Until the last ice age, the basin which is now filled by the San Francisco Bay was a large linear valley with small hills, similar to most of the valleys of the Coast Ranges. As the great ice sheets began to melt, around 11,000 years ago, the sea level started to rise. By 5000 BC the sea level rose 300 feet (90 m), filling the valley with water from the Pacific. The valley became a bay, and the small hills became islands.
How was the San Francisco bay created?
Around 11,000 years ago the earth began to wam and melt the great ice sheets, raising sea levels and filling the valley that the San Francisco Bay now lives in.