The Late Cretaceous Western Interior Basin of North America is generally considered to have been a retroarc foreland basin. Flexural backstripping of the stratigraphic record from 97.2 Ma to 73.4 Ma, along a section perpendicular to the Wyoming-Idaho salient of the Sevier belt, clearly demonstrates that there were components of subsidence in addition to those driven by the thrust and associated sediment loads. The simulation demonstrates that the flexural foredeep is only ∼180–120 km wide and shows a forebulge located near the subsequent Laramide Rock Springs uplift. Foredeep strata between the Rock Springs uplift on the east and the thrust front to the west were mostly involved in the eastward- overlapping thrust belt or deeply truncated by inferred rebound after 78.5 Ma. The difference between the observed, decompacted, cumulative subsidence and the amount of subsidence that can be explained by simple flexural loading, the “residual” subsidence, increases from ∼800 m in eastern Wyoming to ∼1.8 km near the thrust belt. The westward-thickening Upper Cretaceous sediment wedge filled the accommodation space generated by downward tilting to the west of the North American cratonic margin by underplating and mantle flow associated with the shallowly subducted Farallon plate.