The Rocks of Santa Cruz’s North Coast: The Mighty Mudstone

The Santa Cruz Mudstone is the rock type that shapes the landscape characteristics of the northern Santa Cruz coast from the north county line through to the Westside of Santa Cruz to about Swift Street. It’s a fairly hard rock type, though its many layers vary in their hardness and how easily they erode. If it were a much more erodible rock, or did not have layers, the landscape we see today would be very different. It might be without marine terraces, and dramatic sea cliffs with their undulating pocket beaches. The pocket beaches occur where the horizontal layers of the mudstone have fractures or “joints” running roughly perpendicular to the cliffs. The joints are points of weakness and thus the waves erode along the joints much faster than the rest of the layer.

Creation of the Santa Cruz Mudstone

The Santa Cruz Mudstone was created about 7-9 million years ago (late Miocene) by the settling of fine grained silts, clays and the silicate remains of billions and billions of single-celled plankton called diatoms. It varies in thickness but has been measured to be 8,900 feet thick in at least one location.

The sediments accumulated offshore of the ancient (paleo) coast, on the continental shelf or slope, in ocean water about 450-600 feet deep. This is nearly the same type of condition we have today half a mile off our modern coast. The silts and clays came from nearby rivers, and as the amounts coming from each source varied over time, so did the composition of the layers. With time, these sediments became very thick.

Hardening to Rock

Over millions of years, thousands of feet of sediment were deposited on top of the Santa Cruz Mudstone sediments. With burial, the sediment warms up and some of the silica from those billions of diatoms dissolves and later re-precipitates as cement that binds the particles together; this is called lithification.

During and after lithification, the sediments are also subjected to regional tectonic stresses from huge tectonic plates moving around, which can cause folding and faulting. Thus the original sediment composition, the degree of lithification, and the amount of tectonic stress all combine to determine how hard a rock is and how it will erode. In and around Santa Cruz, the Santa Cruz Mudstone is not very deformed, with just some tilting, jointing, and faulting.

Take the Self-Guided Mobile Tour

This piece is part of the North Coast Tour. Download the free app with many tours of the Santa Cruz area and beyond.

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  1. Sources Used

    • Geologic map of Santa Cruz County, California. E. E. Brabb. Department of the Interior, United States Geologic Survey Numbered Series MAP I-1905; 1989.

    • 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.



About The Author

I really enjoy field trips. I love being in a cool place and having someone tell me about it. The problem is, you can’t always find a professor or park ranger-type to tell you all they know about the local rocks, plants, and history. So I decided to combine my love of things natural with mobile technology.

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