Coals in the post-Wapiabi strata of the Rocky Mountain Foothills are found in the upper Campanian (uppermost Belly River and lowermost St. Mary River formations), lower Maastrichtian (upper Brazeau Formation) and lower Paleocene (upper Coalspur Formation) stratigraphic sequences. Large-scale facies relationships within these sequences, combined with sedimentologic data for the coal-bearing strata and their correlatives, indicate that the coal-forming swamps originated in marginal marine, marginal lacustrine, and flood-plain enviornments. The coal-forming swamps developed only when there was a combination of appropriate diastrophic and favorable climatic conditions. This happened twice in the depositional history of the Rocky Mountain Foothills during phases of relative tectonic quiescence (early Maastrichtian and early Paleocene) in the northern, humid part of the basin. At those times the semiarid conditions in the southern part of the basin precluded the formation of coal-forming swamps. The semiarid conditions in this part of the basin were overridden in the late Campanian by the influence of the Bearpaw Sea, which led to the formation of thin coals in the marginal marine environment.