consisting of mostly low-density single-family homes. Natural history Geology and soils Like the rest of the California Coast Ranges, Mount Tamalpais is the result of uplift, buckling, and folding of the North American plate as it slides along the Pacific Plate near the San Andreas fault zone. In 2004, a team of Penn State geoscientists suggested that a blind thrust fault, like the one that caused the infamous Northridge earthquake, lies beneath Mount Tamalpais. This idea was partly based on the steepness of Mount Tamalpais and of nearby Bolinas Ridge, such steepness on the visible surface often being the result of blind thrust faults. Another reason for the suggestion was that the San Andreas Fault creeps more slowly south of Mount Tamalpais than it does in its sections north of Mount Tamalpais and in the Olema Valley, and that the existence of a blind thrust fault may explain the different creeping velocities. If a blind thrust fault does exist under Mount Tamalpais, and if it ruptures, it could be potentially devastating to the North Bay, San Francisco, and any other nearby locale resting on unstable earth and loose fill. Major Mount Tamalpais rockforms include serpentine, particularly evident in outcroppings near the summit and on the north side. A number of serpentine endemic plants grow in the serpentine soils in this part of the mountain. Hydrology Since the steep slopes of Mount Tamalpais force out moisture from passing storms and/or fog, the mountain supports several year-round streams like Redwood Creek on the southern face of the mountain down into Muir Woods. The steep southeastern slopes of Mount Tamalpais drain to Arroyo Corte Madera del Presidio, which in turn discharges to Richardson Bay. Climate With its height, various faces, and proximity to the ocean and bay, the mountain contains many microclimates, ranging from cool and foggy in lower ocean-facing valleys with their redwood forests, to hot and dry on the manzanita slopes, cool and breezy at the summit, and shady on the heavily Douglas-fir-forested north slopes near Alpine Lake. Annual precipitation around Mount Tamalpais varies greatly from around 27.5–31.5 inches (700–800 mm) in the drier, eastern foothills to about 59 inches (1,500 mm) near the Bolinas Ridge, close to the Pacific Ocean. Both Mount Tamalpais and the Bolinas Ridge force moisture out of the air efficiently, since the air is cooled rapidly as it ascends the steep mountain faces and thus Mount