an engineering geologist once told me that slides in California occur where bedding (thick layers) and the fissility (very thin layers) of the San Francisco shale both slope downhill. When rain or snowmelt wet and lubricate this huge, downtilted mass of broken, hardened clay, a slide may occur.

(I stand prepared to learn that there are other rock units behind slides, but this type of explanation is at least a good beginning.)

It shouldn't be that hard to determine where these slopes are, given geologic maps and the help of the state geololgic survey, which is only a phone call away. However, for decades American schools have shied away from any serious teaching of earth science because of the political power of fundamentalists.

Hence we get perfectly normal people who think that their water comes out of underground lakes and that California is floating on the sea. Just last week I heard another one. We had a day of 70-degree weather(Hawaiian type, for those of you who use Celsius), and a bus driver, who had moved from San Francisco, said that this weather would make people afraid in SF, because it would warm up the fault and might trigger an earthquake. This fear is not realistic at all, but it is sad to think that people have such fear gnawing at them for no reason.