The interrelationship between the Upper Cretaceous Mesaverde, Lewis, Fox Hills, and Lance formations of Northwestern Colorado and south central Wyoming, and the position of the Cretaceous-Tertiary boundary in this area, have been studied in detail for over 50 years. With the addition of considerable subsurface control in the past few years, it is now possible to present a more complete picture of the stratigraphy of this part of the section. Additional paleontological collections are needed and, in the deeper parts of the Sand Wash and Washakie basins, there are large gaps in the subsurface control. Detailed surface mapping of the intertonguing relationships of the Upper Cretaceous formations could add immeasurably to the stratigraphic knowledge of this area. The occurrence of gas, and possibly oil, in this part of the stratigraphic column, creates an economic reason for more precise knowledge of environments of deposition and paleogeography. The purpose of this paper is to review the origin of stratigraphic nomenclature now in use and to add new information regarding regional stratigraphy and its relationship to gas entrapment.