Beaches - continued from top.  One type of cell is defined by a long stretch of coastline that begins at a headland and terminates in a submarine canyon, such as at Mugu Canyon in Ventura County and La Jolla Canyon in San Diego County; another cell type consists of a large river delta bounded on either side by rocky headlands, such as at Humboldt Bay; a third type of littoral cell is defined by a crescent-shaped by downcoast of a promontory, like Half Moon Bay in San Mateo County; and a fourth type of cell consists of a rocky headland downcoast of a beach where waves break in a line parallel to the shore, as at Ten Mile Beach in Mendocino County. Finally, lagoons and closed bays with restricted tidal flow create a fifth type of littoral cell, such as Bolinas Bay in Marin.  

Characteristic differences between Northern and Southern California beaches depend upon the directions of prevailing wind and upon local coastal geology. Along California's north coast, cove or pocket beaches are common where the granitic and basaltic rock that composes the sea cliffs has been sculpted by prevailing northwesterly winds and battered by high energy waves over millions of years. In Southern California, beaches often consist of long ribbons of sand interrupted by widely separated rocky points. The bluffs of easily eroded shales and sandstones that edge the coast here continuously crumble away, creating on even coastline over time.  

Some beach types are found along both Northern and Southern California coasts. Narrow cove beaches like those at Laguna Beach in Orange County form where the coast is composed of conglomerate rock and hard sandstone; even when exposed to direct wave attack this rock type is highly resistant to erosion. The narrow beaches formed within there coves often lose all their sand during winter storms, exposing the underlying cobbles, as at Boomer Beach, south of Point La Jolla in San Diego County. Barrier beaches and sand spits are also present along the coast at river mouths, bays, and lagoons; examples are Silver Strand Beach in San Diego, Zuma Beach in Malibu, and beaches at the Smith, Salinas, Pajaro, and Santa Maria River mouths.  

Beaches vary in color according to the mineral content of the sand, which is also a clue to the origin to the eroded sediments that make up the sand supply. Eroded shale cliffs create the charcoal gray beach sand at Shelter Cove in Humboldt County. North of Humboldt Bay, the coarse sands of Agate Beach are multicolored agates that have been ground and polished by the surf. Ground quartz and feldspar mineral make up the white beaches of Carmel, while a few miles to the north in Sand City, amber colored sand indicates the presence of iron mineral. Close inspection reveals that white sand beaches are mosaic of pale quartz grains, pink, green or white feldspar and flecks of black mica.  Beaches are inhabited by a variety of invertebrates and insects.  More