At the Harriet Lake showing, steeply dipping shale and phosphorite of the Jurassic Fernie Formation unconformably overlie siltstone of the Triassic Sulphur Mountain Formation (Spray River Group). The phosphorite is pelletal, has an exposed thickness of 0.55 metres and contains 13.8 per cent P2O5 (Open File 1987-16). It occurs at the base of the Fernie Group.