Greyhound Rock is named for the beach’s large rock formation that sits stoically offshore in the crashing waves. Usually, the rock is connected to the beach by a small sand spit that is part of the Greyhound Rock State Marine Conservation Area near Davenport, north of Santa Cruz, California. The rock was given its name by a Santa Cruz pioneer named Thomas Majors who thought it looked like a hound.
Today, it’s a bit hard for most people to see the hound. Perhaps, over the decades, the ocean surrounding Greyhound Rock County Beach has eroded away any canine resemblance. If anything, the rock looks like a sleeping alligator from above, and from down on the beach, it just looks like a long mound of layered mudstone. That’s very cool in and of itself, hound or no hound.
Formation of the “Hound”
Greyhound rock is made up of Santa Cruz Mudstone. This is the rock formation that shapes the landscape characteristics of the northern Santa Cruz coast, from the north county line (basically at Ano Nuevo State Park) through to the west side of Santa Cruz. It’s a fairly hard rock type, although 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 coastal landscape we see today might be without the many arches and bridges 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, so the waves erode along the joints much faster than the rest of the layer.
Creation of Santa Cruz Mudstone
The Santa Cruz Mudstone was created about 7 – 9 million years ago (late Miocene era) by the settling of fine-grained silts, clays, and silicate remains of 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 era) coast in ocean water about 450 – 600 feet deep. The silts and clays came from nearby rivers. Because the amounts coming from each source varied over time, so did the composition of the layers. With time, these sediments became very thick. This is nearly the same conditions we have today half a mile off our coast.
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 turns into 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 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.
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This piece is part of the Santa Cruz Marine Protected Areas Beaches Tour made possible by the Santa Cruz Collaborative with support from the California Marine Sanctuary Foundation and the Resources Legacy Fund. Download the free app with many tours of the Santa Cruz area and beyond.
- The Rocks of Santa Cruz’s North Coast: The Mighty Mudstone. Mobile Ranger website.
- "Our Ocean Backyard: Exploring the North Coast,” by Gary Griggs. Santa Cruz Sentinel, August 9 , 2013.
- "Our Ocean Backyard: West Cliff – Stepping Back in Time,” by Gary Griggs. Santa Cruz Sentinel, December 14, 2013.
- California Coastal National Monument. National Conservation Lands, California. U.S. Department of the Interior, Bureau of Land Management website, blm.gov.
- The things you learn on a Coastal Geology Walk at Año Nuevo State Park,” by Ryan Masters, Goat Trails blog. Hilltromper website, October 24, 2014.
- Coast Dairies Property: A Land Use History. Excerpt from "Coast Dairies Long-Term Resource Protection and Use Plan: Draft Existing Conditions Report for the Coast Dairies Property," Section 1.0. In Santa Cruz County History - Places, Santa Cruz Public Libraries website.