Science Spotlight: Marine Terraces

Geologic forces and the steady work of crashing waves have sculpted the shoreline of Santa Cruz and San Mateo counties.

by Sarah Derouin

During your travels between Santa Cruz and Half Moon Bay, you’ve likely noticed expansive flat stretches of land above the shoreline usually covered by rows of strawberries, Brussels sprouts or artichokes. Those lovely flat fields sit atop formations that geologists call marine terraces.

Over hundreds of thousands of years, the interplay between colliding tectonic plates and rising and falling sea levels creates flat areas of beach deposits. Steady geologic forces lift these platforms out of the water while beaches are carved by ocean waves. The position of the shoreline changes over time—water levels can rise and fall as much as 100 meters as the climate fluctuates. These processes repeat in a relentless cycle.

“For marine terraces we need two things to happen: slow, constant uplift and an oscillating sea level,” says coastal geologist Gary Griggs of UC Santa Cruz. “The terraces are like a dipstick of two very important processes.”

Terraces only form where the coastal rocks are easy to erode. For instance, much of Santa Cruz County has soft mudstone bedrock, creating multiple terrace platforms along the coast. In contrast, Big Sur has a mix of much harder granitic and metamorphic rocks. Those sturdy rocks create steep, rugged cliffs that drop 200 feet to the sea.

As many as five terraces rise from the coast near Santa Cruz. One place you can easily see them is Wilder Ranch. Trails for biking or hiking will take you across several marine terraces. When you switch from a flat or gently sloping stretch to a steep climb, then back to flat, you are traversing former shoreline platforms and old seacliffs. Indeed, your afternoon bike ride could take you on a geologic journey spanning 600,000 years.

Note from the editors: Science Spotlights are a joint project of the UCSC Science Communication Program and Hilltromper. This article was published in November 2016.