Over 5600 short-period recordings of teleseismic events were used to create deterministic maps of P-wave scatterers in the upper mantle beneath Southern California. Between depths of 50 and 200 kilometers, the southern flank of the slab subducting beneath the Transverse Ranges is marked by strong scattering. The marked scattering indicates that the edge of the slab is a sharp thermal boundary. Such a boundary could be produced by slab shearing or small-scale convection in the surrounding mantle. The northern limb of the slab is not a strong scatterer, consistent with thicker lithosphere north of the Transverse Ranges.