This geologic cross section shows the drill hole and the rocks it goes through. West of the fault is Salinian granite (gr); east of the fault is Franciscan melange (KJf), a thoroughly mixed set of subduction-related rocks. Movement along the fault has brought these very different rock types together. Younger rocks of Tertiary and Quaternary age are in light tan and yellow; these formed in shallow seas and continental basins. The hachured region marks a zone of low electrical resistivity where fracturing and groundwater penetration affect the geophysical character of rocks near the fault zone.
Notice the numerous secondary faults around the San Andreas. Some of these are probably older traces of the fault, and others reflect the pressures across the fault that come and go over geologic time. Today there is compression across the fault causing motions about 10 percent the size of the strike-slip motion along the fault, that is, a few millimeters per year. That accounts for the uplift of hills throughout the length of the San Andreas fault system.