San Bruno Mountain is a fault-block horst in northern San Mateo County, California. Rising to a quarter-mile high peak directly out of San Francisco Bay, it also includes a smaller ridge in San Francisco. Viewed from downtown San Francisco, the mountain occupies the southern horizon. It is surrounded as well by the cities of Brisbane, Colma, Daly City, and South San Francisco, and has an important role in the history and life of these communities.
From atop the four mile long ridge, summit trails afford expansive views of the San Francisco Bay Area. Much of the mountain is in the San Bruno Mountain State Park or the adjoining State Ecological Reserve. Radio Peak (elevation ), the highest point, hosts radio/TV broadcast towers serving the hilly Bay Area.
San Bruno Mountain is not part of the nearby Santa Cruz Mountains, though their northern-most peak, Montara Mountain, is only eight miles from Radio Peak. The Santa Cruz range is on the Pacific tectonic plate, however, while San Bruno Mountain is on the North American Plate.
Distinct geology and weather set San Bruno Mountain apart from other California coastal areas. The mountain soil provides habitat for rare and endangered plants (see below) and butterflies; the Callippe silverspot, Mission blue, and San Bruno elfin butterfly all inhabit mountain slopes. The mountain is also a cradle for economically useful plants; the low-growing evergreen San Bruno Mountain bearberry (Arctostaphylos uva-ursi) has become widely used in drought-resistant landscaping.
Location The southern border of San Francisco runs up the north reaches of the mountain; post-WW2 residential tracts of Southern Hills and Bayshore encircle the mountain-side. Construction there inspired Malvina Reynolds song Little Boxes (a 1964 Billboard Hot 100 hit for singer Pete Seeger). The Bayshore neighborhood features the 1941 Cow Palace indoor arena, and faces a large bay-landfill.
At the western end of the mountain, Daly City's "top-of-the-hill" community was founded on John Daly's dairy farm by San Francisco residents fleeing the 1906 quake. Brisbane is on an eastern prominence, while the oldest residential portion of the community of South San Francisco spills into the southeastern valleys. The large cemeteries in Colma abut nearly two miles of the mountain's south flank; most were developed after the City and County of San Francisco evicted all burial grounds from the county in 1912.
The Guadalupe Canyon Industrial Park abuts the north side of the south ridge; it is home to warehousing, storage, and transportation facilities, to Brisbane's city-hall, as well as a bakery and indoor sports facilities. It also contains a large gravel/crushed-rock quarry which opened in 1895, and which has provided material for SFO runways and US 101. Radio towers occupy the highest (1,319 ft) peak, and may be accessed from the San Bruno County Park center, in the saddle between the north and south ridges.
A series of historic maps and photos of the area may be viewed at the website of the City of Brisbane.
Geology, Topography, & Climate
San Bruno Mountain is a part of the Franciscan Formation; sedimentary rock laid down in the Mesozoic, then uplifted as fault-blocks starting in the Pleistocene; prior to the initial occurrence of the San Andreas transform-fault. The formation is centered on present-day San Francisco, composed of small ranges trending northwest-to-southeast from the peninsula up into Marin County. The southern face of the mountain has a concealed fault-zone (the "Hillside" zone) several hundred feet wide through the mixed substrate, running along a nearly straight course from the prominence behind Kennedy Elementary School into the Bay at Oyster Point and Point San Bruno.
The mountain is largely formed of late Cretaceous graywacke exposed on high ridges and on steep canyon walls. This dark greenish-gray rock is a poorly sorted sandstone with angular fragments of feldspar and detrital chert. The fragments suggest rapid erosion and deposit in a sea-basin, visible fossils are rare. Microscopic fossils, though, abound in the radiolarian chert seen on south facing slopes of San Bruno Mountain and at Point San Bruno. Finally, serpentinite outcrops occur near Serbian Ravine and at Point San Bruno; this California State Rock is a greenish soft metamorphic material. With their unusual and diverse mineral composition, serpentine soils host a variety of rare plants which support even-rarer animal life.
The mountain pairs two parallel ranges separated by the Guadalupe Valley and Colma Canyon and united by a saddle. The lower north range (also called Crocker Hills) attains a peak of . The south range rises abruptly from Merced Valley at the south to the Radio Peak in a horizontal distance of only . Slightly to the east of the highest peak, south of the main range, a small block reaches in South San Francisco, named Sign Hill for the large letters there since the 1920s. An even-smaller block rising is now called Siplichiquin Hill to memorialize the nearby ancient Ramaytush shoreline hamlet. Into the early 1900s, San Francisco Bay still lapped directly against the eastern flank of San Bruno Mountain; but today the entire bay shoreline is fill.
The region is drained by two streams: Guadalupe Valley Creek flows east between the two ranges to reach the Bay north of Brisbane - most of it now runs through underground culverts. Colma Creek rises from a source in San Bruno Mountain State and County Park, runs west into Colma Canyon, then turns southeast and runs south along the base of the mountain (fed by Twelve-Mile Creek along Westborough Boulevard, and at least eight other streams) until it reaches the San Francisco Bay north of San Francisco International Airport. steelhead trout historically swam nearly to the headwaters of Colma Creek to spawn; boats carried supplies up to the Southern Pacific stop (now Molloy's Tavern on Old Mission Road in Colma). Seasonal creeks to drain runoff cut ravines in the south face; after a heavy rain a 12-foot waterfall regularly appears in Juncus ravine behind Hillside School, others may be found in Guadalupe canyon.
An inland sand-dune (Colma formation) is found on the western slope behind Daly City's John F. Kennedy Elementary School, more than two-and-a-half miles from the ocean and elevated more than seven hundred feet above sea-level. Reputedly, an exploratory gold mine exists on the mountain, above the Southern Hills., while the South San Francisco Land And Improvement Company registered a small silver, lead, and zinc mine in 1919.
The mountain's climate is dominated by marine air flow heading inland, temperatures are mild to chilly in all seasons. Summer temperatures are further affected by marine fog shrouding the mountain mornings and evenings between late June and October, particularly the western slopes. Minimum summer temperature may fall to in sheltered ravines.
Wind speed is higher than at surrounding locations; The mountain itself divides winds into two flows; north of the mountain they flow through San Francisco's Outer Mission and India Basin districts, and across the bay through Castro Valley to Altamont Pass. South of the mountain, the wind blows down Colma Creek past SFO across the Bay to Fremont and through Niles Canyon or further down the Bay to Morgan Hill.
Precipitation is similar to surrounding cities, or about per-annum; approximately 66 days-per-annum have measurable rainfall. Very occasionally, the mountain been covered with snow (recorded in December 1932, January 1952, January 1957, January 1962, February 5, 1976, February 26, 2011, and February 5, 2019).
Vegetation and ecology The northern end of the San Francisco peninsula was primarily covered by plant communities of the grasslands, coastal scrub, redwood and mixed evergreen forest types prior to being overtaken by urban development. And, though much of the Pacific side of the Santa Cruz Mountains south of Montara Mountain is forested, grasslands and scrub predominated from Montara Mountain to the Golden Gate. Along the San Francisco Bay, most of the original wetlands have been filled in by sediment from inland human activities (hydraulic mining, dredging, farmland runoff) and by purposeful landfill. More than eighty small streams draining to the Bay provided riparian habitat along the peninsula.
San Bruno mountain is not only one of the largest remaining examples of the original ecosystems, but also encompasses many microsystems in its relatively small area. The mineral content of the underlying rock and the high velocity marine air have evolved unusual varieties of flora and sustained specialized fauna that live among them. There are a variety of habitats in this mountainous area, and notably the following rare or endangered flora: Coast rock cress (Arabis blepharophylla) Franciscan wallflower (Erysimum franciscanum) Montara manzanita (Arctostaphylos montaraensis) Niebla halei lichen Pacifica manzanita (Arctostaphylos pacifica) San Bruno Mountain manzanita (Arctostaphylos imbricata) San Francisco campion (Silene verecunda) San Francisco lessinga (Lessingia germanorum) San Francisco owl's clover (Triphysaria floribunda)
The State Parks system, San Mateo County, private agencies, water-districts, and cities surrounding the mountain have all begun efforts to mitigate effects of invasive species, many of which are successful in crowding-out native flora and fauna. Beginning in 1853, for example, many variants of Eucalyptus were brought to San Francisco. Used for windbreaks, shade, and firewood, their ability to thrive and grow quickly in very dry conditions got them planted throughout California in the late 19th and early 20th centuries; increased populations of the monarch butterfly and several bird species were side-effects. Groves of eucalyptus, however, radically alter subterranean hydrology, they are very susceptible to fire and wind, and their shade and chemical composition have proved to discourage other plants and the fauna those plants support. After the Oakland firestorm of 1991, many localities (including San Bruno Mountain State and County Park) began projects to reduce or eliminate eucalyptus stands. Other non-native flora being targeted on the mountain include common gorse and broom (introduced as a garden shrubs), fennel (brought for food), yellow-blooming oxalis (which spreads rapidly, is difficult to control and attracts gophers), as well as a variety of non-native grasses.
The county also reports that plague was detected in rodents on the mountain during the 1940s.
History The earliest Spanish explorers on the San Francisco peninsula found the north end occupied by Ramaytush-speaking Ohlone peoples. The Yelamu seasonal hamlet of Siplichiquin occupied the southeastern point of the mountain, sheltered from western breezes and facing an oyster-laden cove; the occupants covered their dead with the shells. Tested samples from the Siplichiquin shellmound indicate regular and continuous occupation from 3,200 BCE (contemporary with the earliest dynasties of Ancient Egypt; and possibly a Hokan or early Penutian settlement pre-dating the OhloneUnknown) through AD 1800.
Other prehistoric shell middens (names unrecorded and now unknown) have been unearthed during excavations over the past century - now denominated with CA-SMA numbers 44, 299, and 355. These are on the south flank of the mountain where Colma Creek once entered the salt marshes that formerly lined San Francisco Bay. All of them are presently considered outlying seasonal encampments for residents of Siplichichiquin and Urebure, and none have been extensively studied.
Before the gold-rush