Spatial averaging of short-period teleseismic P wave arrivals across SCARLET constrains upper mantle lateral velocity variations beneath southern California. We compare dT/dΔ, back azimuth, and averaged arrival time estimates determined from the entire network for 154 events to the same parameters derived from small subsets of SCARLET. The resulting relative array diagrams and plots of 'net subarray delays' computed for 171 overlapping subarrays exhibit large anomalies that vary smoothly as a function of subarray position, implying substantial lateral heterogeneity under the network. Patterns of mislocation vectors for more than 100 subarrays permit identification of an east-west striking high-velocity antiform beneath the Transverse Ranges. Further constraints on both the lateral configuration and depth of the anomaly are obtained through thin lens analysis of the net delay data, where all the time perturbations are assumed to originate at a single depth. Projection of 2798 net delays requires a lens deeper than 100 km. A representative lens at 150-km depth features low velocities beneath the Salton Trough as well as a concentrated high-velocity zone between 116.3¿-118.3¿W and 33.7¿-34.8¿N, consistent with the relative array diagrams and the PKP-delay contours of Hadley and Kanomori <1977>. Therefore the bulk of the high-velocity material observed by Hadley and Kanamori may in fact lie or deeper than 150 km rather than 40-100 km. This possibility prompts renewed speculation regarding the relationship of this body to the plate tectonic history of the region.