During 1964 an east–west geomagnetic recording profile was operated in western British Columbia to investigate the "coast effect" in geomagnetic variations. The profile extends earlier work by Hyndman in central and eastern British Columbia. Results of the survey are described and interpreted. The restraints imposed on the choice of models have been based primarily on compatibility with other geophysical information, rather than on agreement with idealized theoretical conductivity structures.The "coast effect" has been interpreted as being due to two different effects: the land–sea interface at periods of less than 10–15 minutes, and an associated upper mantle structure at periods of more than 30 minutes. The latter has been tentatively identified with the discontinuity of the seismic low-velocity layer under continental and oceanic regions.On the basis of additional geophysical information, the interpretation has been extended to include the anomalies in the southwest United States reported by Schmucker in 1964. The inland anomalies in western North America have been interpreted on the basis of a moderately conducting zone at the top of the mantle, tentatively identified with the anomalous seismic low P velocity zone. If confirmed by further observations, these results imply temperature rather than density effects as the cause of the low P velocities.