New SIO deep sea magnetic data from immediately west of southern Baja California are presented. Anomalies in the standard sequence of ages from 25 m.y. to 4 or 5 m.y.B.P. are correlated from north to south within the area. Interpreting the anomaly patterns in light of the assumptions of plate tectonics gives the following picture of evolution of the East Pacific Rise as it approached North America. Before 20 m.y.B.P., two Pacific Ocean plates were spreading east-west from each other, locally unaffected by approach of the continent. By 15m.y.B.P. the ridge had split into a “Y” of three ridges radiating from a point. Three Pacific blocks were present, the southern two still spreading east-west, while the northern one inside the arms of the “Y” had a northward relative component of motion presumably derived from coupling to the continent somewhere to the north. Soon after 12 m.y.B.P., the three-ridge system culminated, followed by reorganization into a single ridge spreading NW.—SE. Since 5 m.y. ago the present system has prevailed, with the remnants of the old Eastern Pacific block fused to the continent north of the Rivera Fracture Zone. The Western Pacific block has spread to the northwest, opening the Gulf of California, and carrying southwestern California and Baja California with it.