ScS-S residuals obtained at stations of the Missouri-to-Massachusetts (MOMA) temporary broadband seismic array are used to delineate variations in seismic velocity structure above the core-mantle boundary (CMB) at scales smaller than observable with tomographic models. South American earthquakes recorded at MOMA reveal a slow-velocity anomaly that is at least as small as the limit of the resolution of ScS waves, about 300 km across. This is modeled as being within a region of fast velocities in whole-mantle models. The slow ScS-S residuals correlate well with a peak in ScS/S relative amplitudes. The small region of slow shear velocity at the CMB could be a pocket to lower mantle rock trapped beneath the descending Farallon slab, or evidence of chemical boundary layer variations. ¿ 2001 American Geophysical Union