Thick cliff forming dolomites and dolomitic limestones of the Helikian Mount Nelson Formation outcrop over 500 square kilometres along the west side of the Rocky Mountain Trench. The unit extends northwestward from Brew Creek along the east slope of the Purcell Mountains for 70 kilometres to Bugaboo Creek.

The carbonates rest on a basal white orthoquartzite member of the Mount Nelson Formation and are overlain by a succession of purple and red shale with dolomite interbeds of the Mount Nelson Formation. The carbonate member varies from 100 to 300 metres thick, while the entire formation varies up to 1950 metres in thickness.

The carbonates are comprised of light grey to purple, reddish buff weathering, thinly laminated to massive dolomite and dolomitic limestone, sometimes interbedded with purple shale. The dolomite commonly contains dark lenses and nodules of chert paralleling bedding. In thin section the rock displays fine grained carbonate, usually with abundant interstitial hematite and scattered silt sized quartz grains. A series of samples taken from various horizons in the formation analysed as follows (Geological Survey of Canada Memoir 369, page 20):

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Sample 286 288 295 300 305 341 383 386

CaO 24.96 27.42 27.70 20.74 15.58 22.86 25.05 22.54

MgO 17.14 19.93 18.64 14.63 11.43 16.40 17.67 15.54

CO2 38.30 42.80 42.75 31.85 24.35 36.30 38.10 34.55

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