This subsurface reef in west-central Alberta, and its platform, form the Swan Hills Formation of the Beaverhill Lake Group, Middle to Late Devonian in age. Structure and isopach maps, and cross sections illustrate the atoll-like geometry and paleotopography. Stromatoporoids and algae were most important reef organisms; brachiopods, corals and other fossils are common. The complex may be subdivided into 7 biotopes, each having distinctive biosomes; and into 5 facies, 11 microfacies, and 32 rock types. A number of diagenetic factors affected the reef limestones, but original sedimentary features are not greatly altered. The geological history of the reef complex has been portrayed in 6 stages: reef platform, biostromal-type carbonate; biohermal reef growth; green shale, in soluble residue; calcarenite bank; reef death, regression and erosion; and argillaceous sedimentation.