The Marble Bay showing is located at the east end of Cowichan Lake, 30 kilometres west-northwest of Duncan.

A 150 to 300 metre thick limestone bed strikes northwest for 4.75 kilometres across the peninsula at the east end of Cowichan Lake. The limestone has been correlated to the Upper Pennsylvannian to Lower Permian Mount Mark Formation, Buttle Lake Group (previously Buttle Lake Formation, Sicker Group). The bed dips between 30 and 72 degrees southwest. The limestone is overlain by Upper Triassic Karmutsen Formation, Vancouver Group basalt and underlain by bedded chert, tuff and breccia of the Upper Devonian McLaughlin Ridge Formation, Sicker Group. A few dykes intrude the limestone.

The formation consists of chert and well-bedded siliceous limestone interbedded with relatively pure, light grey to white, massive, fine to medium grained limestone containing abundant crinoid fragments. Thin sections reveal numerous crinoid stems and sponge spicules. A chip sample taken at 3.0 metre intervals across 61 metres of limestone just north of Marble Bay on the south side of the peninsula contained 51.5 per cent CaO, 0.84 per cent MgO, 5.70 per cent insolubles, 0.28 per cent R2O3, 0.10 per cent Fe2O3, 0.02 per cent MnO, 0.04 per cent P2O5, 0.011 per cent sulphur, 41.4 per cent ignition loss and 0.13 per cent water (Bulletin 40, page 47, Sample 2).