There is a regional unconformity across most of northern Wyoming and southern Montana during the latest Cretaceous, before 67 mya. Sedimentation first resumed in the Big Horn Basin, and extended westward and eastward during the late Cretaceous. Drainage was uninterrupted eastward during the deposition of the Hell Creek formation, indicating that regional uplift was not present at this time (Belt et al. 1997). The uplift of the Big Horn Mountains likely occurred during an interval between 65 to 62 mya (Crowley et al. 2002).