Each of several regional or local unconformities may have been identified locally as the "late Cimmerian unconformity," a supposed major break at the base of the Valhall Formation (or Roedby Formation). Over most of the North Sea the base of the Valhall Formation is isochronous, and conformable with underlying sediments. It is detected on seismic reflection profiles because it represents a widespread facies change marking the late Ryazanian transgression. Most of the unconformities are of eustatic origin and the effects of eustatic sea level changes were never completely masked by local tectonics. Thus, in the modeling of individual oil fields, the possibility of sedimentary breaks occurring can be predicted in part by reference to regional or eustatic events.--Modified journal abstract.