California's Marine Terraces: Staircases Carved by the Sea

San Clemente Island, in the Channel Islands: the stair-step flats are marine terraces of different ages. Dan Muhs, USGS.

Stairs by the sea

Many coasts around the world rise in stair-step landforms called marine terraces, and they make up a large part of coastal California. Scientists study them to learn about earthquakes, past sea levels, climate and soils — and anyone can explore them.

How they form

Marine terraces come from two processes working together over hundreds of thousands of years: uplift of the land, and the natural rise and fall of sea level.

• Carving: as sea level rises, waves push sand and gravel back and forth over bedrock, sanding it into broad, flat platforms.
• Lifting: when the sea falls, those wave-cut surfaces are exposed. Earthquakes on California's coastal faults, and other movements of the crust, raise the land. Where uplift is fast enough, the surfaces rise out of the ocean's reach and are preserved as terraces; where it isn't, the next rise of the sea floods them.
• Not old shorelines at that height: a high terrace doesn't mean the sea once stood that high. The crust rises steadily while sea level goes up and down, and each high stand of the sea cuts a notch into the rising land.

How a terraced coast forms. Waves during past high sea levels cut platforms at sea level; recurring fault movement lifted them out of reach. Older terraces are higher and farther inland. The graph shows global sea level over the past 450,000 years, relative to today. USGS.

Santa Rosa Island: the geologist stands on tilted bedrock. Just above is an old wave-cut surface covered with rounded cobbles — once shoreline sediment — and above those, sand from old beaches and rivers. Dan Muhs, USGS.

Faults and uplift

Uplift near the coast comes from movement on nearby faults, with warping and folding of the crust in places. Scientists estimate uplift rates from the height of ancient shorelines, the terraces' ages, and past sea levels.

Point Arena in northern California: terraces formed across the San Andreas Fault have been pulled sideways by it — older shorelines farther than younger ones. That lets scientists estimate the fault's horizontal slip rate over time and better understand how often earthquakes recur.

Near Point Arena, the 120,000-year-old shoreline (green) has been offset about 1.6 miles (2.5 km) across the San Andreas Fault (red), and the 80,000-year-old shoreline (blue) about 1 mile (1.5 km). USGS.

Channel Islands National Park: terraces give all five islands their stepped look.

• The lowest — youngest — terrace dates to the last major interglacial, about 120,000 years ago, when the sea stood 20 to 26 feet (6 to 8 meters) higher than today.
• On Anacapa, Santa Barbara and Santa Cruz islands it has barely risen; on San Miguel and Santa Rosa, it sits higher — a sign of faster uplift.
• Some of the highest terraces are 1 million years old (San Miguel) and up to 2 million (Santa Cruz).
• Terrace deposits hold plenty of fossil sea creatures, mostly species still alive today, which reveal past ocean temperatures off southern California.

How we know it was once sea level: fossils

Pholad clams (family Pholadidae) bore into rock, wood and sediment in the intertidal zone, between high and low tide. Find their fossils on bedrock away from the sea, and that surface was once at sea level. Fossil clams, corals and snails in the sediment just above a wave-cut surface are more evidence. Look for them in cliffs and roadcuts near the coast — and the holey rocks on today's beaches are often old pholad homes.

Fossil pholad clams in bedrock at Año Nuevo State Park, north of Santa Cruz; they are 1 to 2 inches (2.5 to 5 cm) long. Dan Muhs, USGS.

An uplifted wave-cut platform drilled by pholad clams, Año Nuevo. Dan Muhs, USGS.

Soils: a staircase through time

Once a terrace rises out of the sea, plants and animals move in and soil begins to form — changing steadily with time.

• Soil chronosequence: a flight of terraces carries a predictable series of soils, oldest and most depleted on the highest terrace and youngest and most fertile on the lowest.
• Young soils resemble the original sediment: nutrient-rich, and often farmed. Old soils are heavily weathered — primary minerals dissolved and replaced by clays — low in organic matter, nutrient-poor, and usually not productive enough for row crops.
• Why scientists care: chronosequences show how nutrients, water, organic matter and biology change over time. And California's rainfall gradient — San Diego much drier than Eureka — lets them compare dry and wet ecosystems, and how soils may respond to climate change.

Soil pits at Wilder Ranch State Park near Santa Cruz, about 5 feet (150 cm) deep: on terrace 1 (first), a younger soil with two main horizons; on terrace 2 (second), older, with four — three visible here. Both began as similar sandy marine sediment. Marjorie Schulz, USGS.

Iron nodules: many coastal terrace soils hold hard, oval, dark orange or brown iron nodules — sometimes mistaken for meteorites, but formed in the soil. Iron is an important plant nutrient, so scientists want to know how it moves and concentrates.

• In southern California and the Channel Islands, some soil surfaces are covered with nodules left behind when finer soil eroded away.
• They grow scarcer northward, toward wetter climates — so they form more easily, or survive better, in drier soils.
• How they form is still debated: first thought to come from wetting and drying of iron, they may also depend on fungi and bacteria.
• Look closely at the soil around gopher holes — you may find one.

Nodules exposed on a gopher mound after rain. David Stonestrom, USGS.

Cut open, nodules show complex histories of iron cementation. Marjorie Schulz, USGS.

Pygmy forests on the ecological staircase

Because old terrace soils are poor, different plant communities grow on terraces of different ages — an ecological staircase on top of the geologic one. Fine examples are in northern California's Jug Handle State Natural Reserve, Russian Gulch, Salt Point and Van Damme state parks. On the oldest terraces, heavily weathered, nutrient-poor soil with a dense, poorly draining hardpan stunts the trees into a pygmy forest — home to hardy plants scientists study for how they survive.

The ecological staircase in Mendocino County: five terraces, about 100,000 to 500,000 years old, spanning about 3.5 miles (6 km) and 650 feet (200 m) of height. Vegetation runs from coastal scrub, beach pine, Bishop pine, Sitka spruce, redwood and Douglas fir to pygmy forest. Modified from a figure by Michael Kauffmann; USGS.

A dynamic landscape

Cut by waves, lifted from the sea, then shaped by climate and life, marine terraces sit where tectonics, sea-level change and climate meet — a record for understanding earthquake hazards, past climates and soil development.

Sources

Based on Landscapes from the Waves—Marine Terraces of California, USGS Fact Sheet 2018–3002 (March 2018), by Marjorie Schulz, Corey Lawrence, Dan Muhs, Carol Prentice and Sam Flanagan, U.S. Geological Survey; a work of the United States government in the public domain. Photographs and diagrams are taken from the fact sheet's PDF; a copyrighted aerial photograph of Wilder Ranch and a "where to find marine terraces" map on a commercial base are not reproduced.