San Bruno Mountain - Topography, Geology and Climate

Topography, Geology and Climate

The name "San Bruno Mountains" was first affixed by the Geological Survey of California in 1865, describing the place as a short range extending from Sierra Point nearly to the Pacific Ocean. The mountain itself actually consists of two parallel northwest trending ranges separated by the Guadalupe Valley. These two ranges are united by a saddle at the northern end of Colma Canyon. The northernmost range attains a peak of 850 feet (259 m), while the southern range rises abruptly from Merced Valley at the south to reach Radio peak in a horizontal distance of only 0.8 mile. (1.3 km) This southern range is often referred to as San Bruno Mountain., while the northern range is called the Crocker Hills.

The region is drained by two major streams: Guadalupe Creek flowing through Guadalupe Valley, and Colma Creek from a source in the Flower Garden into the deep cleft of Colma Canyon and thence into Merced Valley.

In the 1850s San Francisco Bay lapped against the eastern sandstone cliff flank of San Bruno Mountain, whereas today the entire shoreline is bay fill.

These two mountains are underlain primarily with late Cretaceous dark greenish-gray graywacke, a poorly sorted sandstone containing angular rock fragments, about ten percent feldspar and detrital chert. The angular unsorted content implies a rapid erosion and burial in a depositional basin, with an outcome of few fossils. Exposed graywacke can be observed on high ridges and on the steep canyon walls. Radiolarian chert is exposed on certain south facing slopes of San Bruno Mountain and at Point San Bruno.

The most important rock type is serpentine, a greenish soft material that is the California State Rock. Outcrops of this rock are found near Serbian Ravine and at Point San Bruno. Serpentine's importance is its unusual and diverse mineral composition which imparts to associate soils the ability to host rare plants, not usually supported on common soils. Therefore, San Bruno Mountain is a habitat for a variety of uncommon plants, which in turn host even rarer animal life.

Since the climate is dominated by marine air flow, temperatures are milder in the winter and summer on these mountains. Furthermore, summer temperatures are further reduced by the annual appearance of marine fog enshrouding the mountains most mornings between late June and late August; this fog is particularly pronounced on the western slopes. Minimum credible temperature might extend as low as 20°F (-6.6°C) in the sheltered valleys. Winds are higher than on reference locations at surrounding points; in fact, on ridges it is not uncommon for the most fierce winter storms to produce gusts from 50 to 80 miles-per-hour (80.5 – 129 km/h). Precipitation is similar to surrounding cities, or about 22 inches (56 cm) per-annum, with approximately 66 days-per-annum realizing perceptible rain. On several occasions the mountain has been temporarily covered by snow, including December 1932, January 1952, January 1957, January 1962, and February 1976.