Have you ever wondered why much of the Westside of Santa Cruz and all the agricultural fields just north of it are so flat, or why many of the hills climb upwards like a staircase to flat tops? It’s because the local landscape is made up of ancient marine terraces, which formed as sea level rose and fell over the millennia. They are clearly visible from Western Drive north to Año Nuevo State Reserve.
The Santa Cruz marine terraces are a series of platforms cut into the ocean-facing slope of our local mountain (Ben Lomond Mountain). Each terrace is an ancient ocean floor that has since been slowly uplifted by large-scale tectonic movements. The higher the terrace, the older it is.
The first marine terrace is 105,000 years old and the fifth is about 550,000 years old. The steep surfaces that separate each terrace are old beach cliffs eroded away by waves during high stands in global sea level that occur during glacial cycles. The Westside of Santa Cruz and the many agricultural fields to the northwest are on the first marine terrace (also known as the Highway 1 Terrace). The second terrace parallels Highway 1 on the inland side all the way to Davenport. It is very easy to spot once you know it.
How Do They Form?
Marine terraces can be found in many places around the globe but just the right conditions must be present for them to form. These are:
- Slow and steady tectonic uplift (rates of about 0.02 inches per year)
- Repeating cycles of rising and falling sea level (rise and fall of up to 400 feet!)
- Just the right rate of erosion for a wave cut platform to form and a sea-cliff to be preserved
To understand how marine terraces are created, begin by thinking about the beaches of today. Each of the terraces you see once looked like a modern beach.
Normally, most of the ocean’s wave energy is used in leveling the wave-cut platform. But during winter storms and high tides large energetic waves erode into the sea cliff. In order for a marine terrace to form from a modern sea cliff, two things must happen:
- Sea level falls by 10 to 100s of feet, leaving the cliff free from further wave erosion.
- Continuous tectonic uplift then isolates it from the next high sea level stand.
Once the ocean recedes during a low stand in sea level, the process starts over again. The beach slowly rises becoming the new elevated sea cliff and terrace platform, and when the ocean advances again it slowly cuts a new wave-cut platform into the terrace. Erosional processes during uplift round out and bury much of the original beach deposits and sharp contours.
Another key factor is the erosion rate of the rock that makes up the mountain. It has to be hard enough to hold a cliff edge. In Santa Cruz, it is the Santa Cruz Mudstone that forms the modern sea cliffs and most of the marine terraces. It makes a hard and durable sea cliff that takes time to erode. In fact, the waves need to be focused in one location for a long time — thousands of years — to form a sea cliff. Also, sea level must rise at a rate just slightly greater than the tectonic uplift rate. If sea level rises too fast or too slow, wave action will not be focused on one location long enough to create a wave-cut platform and a sea cliff.
Geek Details: How Many Terraces and How Old?
The number of marine terraces found on Ben Lomond Mountain is still debated in scientific papers. Papers in the 1960s reported up to nine. Since the mid-1970s, five or six are generally reported. Part of the reason for the disagreement is that the higher terraces are older, more modified by erosion and harder to identify conclusively. Also, not all of the terraces run contiguously along the length of the coast. For example one terrace is found only in the Davenport area. It is above the Highway 1 Terrace but below the second terrace (as it is shown in the image above).
The ages of the terraces from youngest (lowest) to oldest (highest) are reported to be 105, 213, 320, 430 and 545 thousand years. Terrace age is debated in scientific studies, in large part because of the different dating methods used:
- Uranium series dating, which measures the amount of short-lived decay products resulting from radioactive decay of Uranium in fossil corals and mollusks over time
- Amino-acid racemization, which measures the degradation of proteins in fossilized organisms into suites of amino acids over time
- Cosmogenic dating, which measures the buildup of beryllium-10 and aluminum-26, naturally occurring isotopes found in dust and known to be created at a fairly constant rate
Each method includes some unavoidable assumptions and has limitations. Their application to the terraces results in ages that vary 20-50 thousand years for the first terrace and 200+ thousand years for the third and higher terraces. I have given the ages from a 2001 paper by Gerald Webber and Alan Allwardt because the authors discuss all of the available studies and make a good case for their choices.
- Chemical weathering of a marine terrace chronosequence, Santa Cruz, California I: Interpreting rates and controls based on soil concentration–depth profiles. A F White, M S Schulz, D V Davison V. Vivit, et al. Geochimica et Cosmochimica Acta. 2008;72:36–68.
- The Geology from Santa Cruz to Point Año Nuevo—The San Gregorio Fault Zone and Pleistocene marine terraces. By Gerald Webber and Alan Allwardt. In: Stoffer PW, Gordon LC, eds. Geology and natural history of the San Francisco Bay Area: a field-trip guidebook: 2001 Fall Field Conference, National Association of Geology Teachers, Far Western Section: September 14-16. USGS Bulletin 2188; 2001:194.
- Living With the Changing California Coast. Gary B. Griggs, Kiki Patsch, Lauret E. Savoy. University of California Press; 2005.
- The Natural History of the University of California Santa Cruz Campus. Tonya M. Haff. Environmental Studies Department, University of California, Santa Cruz; 2008.
- The thaumatology of moving edges. Friends of the Pleistocene Worldnews.com.