Fort Point and the Geology of the Golden Gate

From the National Park Service's Geodiversity Atlas. The atlas page for Fort Point describes the whole of Golden Gate National Recreation Area, of which the fort is part.

Fort Point's parade ground, with the Golden Gate Bridge above. NPS.

Golden Gate National Recreation Area

Golden Gate National Recreation Area is one of the largest urban parks on Earth, spanning the San Francisco Bay area in Marin, San Francisco and San Mateo counties. Established on October 27, 1972, it covers about 33,195 hectares (82,027 acres) and preserves:

• coastal scenery, including the Marin Headlands;
• beaches and lagoons, and more than 160 km (100 mi) of trails;
• Alcatraz Island's notorious prison;
• the giant trees of Muir Woods;
• former U.S. military fortifications;
• the Presidio, site of the first Spanish fortification in San Francisco.

It is part of the UNESCO Golden Gate Biosphere Reserve, home to more than 1,250 plant and animal species, including some of the rarest, threatened and endangered species in the national parks. "Golden Gate" is the narrow strait joining San Francisco Bay to the Pacific; the bay, its islands and its rugged headlands made a landscape valuable for trade and for defence. The area also includes two other park units: Fort Point National Historic Site and Muir Woods National Monument.

An active landscape

The Golden Gate lies in one of the most geologically active regions of the United States, a natural classroom for plate tectonics.

• An old subduction zone: the rocks go back to the Jurassic, about 160 million years ago, when North America's western edge was a convergent boundary. The denser oceanic Farallon Plate sank beneath the North American Plate, forming what geologists call the Franciscan subduction zone, active from about 160 to 50 million years ago.
• Where the rocks formed: the basement rocks formed, or at least were assembled, in settings tied to that subduction — an ocean trench, a forearc basin and a volcanic arc.
• Three groups of formations: rocks of the Jurassic–Eocene Franciscan Complex; Jurassic and Cretaceous intrusive and volcanic rocks; and Cenozoic units — the Whiskey Hill, Purisima, Millerton, Olema Creek, Merced, Santa Clara and Colma formations.
• The San Andreas fault cuts through the recreation area: a transform boundary between the North American and Pacific plates. The modern transform boundary began in Southern California about 28 million years ago.

Region, maps and reports

Fort Point is part of the Pacific Border province and shares its geology with a much wider region. The Geologic Resources Inventory report covers Golden Gate, Fort Point and Muir Woods together.

Detail of the Geologic Resources Inventory's map around Fort Point and the Presidio. NPS.

Sources

• National Park Service, "NPS Geodiversity Atlas—Fort Point National Historic Site, California." https://www.nps.gov/articles/nps-geodiversity-atlas-fort-point-national-historic-site-california.htm — which cites Elder (2013), National Park Service (2017) and Port (2016). The source's account of the San Andreas fault breaks off mid-sentence, on the NPS page as well as in the import; this page stops where the text is complete.
• Rewritten in hubnx's own words.