The presence of a considerable variation in the thickness of the interval from the base of the Eagle sandstone of Montana time to the top of the Kootenai or Cloverly of Lower Cretaceous age has long been recognized over east-central Montana. As far as is known, however, no systematic study has been published showing the regional variation in thickness or the possible causes of such a phenomenon. This article is therefore an attempt to show by discussion and graphical representation the regional variation of the interval previously described, an interval formerly supposed to include only the Colorado formation of Upper Cretaceous age, but now known to embrace, over the southern part of the state at least, sediments of Montana time. There is also shown overloading of sedime ts over certain parts of the area and the resultant effects on diastrophism and sedimentation, from which additional light is shed on the problem of correlation of the sediments grading from Colorado to Montana time. In order to show some effects of the difference in sedimentation, it has been thought advisable to outline roughly the probable conditions immediately prior to Dakota time, as well as the continued deposition into Montana time, as criteria upon which to draw generalized conclusions. It is suggested that more detailed study of isopach lines may throw more light on proper subsurface mapping, especially of relatively large areas, as well as being an aid in the study of the deposition of sediments.