Five Standout Specimens
The temperatures have dipped, the skies are increasingly gray, and the foliage is shifting colors and falling. While the summer months are behind us, one garden on campus remains unchanged: The California Rock Garden.
Surrounding the Earth and Physical Sciences Building, the California Rock Garden is a collection of over 50 boulders and core samples that showcase the state’s geological diversity. Specimens were sourced from the coast to the Sierra Nevada mountains, and from Mountain Gate in the north to as far south as the Chocolate Mountains.
Open year-round, the California Rock Garden is a viewable time capsule of California’s unique geology and a learning lab for UC Davis geology students.
Take a walk with us through billions of years of Earth’s geological history as we highlight five specimens in the UC Davis California Rock Garden.
Serpentinite: California’s state rock
The official state rock of California, serpentinite is visually characterized by its namesake. The metamorphic rock has a mottled blue-green pattern, resembling snakeskin, and its surface is slippery smooth. Widespread throughout the state, hence its state rock status, serpentinite contains green veins of the mineral serpentine.
“Take the constituents of what makes up oceanic crust and the upper mantle beneath it, alter it with fluids and you create serpentinite,” explained Michael Oskin, a professor of earth and planetary sciences.
Serpentinite specimens in the California Rock Garden were formed during the Mesozoic Era, roughly 65-251 million years ago. They were sourced from the Idaho-Maryland Gold Mine in Nevada County.
Pillow lava: Formed underwater
What happens when an underwater volcano erupts? You get pillow lava.
Colored gray-green from chlorite, rusted red from oxidized pyrite and spiderwebbed with white veins of quartz, the surface of pillow lava resembles the bubbled skin of the Marvel superhero character "The Thing.“
When that erupted lava stays underwater, the outside of it freezes as it fills up, like a balloon, and that makes a pillow,” Oskin said. “Eventually, that balloon cracks and the lava spills out and you make another pillow. This is how the ocean floor is made.”
Like serpentinite, the pillow lava in the California Rock Garden is from the Mesozoic Era, formed on the sea floor about 160 million years ago. However, it is an igneous rock rather than metamorphic.
Pillow lava specimens in the California Rock Garden were sourced from the Keithly Ranch/Highway 175 Quarry in Lake County.
Volcanic bombs: Solidified in mid-air
Organized in a shape resembling an abstract trefoil knot, the three volcanic bomb specimens in the California Rock Garden are among its youngest. Ruddy in color and jagged in texture, the igneous rock specimens most likely erupted within the last 20,000 years from a basalt cinder cone volcano.
“This is liquid when it was ejected and it cooled mid-air,” Oskin said. “It’s kind of like a big football shape that was streamlined on its sides as it was flying through the air.”
The holes in the rock’s surface are the result of gas bubbles trapped in the molten lava as it cooled.
The volcanic bombs specimens in the rock garden were sourced from Clearlake Oaks in Lake County.
Sandstone: A Paleocene-era time capsule
Resembling a gigantic, dried dirt clod, the rock garden’s sandstone specimen is a sedimentary rock from the Paleocene Epoch, roughly 57 million years ago. Sandstone forms when sand is cemented together over time.
The sandstone specimen in the California Rock Garden is from Cement Hill in Solano County and most likely accumulated in shallow water on an ancient shoreline, according to the specimen’s plaque.
“This specimen has fossilized shells from a spiral-shaped sea snail called Turritella in it,” Oskin pointed out.
Speckled along the sandstone’s edges, the white fossil flecks and cones are easily discernible against the rock’s otherwise shades of brown surface.
Greenstone: A polished geological story
And finally, the pièce de resistance of the rock garden: a polished specimen of greenstone prepared and donated by Ralph Mullican of Yuba Blue Boulders.
A metamorphic rock that formed during the Mesozoic Era, the greenstone boulder showcases different forms of underwater basalt lava flow. On the rock’s polished sides, green-colored veins of chlorite outline oval-shaped pillows and red hydrothermal deposits.
The polished surface captures a fragmentary story, showing shifts in how its constituents accumulated, from sheet- and lobe-shaped surfaces at the bottom to pillow shapes in the upper right corner to breccia (broken) shapes in the upper left.
“This is a really nice rock,” Oskin said.
Bonus specimen: Marble
The darling of architects, sculptors and interior designers, marble also has a presence in the California Rock Garden. Although, it’s of the unpolished variety.
Beautifully striated with grays, whites and pinks (the latter evidence of a mineral called rhodochrostite), this marble specimen was formed during the Paleozoic Era, roughly 251 to 542 million years ago and was sourced from the Lucerne Valley in San Bernadino County.
“If you take a limestone and you deform it and heat it up, you make marble,” said Oskin, who noted that the limestone was formed in deep waters and later metamorphosed due to high pressures and temperatures. “It metamorphoses well over 5 to 10 miles down and then they get returned back up and they make these chunks of rock.”
How the California Rock Garden came together
The California Rock Garden was built thanks to the generosity of UC Davis alumni, faculty, staff and friends, including geology alum Jeff Light, who secured around 30 specimens through his work as a geologist with Granite Construction. Homestake Mining, the McLaughlin Natural Reserve and Emgold Mining, among other organizations, also donated specimens, as did individuals Donn Ristau, Skyler Phelps and Jim Wood, among others.
Learn more about the California Rock Garden on the Department of Earth and Planetary Sciences’ website.
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