Bouguer values are presented for a strip across the southern part of the Canadian Cordillera (49 °N to 51 °N and from 112 °W to 132 °W). The Bouguer anomalies outline density variations in the surface rocks, but where there are regional changes in the topography, these anomalies are distorted by the relationship between Bouguer values and elevation. A Bouguer anomaly profile at 50 °N has been interpreted in terms of Woollard's average crustal model which assumes Airy-type compensation for the topography. Residual anomalies are related to lateral density variations in first the crust and then the upper mantle. Comparison between these models and other geophysical data suggests there is a decrease in the density of the crust and of the upper mantle west of the Rocky Mountains; the transition is related to the western edge of the Precambrian basement. The large residual anomalies in the vicinity of Vancouver Island are attributed to either very dense crustal material or low density mantle material. It is concluded that the anomalous density distribution below Vancouver Island is due to the presence of a remnant plate of oceanic lithosphere below the island which extends eastwards below the central part of the Cordillera. The suggested northern limit of this plate is 51 °N and arguments are presented for the plate being a relic of late Cenozoic convergence between the Juan de Fuca and American plates.