A regional unconformity lies beneath Harebell-Lance-Hell Creek nonmarine facies and above much older Meeteetse and Fox Hills marginal marine facies. This pre-Lancian unconformity (U 1 ) extends eastward from the Sevier overthrust belt in northwestern Wyoming to the southwest margin of the Williston cratonic basin in the tri-state area of Montana and the two Dakotas. Chronostratigraphic data from five areas scattered from west to east across 600 km of the Western Interior Basin bracket the ages of this unconformity with precision that is based on isotopic ages, magnetozone boundaries, and ammonite range zones. Deposition on top of this unconformable surface occurred 3.5 Myrs earlier in NW Wyoming than in SE Montana. The erosion represented by the unconformity migrated eastward with time, generally removing 2 Myrs of strata as it went. A tectonic cause is more probable than a eustatic one for the U 1 unconformity. If an erosional gap resulted from lowering global sea level, a larger time gap would have been produced on the eastern side of the Western Interior Basin than on the western side. Our data show the opposite pattern of ages: older and wider time gaps occur in the Jackson Hole Basin of NW Wyoming than are found in north-central Wyoming or in SE Montana. Sedimentological studies in the tri-state area in this and previous research shows that at the beginning of Lancian time, neither the Big Horn Mountains nor the Black Hills were uplifted source areas. Detailed sedimentological investigation has identified a trunk paleodrainage of the Hell Creek Formation in southeastern Montana (67 Ma to 65 Ma) that flowed to the east and southeast. Hell Creek and Lance drainage in central Montana and central northern Wyoming flowed northeast to east (Connor, 1992; Lillegraven and Ostresh, 1988) and represented tributary drainage to the trunk system of eastern Montana. The position of this trunk drainage follows the axis of the foreland basin, which by 67 Ma had shifted eastward into eastern Montana, half way between the Sevier overthrust belt and the North American craton. The Big Horn Mountains were uplifted approximately at the time of the K/T boundary, but this uplift lay sufficiently to the west that it did not divert southeast flowing Puercan drainages of the Tullock and Lower Ludlow members of the Fort Union Formation. At some time after the Puercan and prior to the mid-Torrejonian, the Black Hills were uplifted. This uplift produced a local unconformity (U 2 ) along the Miles City Arch, a structure that is co-axial with the Black Hills. This uplift diverted the regional paleodrainages of eastern Montana and western North Dakota from southeast to northeast. Early Paleocene Cannonball oscillatory, but generally westward transgressing changes in sea level seem to have been independent of the Black Hills uplift event that produced the U 2 unconformity along the Miles City Arch. Transgressions and regressions of the Cannonball sea were also apparently independent of the U 3 unconformity, and its correlative conformity that regionally lies beneath the Tongue River Member of the Fort Union Formation, and above nonmarine facies of the Lebo and Ludlow members in eastern Montana and western North Dakota.