Regional correlations in the Devonian have been obscured by diversity of treatment and the erection of numerous names for local lithologic developments. Broad-scale studies suggest that four operational rock units can be carried over much of the area and that these form a rational basis for analysis of the complex stratigraphic problems involved. Four units are recognized.

(1) A basal unit including the "Basal Devonian unit" and "Dc" unit of Montana. Ghost River formation of the Alberta mountain front, and Elk Point and equivalent units in the Alberta-Saskatchewan subsurface.

(2) A lower limestone unit including the limestone member of the Jefferson formation, the Fairholme formation (minus uppermost beds), and lithologic equivalents in the subsurface.

(3) A dolomite-evaporite unit including the dolomite member of the Jefferson (plus evaporite-breccia zones in the lower part of the Three Forks), the lower part of the Darby formation of Wyoming, the Potlatch anhydrite, Morro member of the Palliser and Alexo silt of the Fairholme, and lithologically equivalent subsurface units.

(4) An upper post-evaporite unit including the upper part of the Three Forks formation, the

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upper Darby, the Costigan member of the Palliser, and equivalent clastic and non-clastic beds between the top of the anhydrite and the base of the Exshaw shale in the subsurface.

An attempt is made to refine correlations by tracing of cyclical evaporite deposits, widespread clastic zones, and biostromal and fossiliferous-fragmental limestones.

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