Secrets of the Greenbelt: Frank’s Fault

You know the feeling: The house starts to shudder, maybe it jumps a bit. Dishes rattle, pictures on the walI tilt. If you’re outside in a quiet place, you may hear a rumbling. We live in earthquake country, and somewhere, near or far, titanic rock masses have shifted relative to each other across a fault line.

We ride on the thin rocky crust of the Earth, on a giant tectonic plate moved by the great heat engine of the Earth. The Pacific plate, on which Southern California rides, is moving north at about one inch per year relative to the North American plate, where the rest of the U.S. is.

It’s only a matter of time before Laguna is a suburb of San Francisco.

Getting there will take a lot of earthquakes along the San Andreas fault.

Besides those striking aerial photos of the San Andreas, how many of us have ever seen a fault? Geologists can find them, but their eyes are better trained than ours. Ignoring shrubs and soil, they see the rock formations beneath the surface, tracing separations all but invisible to the layperson.

Sometimes, however, a fault is easily visible, defined by an abrupt change in grade. In Laguna Canyon, the backbone ridge at the Jim Dilley Preserve is crossed by the Shady Canyon fault. At that point, there was a sudden drop-off in the ridgeline. You can’t see it anymore because the San Joaquin Toll Road was built right atop the fault.

Across Laguna Canyon Road, the fault parallels the toll road, a bit to the north (inland), up Camarillo Canyon. Its trace is marked by the steep dirt road that leads to the Four Corners intersection. Rocks to the north of the road are much older than rocks to the south -- what geologists call a discontinuity. This is a sure sign of a fault.

Nearby, there’s an even better sighting. On the Laurel Canyon trail you can step right over the tip of a normal fault, taking a big rocky step down in the process.

A normal fault is one where the rock masses on either side of the fault are moving vertically with respect to each other. During an earthquake, one side goes up relative to the other.

Retired oil geologist Frank Weagant was the first to recognize that the uncomfortable rocky bump on the Laurel Canyon trail is the tip of a fault. He explained to the lucky students on his field trip that day how it has all the hallmarks of a fault: the sinking rock mass polishes and grooves the reddish face (known as slickensides) of the adjoining rock. The whitish mass on the lower side is crushed rock (fault gouge) that accumulates along the fault from the movement of the rock masses.

Local geology maps show this fault, along with several others, but finding one exposed like this is unusual.

This fault helps explain how it is that the Laurel Canyon sandstone, hundreds of feet high, is studded with seashells and fossils of ocean life. The rocks formed underwater and have been uplifted by earthquakes over millions of years.

Frank’s Fault adds to the silent drama of our canyon. Make a New Year’s resolution to go out and see it.

* Elisabeth M. Brown is president of Laguna Greenbelt Inc.

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