Time varying geomagnetic fields observed in southern British Columbia and Alberta are analyzed using optimum "transfer function" techniques for periods from 4 to 240 min. Estimates of the frequency and spatial dependence of the "coast effect" are obtained. Inland anomalies are delineated and attributed to variations in conductivity in the upper-to-middle crust coupled with an abrupt east–west transition in deeper conductivity. Regional attenuation of the uncorrelated portion of the vertical magnetic component is indicative of high conductivity near the crust mantle transition zone under the western part of the Cordillera. The conclusion is in agreement with magnetotelluric observations and other geophysical data.