Seismic and gravity data were used to construct crustal and subcrustal sections from the Tufts Abyssal Plain, west of the coast of Oregon, to southern Idaho. Nearly the entire area appears to be in isostatic equilibrium. Mantle depths west of the Juan de Fuca Ridge, in eastern Oregon, and in southern Idaho are normal for the surface elevations or water depths at these locations, and the corresponding mantle densities and velocities appear to be 'normal'. The mantle under the Juan de Fuca Ridge and the Cascadia Basin (west of Oregon) is relatively shallow and its density appears to be below normal to a considerable depth. Two different mantle structures are considered that are consistent with the seismic and gravity data. Magnetic data were not found to be diagnostic in differentiation between these two structures.Across the continental margin the Mohorovicic discontinuity dips sharply under the continents, being at a typical depth of 18 km near the coast line. The discontinuity is relatively shallow under western Oregon (as has been proposed previously), and the corresponding mantle is described in terms of apparent velocity and density.