At the Fort Steele showing, a bed of coarsely crystalline, light creamy-grey magnesite, approximately 45 metres thick, occurs within the Lower Cambrian Cranbrook Formation. Near the basal and upper contacts, the layer contains quartz and calcite but usually has a core zone of relatively pure magnesite in the order of 12 metres thick. The magnesite weathers to a rough surface with a light buff colour.

The magnesite grades upwards into a series of green quartzites with well-rounded quartz grains; accessory minerals include chlorite, serpentine and talc with minor hematite, sphene and zircon. In general it overlies light colored quartzites but in the Boulder- Wallinger creeks area these basal quartzites are, at least in part, replaced by coarse conglomerates resting unconformably on the Siyeh or Gateway formations.