Geology
San Francisco Bay is thought to represent a down-warping of the Earth's crust between the San Andreas Fault to the west and the Hayward Fault to the east, though the precise nature of this remains under study. During the last ice age, the basin now filled by the bay was a large linear valley with small hills, similar to most of the valleys of the Coast Ranges. The rivers of the Central Valley ran out to sea through a canyon that is now the Golden Gate. As the great ice sheets melted, sea level rose 300 feet (91 m) over 4,000 years, and the valley filled with water from the Pacific, becoming a bay. The small hills became islands.
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