Vignette 295 > Physiography

The city of Mill Valley is equidistant at eight miles from both the San Andreas and Hayward faults. It straddles several ridges and canyons on the southeast flank of Mt. Tamalpais, and extends eastward into lowlands at the northwest end of Richardson Bay. With a minor exception, its watershed is drained by Arroyo Corte Madera del Presidio into Richardson Bay.  Its geology consists of sandstones, mudstones and shale of the Franciscan mélange. Graywacke, a type of sandstone, is the single most abundant rock. Outcrops of chert and greenstone (a partially metamorphosed igneous rock) are also common. Mill Valley has what is called a Mediterranean climate, characterized by moderate year-round temperatures, wet winters and dry summers.  Such climates are also found along certain coastlines of Chile, Australia, New Zealand and South Africa, as well as around the Mediterranean Sea.  Mill Valley’s average high temperature is 76 degrees, and its average low is 43 degrees.  Microclimates, areas of differing sunshine, wind, temperature, fog and rainfall, are widespread throughout the canyons and hillsides. Annual rainfall averages about 35 inches, but it is typically almost double that in Cascade and Blithedale canyons.  Measurable rainfall is rare during the summer.