Geologists say the coastal cliffs near Pillar Point have retreated 200 meters in the last 70 years. In essence, more than a tenth of a mile deep has crumbled away.
Dave Reid spent a decade researching California's coastline for the United States Geological Survey. He says cliff retreat is not a gradual phenomenon.
"An annual rate for cliff erosion is not necessary the most accurate way to describe what's happening. You get these big events, these singular events, where you might lose tens of meters of a cliff edge at one time and then nothing happens for 10 to 20 years," says Reid.
There have been sudden events that are causing some concern. Earlier this month, a girl was buried up to her neck in a landslide at a Pacifica Beach. In 1998, several homes slid into the ocean after a cliff failure.
One culprit is what's called 'wave attack'. It happens when wave action eats away at the base of a cliff compromising it.
Water also compromises cliffs from the inside out. That's what happened at Seacliff State Beach, prompting a landslide over the weekend. Reid says the cliff was likely saturated from earlier rainfall.
"That first winter storm in January, where we got multiple inches of rain in 24 to 72 hours. Those types of events can lead to these types of cliff failures," says Reid.
Reid co-authored the recent USGS report which documents California's coastal cliff retreat. He says understanding the often sudden nature of cliff failure can help in planning where people build and even where they play.
"There's certain places where it might not be the safest place to sit at the base of a cliff. If you understand the types of processes going on in your environment, you might not put yourself in a dangerous situation," says Reid.
Dave Reid says a historical look at California's coastline shows a cliff retreat, on average, of 17.7 meters over a 70 year period.
