Twin Peaks, San Francisco

Twin Peaks, San Francisco

Located in San Francisco, the Twin Peaks are two elevations with the height of about 920 feet (280 m) each. They are near the city’s geographic center. The hills are the highest elevations in the city except for Mount Davidson. The North and South Twin Peaks have their own names: Noe Peak/South Peak and Eureka Peak/North Peak (Diamond Heights and Twin Peaks). The Eureka/North Peak is a popular observation point, which is known in the area as ‘Christmas Tree Point’. It provides a fine view of the San Francisco Bay and most San Francisco. The hills were dubbed “Twin Peaks” in the nineteenth century when the city passed under American control.

San Francisco lies on the foundation of Franciscan rocks. Despite the fact that the Franciscan rocks come in many types, they share a common trait in that regard that they were all joined in the zone of the Franciscan subduction in the time of the collision between the North American plate and the Farallon plate in Cretaceous and Jurassic time around 65 to 175 million years ago. The geologists divide the Franciscan rocks in the San Francisco area into five separate rock units.

The Twin Peaks consist of red chert and pillow basalt of the Marin Headlands terrane. Those rocks formed the upper part of the Farallon plate some time ago when it was at a distance of several thousand miles west of San Francisco. In the San Francisco area, the Franciscan rocks are found in five bands tilting in the northwest direction. They cut diagonally across the city. If traced deep below the earth’s surface, one will find that these rocks form layers that are placed on one another like giant pancakes and tilt in the northeast direction. The visible bands of rocks that appear on the surface present inclined edges of those rock layers.

The Twin Peaks, a San Francisco’s famous tourist area, were formed from red chert and pillow basalt which make the Marin Headlands terrane lying in the Franciscan geological area. The pillow basalt and red chert make up much of the Marin Headlands terrane. They are visible along the road, which goes around the top of Twin Peaks. The Twin Peaks are not quite identical hills. The southern peak is made of pillow basalt and the northern peak is made of red chert. The red chert can be seen in layers, which are about one or two inches thick. This chert is very hard and resists weathering. It tends to form hill tops and ridges. The composition of the southern peak is quite different. It consists of pillow basalt, which, where exposed, looks yellow-brown and chunky.

The pillow basalt and red chert were formed along the spreading of the East Pacific Rise in the Jurassic period. It was when the spreading center located far to the west of North America. During the formation of rocks, they were moved to the upper part of the Farallon plate and were moved several hundreds of miles the east into the zone of the Franciscan subduction (San Francisco 48).

The area of San Francisco lies within the seismically active Bay Area of Northern California. The Franciscan Formation is the dominant geologic formation of the San Francisco Peninsula. Franciscan rocks are primarily made of various kinds of sandstone. Other common types of rock in the area include shale, chert, and conglomerate. Soils, which are present in the area, reflect the conditions of local bedrock. They represent thin and rocky residual soils found over bedrock, which are predominant in some areas.  At Twin Peaks, the soil complex is represented by a mixture of fine sandy loam and very gravelly sandy loam. During surveys, disturbed soils were detected, which are represented by cut and fill material. They are found where roads or other structures were built.  Soils in some areas are represented by thicker loamier soils. They consist of a mixture of sandy loam, fine sandy loam, and gravelly loam. The significant physical process that affects the area is erosion. Majority of erosion areas are not active around the year. That happens because of the seasonal input of water. Such problems remain and are aggravated by road cuts, steep slopes and other man-made surfaces. Most of San Francisco westward of Twin Peaks has dune sands under surface. The sands are wind-blown and relatively clean deposits (Geology, Soils and Hydrology).