A sedimentary outlier, containing beds of mudstone/shale, outcrops over a 15 square kilometre area on Blue Mountain, 3 kilometres east-southeast of the south end of Alouette Lake and 11.2 kilometres north of the village of Whonock.
The outlier contains up to 150 metres of moderately to poorly indurated conglomerates, sandstones and mudstones/shales of the Lower Cretaceous Gambier Group unconformably overlying Jurassic(?) diorite of the Jurassic to Tertiary Coast Plutonic Complex. The strata dip 5 to 25 degrees south to southwest. The sequence consists of a basal boulder-rich conglomerate, 5 to at least 60 metres thick, overlain by pebbly lithic wacke or by 3 metres of bluish-grey sandy shale that contains abundant biotite flakes. This succession is overlain by a bed of red-brown blocky mudstone/shale, 15 to 30 metres thick. The mudstone is dense, massive and slightly tuffaceous with rare wispy laminae. The unit fines upward from a siltstone rich base to claystone in the upper few metres. The mudstone is overlain by up to 65 metres of interbedded pebble conglomerate, lithic arenite and mudstone within repeated fining upward cycles generally 1 to 2 metres thick.
Ceramic tests carried out on samples of the mudstone\shale indicate that it could be used for manufacturing facebrick, common brick or sewer pipe (Geological Survey of Canada Memoir 65, pages 2 to 15).
Blue Mountain Explorations Inc. drilled nine diamond drill-holes in 1990 to define reserves of shale for open pit mining.