A detailed comparison between the pre-Cretaceous stratigraphic record in the Williston Basin and the available information about Phanerozoic sea-level changes indicates that most stratigraphic breaks in the basin reflect eustatic control. The basin appears to have been subsiding continuously and steadily about the same centre for most of the Phanerozoic above a thick (55 km) continental crust. Deposition kept up with subsidence, and most of the sedimentary record was laid down close to sea level. Erosional breaks in the stratigraphic record may represent periods of removal of strata during episodes of low sea level. A possible explanation of this essentially steady, nonexponential subsidence record is that a mafic subcrustal body is undergoing transformation to eclogite.