San Francisco — John Tinsley, a geologist with the United States Geological Survey, describes what made beach erosion so bad this time: ''For most of the year, waves come into the California coast from west-northwest or northwest. Beaches have formed as the result (a) of long-term pattern with respect to resistant headlands on a coast running north-south. The northeast end of the wave front starts dragging on the bottom before the southwest portion, which is still in deeper water. That is the point at which the wave front starts moving sand as well as changing its own direction - or refracting - because in the deeper water the velocity of waves is greater. The wave front sweeps around the resistant headlands, bearing sand with it. That's why Stinson Beach (north of San Francisco) is where it is - a sand spit in lee of a headland. This headland-beach pattern is repeated all down the California coast.
''This year the storm track shifted, and a great number of storms came from almost due west, or a bit southwest. A lot of high waves came into beaches that ordinarily are in the lee of major storms. There was very little protection for these easily erodible deposits.''