If either two stratified geochemical reservoirs or a mineralogical phase change in perovskite exist in the mantle, the boundary between the geochemical layers or mineralogical phases may lie at a depth between 1800 km and the core-mantle boundary (CMB). We searched for the ScP precursors (S-to-P reflections) that would be generated at the boundary by stacking hundreds of short-period, vertical component teleseismic waveforms from the dense Pacific Northwest Seismic Network (PNSN). PNSN recorded clean ScP phases and saved the P to ScP time window for four earthquakes. We estimate our detection threshold to be impedance contrast >4%. None of the data show evidence of sharp structure between 1800 km and the CMB, suggesting that a sharp seismic discontinuity does not exist between 1800 km and the CMB under the Gulf of Alaska and Mexico: any discontinuity would need to be diffuse, have large topography, or have a small impedance change. While not conclusive, this observation is consistent with an Earth model lacking a global boundary separating geochemical reservoirs and lacking a phase change in this depth range.