Beachcombing: Southern California's geologic history, as told by the sand on our beaches

This story is part of our summer series "Beachcombing," in which KPCC reporters will explore  the ecology, economy and culture of Southern California's beaches and coast. Let us know what you think in the comments below or on KPCC's Facebook page.

If you’ve walked on a southern California beach, you’ve made direct contact with the region’s ancient geology. 

“I suppose I take it for granted,” says UCLA geography professor Tony Orme. He’s spent 40 years studying the way this coast has been shaped. Over millennia, sediment on beaches came down to the coast through mountain watersheds, and eroded off of coastal cliffs.

The sand on beaches between Point Dume and Redondo Beach comes in part from the San Gabriel Mountains: silvery mica, milky quartz, whitish-gray granite with flecks.

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“Sediment that’s derived from granite type watersheds is generally comprised of a lot of quartz,” says Orme. “It tends to be light in color.”

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That light color is visible on Oxnard-area beaches, too, north of Point Mugu. But between Mugu and Dume, black and red flecks in sand are remnants of ancient volcanoes. Volcanic eruptions started around 24 million years ago, eventually forming the Santa Monica Mountains.

“This is one of the more distinctive features here,” Orme says.

The volcanoes erupted largely underwater. Their deposits formed the Conejo Valley, land under what’s now Agoura and Thousand Oaks.

“The sediments on those beaches … contain a lot of volcanic material, which is quite recognizable,” Orme says. “A lot of heavy minerals, dark minerals.”

Among people who know sand well, on Surfrider Beach in Malibu, geological characteristics may be recognizable, but aren’t often noticed.

Surf instructor and musician Chris Crash Carson is a barrel-chested giant in a wetsuit. His hair is sun bleached and straw-stiff with salt. Between sets, he’s had plenty of time to look down at what’s between his toes.

“Some people love the beach and they hate the sand. I personally love the sand,” he says.

He points out that geology and geography make the waves bigger at Surfrider’s third point.

All rocks

“On this break it’s all rocks,” Carson says. He watches a surfer catch an edge and flip. “Ooh, look! She ate it.”

One of his students, 13-year old San Fernando Valley resident Kaitlynn Dorff, lets beach sand filter through her fingers, describing the colors she sees.

“I see a little red, I see brown, goldish, black, grey, did I say grey yet? and a little bit of orange,” Dorff says doubtfully.

Surfrider’s part of the Santa Monica littoral cell, a term geologists use to designate areas in which sand moves. Close to Point Dume, a little bit of sediment that’s volcanic in origin makes its way around the point to Surfrider. Dorff’s favorite beach is Zuma, a few miles north of here.

“Right when I get onto it lots of times it’s super hot,” Dorff says. She says she loves that about Zuma. “Because it kind of gets your attention.”

Darker minerals hold more heat, so it’s possible to perceive the difference between lighter-sand beaches and darker-sand ones. UCLA’s Orme remains skeptical that beachgoers notice that much. He says he suspects what people say they feel in sand is strongly influenced by what they see around them, and what they think they know.

The geologically young volcanic sediment in Malibu and points north tends to be more coarse. 

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“You know, chicks in Hollywood pay top dollar to get their feet exfoliated,” Crash Carson says. “You walk along the beach every day, your feet become smooth. “

But Carson doesn’t want to overthink the texture of sand, the age of the rocks, or the source of beach sediment. He’s just glad the beach’s benefits are available to everybody.