One of these times, you might find yourself in the city of San Francisco. It's a beautiful city built on a number of hills on the San Francisco Peninsula with a gorgeous stretch of beach, and a complex mix of rocks and sand dunes. If you do visit the city, try and take the time to seek out one of the unknown wonders of the city. As early as 1917, talk began of building a bridge across the opening of San Francisco Bay to the Marin Headlands. Much to the surprise of many visitors to the city, the bridge was actually built in 1937, providing a quick shortcut to the Sonoma/Napa Valley region, another part of California that no one has heard much about.
|The bridge as seen from Fort Baker on the north side.|
|The bridge from the Marin Headlands.|
important fault systems cut across the Bay Area, including the well-known San Andreas, but also the less familiar Hayward, San Gregorio, Calaveras and many others. The proximity of these faults to the bridge is cause for some concern to the engineers who desire that the bridge would survive a major seismic event (retrofits have recently been completed). Part of the problem is the that the southern abutment is situated in serpentinite, a notoriously unstable rock (the cliff just visible on the right of the picture above is Baker Beach, which is made up of numerous landslide blocks).
|The Golden Gate as viewed from the summit of Mt. Diablo in the east bay region.|
|Source: National Park Service|
The rugged cliffs of the Marin Headlands (seen in the picture below from Fort Point) will be the subject of a follow-up post soon, but in short they are made of a complex mix of ocean floor basalt and reddish chert, a quartz-rich sedimentary rock that formed originally on the sea floor as diatomite. The diatomite was composed of trillions of microscope shells of one-celled creatures called (you guessed it) diatoms. The rock forms a solid foundation for the northern bridge abutment.