The Maw showing is located on a cliff 5 kilometres northwest of Armstrong.

In this area, sedimentary and volcanic rocks of the Upper Triassic to Lower Jurassic Nicola Group are in fault contact to the north with Cambrian-Ordovician volcanic (Tsalkom Formation) and sedimentary (Sicamous Formation) rocks. To the south, the Nicola Group is in probable unconformable contact with Devonian to Triassic sedimentary and volcanic rocks of the Harper Ranch Group. Cretaceous granodiorite plugs of the Salmon Arm Intrusions intrude the Nicola, Sicamous and Tsalkom rocks. Outliers of Eocene Kamloops Group volcanic rocks are present in the area.

A marble unit in the Tsalkom Formation comprises blue and white medium-grained calcium marble. The unit has a massive appearance, but is thin-bedded and up to 15 metres thick. Thin schistose sheets occur between some of the beds. Scattered siliceous stringers and inclusions stand out on weathered surfaces.

Sampling of marble in 1944 returned values of 94.2 per cent calcite, with a CaO grade of 52.8 per cent. Sampling in 1961 assayed 54.4 per cent CaO, 0.23 per cent MgO, 0.09 per cent Fe203 and insolubles at 1.6 per cent (Minister of Mines Annual Report 1961, pp. 146, 148).