Eventually, streams cutting through the landscape can erase terraces entirely. For the Santa Cruz coast, the estimated “forget time,” the period it takes for erosion to completely remove a terrace, is around 500,000 years.
Erosional vs. Constructional Terraces
Most marine terraces along rocky, tectonically active coastlines are erosional: they’re carved directly into bedrock by wave action. These are the classic “California-style” terraces found along the Pacific coast from Washington state down through Baja California. But in warmer tropical waters, terraces can also be constructional, built upward by coral reef growth rather than carved downward into rock. Along the southern Gulf of California, for example, both types exist side by side. The formation process differs, but the result is similar: a flat surface at a former sea level that records past ocean conditions.
Reading Earth’s History From Terraces
Marine terraces are one of the best natural archives of past sea levels and climate conditions. Scientists date them using several techniques. Uranium-series dating measures the radioactive decay of uranium absorbed by corals and shells that grew on the terrace when it was still underwater. Amino acid racemization works on a different principle: after a mollusk dies, the proteins in its shell slowly change their molecular structure at a predictable rate. By measuring how far this conversion has progressed, researchers get an estimate of when the animal lived. Radiocarbon dating works for younger terraces, though its useful range only extends back about 50,000 years.
Along California’s San Luis Obispo coast, these dating methods have identified terraces corresponding to several high-sea stands within the last interglacial period, roughly 130,000 to 80,000 years ago. The fossils preserved in terrace deposits reveal not just the age of each surface but what ocean conditions were like at the time. One terrace there contains a mix of warm-water and cool-water mollusk species, indicating it accumulated shells during two separate high-sea stands with very different ocean temperatures. During the warmest phase around 125,000 years ago, sea surface temperatures along much of California were higher than today. During a later, cooler high stand around 100,000 years ago, temperatures were similar to or below present levels.
By combining terrace elevations with age estimates, scientists can also calculate long-term tectonic uplift rates for a given stretch of coast. If you know how old a terrace is and how high above sea level it sits, and you can estimate where global sea level stood at that time, the math gives you the rate at which the land has been rising. These uplift rates feed into earthquake hazard assessments and help geologists understand the forces shaping coastlines over geologic time.