What is san fransisco built on?

The city of San Francisco is built on a peninsula at the northern tip of the San Francisco Bay. The peninsula is bordered by the Pacific Ocean to the west and the bay to the east. The city's topography is made up of rolling hills, with some areas reaching elevations of over 900 feet above sea level. The Golden Gate Bridge is one of the most famous landmarks in San Francisco, and it connects the city to Marin County across the bay.