Cretaceous-era sandstone visible from a gas station near the Moraga Drive on-ramp of the 405. Roughly 100 million years ago, this rock was a bank of sand on the floor of a giant inland sea that covered much of the western United States. It took millions of years for the sand to be compressed into sandstone.
Since the 405 expansion project started in the summer of 2010, construction workers have cleared brush, cracked rocks and sliced into the Santa Monica Mountains at the Sepulveda Pass.
The project is moving slower than expected: it's already more than a year behind schedule, and L.A. County Metro says it likely won't be finished until the middle of next year.
Bad news if you are a commuter, but good news if you are a geologist like Arthur Sylvester.
Sylvester is a retired U.C. Santa Barbara geology professor. He says the construction has given people a rare chance to see the insides of the ancient Santa Monica Mountains.
LA's watery past
On the south-west side of the Sepulveda pass for example, Sylvester says you can see some light tan sandstone rocks that date back to the Cretaceous age, 90 to 100 million years ago. (See Slide 2 above.)
Back then L.A. was a very different place, Sylvester says.
"This was a big seaway," he said while surveying area near the freeway now full of gas stations and restaurants. "So those rocks over there are marine rocks; they were deposited on the sea floor."
The sea eventually retreated, and these sandstone remnants of L.A.'s watery past were hidden for millennia. They were unearthed during the initial construction of the 405 about half a century ago.
A little further north on the freeway a different kind of stone enters the picture.
A little past the Getty Center on the east side of the 405, Sylvester points to a darker stone covering most of the mountain side.
"See! There's the Santa Monica slate over there, see it?"
Santa Monica slate is a gray stone that is chipped and flinty. (See Slide 4 above.)
"It is also a very weak rock, it slides up easy, it breaks up easy," he said, adding that most of it is held up by massive retaining walls along the 405 freeway.
Sylvester says this formation dates back to the Jurassic era, 165 million years ago. When this rock last saw the light of day, dinosaurs still roamed the earth.
He admits it's not the prettiest stone to look at, but he says it has an epic origin story.
Moving on up
Santa Monica slate, and in fact the entire Santa Monica Mountain chain, started down by San Diego and San Clemente, says Sylvester.
"And then about 20 million years ago, a big slab broke off," he explained.
That slab was on the tectonic Pacific Plate which is slowly pushing up the California coast at an average rate of two inches a year. (The seam between the Pacific Plate and the neighboring North American Plate forms the San Andreas Fault.)
Slowly, the plate dragged the mountains that would become the Santa Monica and San Gabriel ranges north. Sylvester says at some point the mountains got snagged on some feature of North American Plate and were forced to rotate about 110 degrees.
"So that now, the Santa Monica Mountains, which use to be parallel to the San Diego coast line are now east-west mountains!"
He says it's one of only a few east-west ranges in the country.
Just past Mulholland Drive heading North, a group of light brown rocks called the Modello formation comes into view. (See Slide 6 above.)
These are young rocks, relatively speaking, only six to 12 million years old.
"It's finer grain" Sylvester said of the Modello rocks. "Its beds dip towards the Valley and therefore they can slide into the Valley more easily."
Which they did following heavy rains in January of 1952. The landslides caused more than $7 million in damages to newly built homes.
Because of that, the City of L.A. now requires a geologist to inspect every hillside property to keep a similar disaster from happening.
Sylvester says knowing about the geological history exposed by the 405 construction might make your commute a little more interesting.
But he's quick to point out, don't go rock-watching alone.
"Watch where you are driving, don't try to look at the rocks."
Instead, have someone else do the driving. Not only will that keep you safe, but eventually it'll grant you access to those long awaited carpool lanes that are part of the expansion project.