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 of lower mantle rock trapped beneath the descending Farallon slab, or evidence of chemical boundary layer variations.