A 30 km-long N-S seismic reflection line was shot by California Consortium for Crust Studies (CALCRUST) across the southern Mojave Desert and onto the northern flank of the San Bernardino Mountains in southern California. On the northern end of the seismic section, the reflectivity increases markedly in the midcrust at a depth corresponding to a two-way travel time of 4 to 5 s (12--15 km), suggesting a transition between nonreflecting brittle upper crust and reflecting ductile lower crust. The high reflectivity disappears at about 8 s (24 km) and may be correlated with a change in seismic velocity in the lower crust from 6.3 km/s to 6.8 km/s. A band of reflectivity between 9.5 and 10 s (27--30 km) is believed to represent the Moho. The midcrustal reflectivity transition and Moho both deflect downward toward the San Bernardino Mountains uplift over the entire length of the profile. The deflection of the midcrustal transition (12¿) appears greater than that of the Moho (6¿), resulting in a thinning of the lower crust to the south beneath the uplift. In addition, the midcrustal transition coincides with the base of the seismogenic zone (brittle-ductile transition?) which is also dipping southward beneath the San Bernardino Mountains, while the Moho deflection is consistent with elastic flexure resulting from edge loading by the San Bernardino Mountains which have been thrust over the Mojave block. It is suggested that the thinning of the lower crust beneath the San Bernardino Mountains is a result of north directed ductile flow in response to loading by the over thickened upper crust. Since a portion of the load is transmitted through the lower crust to the Moho, the time constant for flow equilibrium must be of the order of or greater than that for the time of uplift (≥2 m.y.). ¿ American Geophysical Union 1992