At the Boulder Creek 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 unit is intermixed with quartz and calcite but usually it 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 color.

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-coloured 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.