A study of the Bouguer gravity field from eastern Washington to western Montana provides new constraints on drawing accurate crustal sections and insights into the tectonic history of the northern Rocky Mountains. A forward gravity model along a 500 kilometer profile reveals an undulating Moho, a crustal root beneath the Idaho Batholith, a sliver of continental crust beneath the accreted terranes and a steeply dipping fault along the Western Idaho Suture Zone. This model indicates that the crust/mantle boundary rises to the northwest and southeast from a depth of 42 kilometers beneath the Idaho Batholith to about 32 kilometers in eastern Washington and western Montana. This crustal root is expressed as a regional gravity low of about 300 kilometers wavelength. West of the Western Idaho Suture Zone, a sliver of continental crust is wedged beneath accreted terranes. During Late Cretaceous to Paleocene shortening, the lower continental crust detached from its cover and was over ridden by exotic terranes. Examination of the free air and Bouguer anomalies plotted versus elevation clearly show the distinction between the two lithospheric plates along the suture zone. These plates appear to be in approximately isostatic equilibrium. The present-day Andes serve as a geotectonic model of the Late Cretaceous Rocky Mountains.