Black Mountain, a 860 m high wedge of Franciscan formation mostly comprising basic-ultrabasic rock SW of Palo Alto and just NE of the San Andreas fault, marks an abrupt bend in the fault at the northern end of a 100 km long segment of the fault that strikes 9¿ more E-W than the fault to the north or south. It also bounds a marked change in the physiographic setting of the fault, which to the north follows a well developed linear fault valley and to the south follows a poorly defined topographic trace that traverses the rugged Santa Cruz Mountains. The bend at Black Mountain had a profound effect on the 1906 rupture: the 75 km section to the SE of this point slipped only 1--1.4 m, as compared to the 2.5--4 m typical of the rupture on the San Francisco Peninsula to the NW. This 75 km long slip deficit region from Black Mountain to San Juan Bautista, if ruptured in its entirety would produce a Ms=6.9 earthquake: the conditional probability of this rupturing in the near future is the highest of any section of the San Andreas fault except Parkfield. This earthquake would rupture 30 km farther northwest and be about 3 times larger than that previously proposed by others and this constitutes a greater risk to the southern San Francisco Peninsula than previously expected.