Fort Point, Golden Gate

Fort Point, Golden Gate

by | Nov 4, 2021

Fort Point is a promontory where a historical masonry fortification, and later the southern approach of the Golden Gate Bridge, were constructed on the south side of the Golden Gate at the entrance to San Francisco Bay, about 4 miles (6 km) west-northwest of downtown San Francisco and 3 miles (5 km) south of Sausalito, California. The point is composed of a rock called serpentinite, which is part of a mélange situated on the boundary between the North American and Pacific Plates. This plate boundary is a now transform fault called the San Andreas Fault Zone where the plates are sliding past each other at about 1 inch (2.5 cm) per year. The fault zone is infamous for producing large earthquakes that release the strain between the tectonic plates. The geology of the western boundary of the fault zone at the Golden Gate is dominated by rocks of the Franciscan Complex, a large-scale mélange that formed during the late Mesozoic and consists of altered oceanic crust and blocks of continental slope sediments. The Franciscan Complex is dominated by greywacke sandstones, shales, and conglomerates that have experienced low-grade metamorphism resulting in a heterogenous assortment of rocks that include chert, basalt, limestone, and serpentinite. The serpentinite at Fort Point is highly fractured and since it is the foundation for the south tower of the Golden Gate Bridge, there are concerns about its integrity during a large earthquake. Weathering and degradation of serpentine rocks into constituent serpentine minerals create soil toxic to many plants because of high levels of nickel, chromium, and cobalt. These areas are called serpentine barrens with distinctive plants forming shrubland and open conifer forests. In 1769, when the first Spanish explorers arrived, this promontory was a serpentine barren of windswept rock overlaid with sand about 100 feet (30 m) above the ocean which they called Punta del Cantil Blanco.

Concerns about the intrusions of British and Russian merchant ships into Spain’s colonies in California prompted the extension of Franciscan missions and presidios into Alta California. In 1769, the Inspector General of New Spain, José de Gálvez, planned an expedition, consisting of three units by sea and two by land, to start settling Alta California. Gaspar de Portolà volunteered to command the overland expedition, and in 1769, arrived at the Golden Gate and claimed the area for New Spain. In 1775-1776, Juan Bautista de Anza led an overland expedition from Arizona to California with 240 friars, soldiers, and colonists. They took 695 horses and mules and 385 longhorn cattle with them. They established the Presidio of San Francisco, followed by the Mission San Francisco de Asís. To protect against encroachment by the British and Russians, a fortification called Castillo San Joaquin was built at the narrowest part of the bay entrance on a promontory called Punta del Cantil Blanco. It was an adobe structure housing nine to thirteen cannons. In 1821, Mexico won independence from Spain and gained control of the region and the fort, but in 1835 the Mexican army moved to Sonoma leaving the Castillo’s adobe walls to crumble in the wind and rain. On July 1, 1846, one week before the start of the Mexican-American War, Captain John Fremont with Kit Carson, Lieutenant Archibald Gillespie, and twenty Delaware Indians borrowed a longboat at Sausalito and rowed south across the bay to where the Presidio was in total abandonment as was the Castillo. They disabled five bronze cannons to ensure that the ordnance would not be used. It was on this mission that Fremont was supposed to have named the mouth of San Francisco Bay the “Golden Gate”. In 1848, California was annexed by the U.S. in the Treaty of Guadalupe Hidalgo. In 1851, the U.S. military recommended a series of fortifications to protect San Francisco Bay and Fort Point was selected as a site that would defend the maximum amount of harbor area. In 1853, the U.S. Army Corps of Engineers began work on a new fort using a design that featured thick, smooth masonry walls, iron shutters, and multiple tiers of heavy artillery. The design specified that the lowest tier of artillery be as close as possible to water level so cannonballs could ricochet across the water’s surface to hit enemy ships at the waterline. To achieve this, workers had to first blast the 90 feet (27 m) high white cliffs down to 15 feet (4.6 m) above sea level. The size and layout of the fort became obsolete by the close of the American Civil War era as new advancements in military construction emerged, but the military intermittently used the fort as army barracks and storage. In the 1930s, plans for the Golden Gate Bridge called for demolition of the fort, but Joseph Strauss, the Chief Engineer on the project, recognized the historic significance and redesigned the bridge’s southern footing to arch over the fort. On October 16, 1970, Fort Point became a National Historic Site and today the fort is a popular tourist attraction within the Golden Gate National Recreation Area.

The present Fort Point Lighthouse is the third navigational aid structure to mark the south shore of the Golden Gate. Construction of the original lighthouse and its twin on Alcatraz Island began in 1852 and was completed in 1853. It was designed in the Cape Cod style with an integral tower, but it stood for only three months and was never used. While awaiting the arrival of its Fresnel lens from Paris, it was torn down to make room for the new fort. In 1855, the second lighthouse at Fort Point was completed with a fourth-order Fresnel lens on top of a squat wooden tower 36 feet (11 m) high with four sides that sloped up to a square watch room. It was built on the narrow ledge between the fort and the water. Erosion undermined its foundation, and in 1863 it was torn down to make way for a bigger seawall. In 1864, Fort Point’s third lighthouse was built atop the wall of the fort. It was an iron skeleton tower with a spiral staircase 27 feet (8.2 m) tall. A fifth-order lens was originally fitted, but in 1902 the lens was upgraded to a fourth-order lens, which produced alternating white and red flashes. In 1933, when work on the Golden Gate Bridge began, a fog signal and navigational light were placed at the base of the bridge’s south tower. On September 1, 1934, after the towers for the Golden Gate Bridge were completed, the lighthouse was deactivated. The bridge would block much of the light from the lighthouse, and as the towers were 740 feet (226 m) tall, they provided a more visible warning for mariners. Read more here and here. Explore more of Fort Point here:

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This ‘warming stripe’ graphic is a visual representation of the change in global temperature from 1850 (top) to 2019 (bottom). Each stripe represents the average global temperature for one year. The average temperature from 1971-2000 is set as the boundary between blue and red. The color scale goes from -0.7°C to +0.7°C. The data are from the UK Met Office HadCRUT4.6 dataset. 

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