This is a story in essence that began 15 million years ago as the Earth entered a period of great volcanic activity. Over the next six million years, massive eruptions occurred over much of the earth’s surface. Where ash fell into the ocean it settled to the sea floor and formed layers.
Under pressure and other sediment those layers turned into clay called bentonite. And as the ocean receded and the continent shifted, the seafloor became the land where the bentonite clays now exist between layers of sedimentary rock that make up the California coast.
In areas such as Palos Verdes where the layers are dipping toward the ocean, these bentonite clays are like the icing of a wedding cake getting chipped away from beneath.