Travel-time data obtained from both natural and artificial events occurring in southern California indicate a major, lateral crustal transition within the Transverse Range Province. The eastern crust is very similar to the adjacent Mojave region, where a crustal velocity of 6.2 km/sec is typically observed. The western ranges are dominated by an extensive 6.7 km/sec layer. Pn velocity beneath the western Mojave, Transverse Ranges, and northern Peninsular Ranges is 7.8 km/sec. The crustal thickness of these provinces is 30 to 35 km. The Transverse Ranges do not have a distinct crustal root. Unlike other provinces within southern California, the Transverse Ranges are underlain at a depth of 40 km by a refractor with a P-velocity of 8.3 km/sec. P-delays from a vertically incident, well-recorded teleseism suggest that this velocity anomaly extends to a depth of 100 km. These data indicate that this high-velocity, ridge-like structure is coincident with much of the areal extent of the geomorphic Transverse Ranges and is not offset by the San Andreas fault. Four hypotheses are advanced to explain the continuity of this feature across the plate boundary: (1) dynamic phase change; (2) a coincidental alignment of crust or mantle anomalies; (3) the litho-sphere is restricted to the crust; (4) the plate boundary at depth is displaced from the San Andreas fault at the surface. Within the context of the last model, we suggest the plate boundary at depth is at the eastern end of the velocity anomaly, in the vicinity of the active Helendale-Lenwood-Camprock faults. The regionally observed 7.8 km/sec layer is suggested as a zone of decoupling necessary to accommodate the horizontal shear that must result from the divergence of the crust and upper mantle plate boundaries. The geomorphic Transverse Ranges are viewed as crustal buckling caused by the enhanced coupling between the crust and upper mantle which is suggested by the locally thin, 7.8 km/sec layer.