San Francisco is a "good place to sit and watch the plates move," wrote John McPhee in Assembling California. The San Andreas Fault marks the boundary of the Pacific Plate to the west and the North American Plate to the east, each moving inexorably past one another at a rate of about two inches per year. These plates lurched twenty feet laterally April 18, 1906, forty miles north of San Francisco at Tomales Bay, destroying buildings and rupturing gas and water mains in the city. Fires destroyed about three-quarters of the city through ignited gas mains and lack of water to extinguish the fires. (The city was rebuilt in three years.) Along the northern San Andreas large-magnitude earthquakes such as that of 1906 recur frequently, perhaps as often as every 200 years. Earthquakes will always be of serious concern to those who live in the Bay Area. Google Earth and the U.S.Geological Survey have created an interesting virtual helicopter tour of the nearby Hayward Fault, including photographs of sidewalks, building walls and streams offset by fault movement. You will find the tour at http://quake.usgs.gov/research/geology/hf_map/GE_helicopter.htm.

The San Francisco Bay, one of the finest harbors in the world, is the drowned mouth of the Sacramento and San Joaquin rivers. At the end of the last glacial period, some 12,000 years ago, continental ice sheets were melting and draining into the world’s oceans, and would eventually raise sea levels by 300 feet and flood low-lying coastal regions. Prior to this, the Sacramento- San Joaquin outlet, presently found in the bay, was some thirty miles offshore near the Farallon Islands. During that time vegetation of the "bay" was riparian woodland and wetlands. Today, with sea levels 300 feet higher, the sediment transported by the rivers is deposited and accumulates in the bay. Barring tectonic subsidence, sea level rise, or continued dredging, millennia from now the San Francisco Bay will fill with sediment and the rivers could again meet the Pacific somewhere west of the Golden Gate.

Gold Rush era hydraulic mining ravaged the Sierra Nevada Mountains and washed mountains of gravel into the rivers, vastly accelerating the natural rate of sedimentation into the bay. This made the rivers and the bay increasingly treacherous to navigate. Hydraulic mining was subsequently banned in 1884. Geomorphologist G. K. Gilbert calculated that more than one billion cubic yards of material made its way into the northern San Francisco Bay via the rivers as a result of hydraulic mining, producing several feet of new sediment. These Gold Rush sediments have now worked their way through the system and have been dredged from the bay to maintain navigable waterways for ship traffic. The trademark hills of San Francisco, where many a heart has been left, are of California's Coast Range system, a geomorphic feature resulting from movement along the San Andreas Fault and subsequent folding and shearing of 100 million year old lithified mud known as Franciscan sediments. There are many other rocks as well, too numerous to name here. Western portions of the city are built on treeless sand dune fields, particularly Golden Gate Park, the Western Addition, and the Sunset and Richmond districts. Although hidden under pavement and buildings, the dunes can still be seen in Golden Gate Park where in some open parkways they are covered by a thin veneer of irrigated lawn. Slightly less easy to visit, very large sand dunes are also submerged in 100 to 350 feet of water just west of the Golden Gate Bridge, covering about two square miles of sea floor. These recently discovered submerged sand dunes, called "sand waves," are among the largest in the world, reaching 700 feet in length and over thirty feet in height.

There is not space enough here to discuss all of San Francisco's physical landscapes— this is but the tip of the mountain. One of the best ways to discover more is to make time during your visit to explore and enjoy the remarkable physical geography of San Francisco.

Please direct all other queries to:
Association of American Geographers
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Washington, DC 20009
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