Coal-bearing beds in Alberta occur in three different geological horizons namely, the Edmonton, the Belly River, and the Blairmore-Kootenay. The first two horizons are in the Upper Cretaceous formation and the third in the Lower Cretaceous.
Coal was formed largely from plant material. The accumulated plant material becomes more compact with time so that coal is a mineral product, which is always getting older, and with age the character of the coal changes. It is important to understand that the chief factors that determine the character and therefore the classification by rank of the coal are age and pressure. In Alberta the Blairmore-Kootenay coals of Lower Cretaceous age are more mature than most of the Belly River or Edmonton coals that are of Upper Cretaceous age. That is why the rank of most Alberta coals increases from east to west, that is, from the plains to the foothills and mountains where the coal seams have been affected to a greater degree by mountain-building forces. This explains why the Lethbridge coal is of higher rank than the Redcliff coal, although the coals in the two areas are of the same age.
Stansfield, E. and Lang, W.A. (1944): Coals of Alberta, their occurrence, analysis and utilization; Research Council of Alberta, RCA/AGS Report 35, 174 p.