Deep seismic reflection data have imaged a crustal-scale, west-facing ramp or ramp system in the subsurface of western Canada. In northwestern Canada the ramp is within Proterozoic crust east of the Cordillera and is unconformably overlain by Paleozoic sedimentary rocks, indicating that it was formed during the Proterozoic in this region. Similar structures are visible within the Cordillera in southern Canada and the northwestern United States along a south projection of the ramp observed in the north. In the Monashee Complex of British Columbia and in the Priest River Complex in northern Washington, reflections are visible that dip westward from the surface to near the base of the crust and are structurally discordant with underlying more horizontal reflections, thus outlining footwall ramps. We propose that the Proterozoic crustal ramp in the north and the footwall ramps in the Cordillera probably coincide with a Proterozoic crustal transition from thick craton on the east to thin crust on the west. This transition may have influenced sedimentation patterns during deposition of Middle Proterozoic Belt–Purcell and Wernecke strata (ca. 1.6–1.3 Ga) and probably controlled the arcuate shape of the Mackenzie Mountains during their formation in the Mesozoic. This interpretation is consistent with the notion that thrust sheets of the Mackenzie Mountains did not extend onto the craton a sufficient distance to produce flexural subsidence of the thick crust and associated foreland basin development, whereas thrust sheets of the southern Canadian Cordillera were driven eastward and loaded the thick craton, causing crustal flexure and development of the Alberta Basin.