A band of limestone of the Upper Triassic Vancouver Group, Quatsino Formation extends east-northeast for 5 kilometres from the east end of Pipestem Inlet to the west shore of Effingham Inlet, 35 kilometres southwest of Port Alberni. The band widens from 500 metres at Pipestem Inlet to 2.0 kilometres at Effingham Inlet. To the south, the band is in fault contact with hornblende gneiss and quartz diorite of the pre-Jurassic Westcoast Complex and volcanics of the Lower Jurassic Bonanza Group. To the north it is bounded by basalts and andesites of the Upper Triassic Vancouver Group, Karmutsen Formation.

At Pipestem Inlet, the band is comprised of fine-grained, blue limestone, while at Effingham Inlet to the east, it consists of white crystalline limestone cut by numerous dykes. In thin section the limestone at Effingham Inlet displays bands of reddish-brown ferruginous mud and unidentified fossil structures cemented by recrystallized calcite and fine limy mud. A series of 26 samples taken across the entire deposit, in a northwest direction, averaged 53.37 per cent CaO, 1.28 per cent MgO, 1.54 per cent SiO2, 0.18 per cent Al2O3, 0.19 per cent Fe2O3, and 43.49 per cent ignition loss (Industrial Mineral File - "Limestone Deposits of the Pacific Northwest," pages 2-3).

The limestone at Effingham Inlet is reported to have been quarried for marble up to 1902 (Geological Survey of Canada, Summary Report 1902, page 63).