The cobblestones that have engulfed a number of North County beaches are remarkably mobile. Sometimes, they make a muffled noise as they roll along.
Marine geologist Gerry Kuhn said the stones have journeyed from riverbed to ocean to beach.
"People used to think that the cobbles were buried beneath the sand, but we've been able to trace them to the rivers," he said.
Cobbles washed down the rivers from as far inland as the Pala Indian Reservation during flooding that occurred earlier in this century and in previous ones, he said. The water carried them out to sea, and they eventually returned to shore.
Swells move the stones toward shore when there is little or no sand to keep them at bay.
Just as marbles move easily over a hardwood floor but not carpet, the beach cobbles move more easily over exposed bedrock and reef than they do over sand.