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Jurassic Los Angeles


By Arthur G. Sylvester M.A.'63, Ph.D.'66

Published Aug 13, 2013 7:23 AM

The massive 405 freeway expansion project has provided a window into the ancient geology of the Sepulveda pass.


Photo courtesy of: Zócalo Public Square.

If you’re among the 300,000 or so commuters who crawl through the Sepulveda Pass every day in the midst of the $950-million 405 freeway construction project, you surely have seen the giant earthmovers slicing through the mountain. You have also likely looked at the eight-story-tall retaining walls doing their best to hold back the Santa Monica Mountains.

As a geologist, I’m thrilled to see those new cuts through the mountains because they give me a temporary glimpse into the anatomy of the mountain range that separates West L.A. from the Valley, downtown from Burbank. The last time some of these rocks touched air, dinosaurs walked the earth.

You can take a quick tour of the geology of the pass from your driver’s seat. Start by heading north on the 405. About a mile past the Wilshire Boulevard on-ramp, before the Sunset overpass, glance out your passenger-side window to see two gargantuan Caltrans retaining walls. During the construction, you may have noticed their cageworks of reinforcing steel, fortified with concrete. The walls are anchored to the rocks with Brobdingnagian nails, some as long as 85 feet and with the diameter of salad plates. These walls need long, thick nails because the rocks here are weak and prone to collapsing.

As you continue driving north, you pass the Hotel Angeleno on your left, and in the mountain just beyond it, you can see a big, light-yellowish-brown outcrop. This is Cretaceous sandstone. About 100 million years ago, this rock was a huge pile of sand at the bottom of a great inland sea covering much of the western United States. Over millions of years the sand was compressed into sandstone, and then uplifted along the Benedict Canyon fault. That fault stretches northeast from Brentwood, crosses the 405, continues east to the mouth of Laurel Canyon, and then cuts over the north edges of Forest Lawn and Griffith Park. You can’t see it from your car because it’s covered by brush.

Cretaceous sandstone is not prone to landslides, so it doesn’t need great walls to hold it back. But in much of the Sepulveda Pass a different kind of rock forms the core of the mountains, including the stretch from Getty Center Drive to the Skirball exit. These dull gray rocks, called the Santa Monica Slate, are from the Jurassic period, when stegosaurs and allosaurs roamed the earth. About 165 million years ago, they began as mud that formed an underwater seabed. Jurassic slate is weak, crumbly, and difficult to build on.

As you ease under the new Mulholland Bridge and crest the top of the pass, you see the San Fernando Valley, where the old Santa Monica Slate is overlain by a thin, younger cover of weak, sedimentary layers called the Modelo Formation. These rocks were deposited during the Miocene epoch between about 12 and 6 million years ago.


Photo courtesy of: Zócalo Public Square.

Caltrans engineers have engraved wide, gently sloped road cuts in the Modelo Formation near the top of the pass to reduce the possibility of landslides and put up retaining walls farther down the pass to hold back the deeper and steeper cuts. The sandy, silty rocks in the Modelo Formation tend to accumulate groundwater. All mountains contain groundwater, which usually leaks out naturally in springs and ground seepage. But retaining walls seal the springs, so the water cannot escape. Over time, the increasing water pressure can cause walls to fail, so here the engineers have placed drain pipes deep beneath the retaining walls to relieve the pressure.

The Modelo Formation is tilted. Its thin layers slope toward the valley, like shingles on a peaked roof. Imagine if those shingles, sloped at a 25-degree angle, were cut somewhere in the middle. The unsupported layers above would slide down off the roof. Now picture those shingles as thousands of tons of rock, sliding onto houses, and you understand the potential problem.

Sadly, that is exactly what happened following heavy rains in January 1952. Massive landslides caused $7.5 million in damage to hundreds of recently built homes. So the City of Los Angeles adopted a tough ordinance requiring a geologist to inspect every hillside lot, and the surrounding rocks to be bolstered with a technique called keying and benching, which essentially locks each layer to the one below it.

Other Southern California municipalities soon followed suit, and today L.A.’s hillside homes are much safer.

You likely have noticed that the concrete walls are stamped with a faux-stone pattern that landscape engineers say is meant to resemble the structure and color of the local bedrock. I don’t think the result looks anything like the dull, ancient Santa Monica Slate, the Cretaceous sandstone or the Modelo Formation.

But in a few million years, geologists may be able to determine whether the landscapers got it right, even if the color has faded and the faux pattern eroded. That’s because California’s coast is sliding along the San Andreas Fault toward Alaska at the rate of about two inches per year. In our geologic future, the surrounding plants and animals will no doubt look very different, and we may be mere fossils, but those walls will be available for re-discovery somewhere beneath the Aleutian Islands. I bet they’ll still be intact.

Arthur G. Sylvester is a retired UC Santa Barbara geology professor. A longer version of this article appears at



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