The Tzartus Island limestone showing is located on the northeast corner of the island, 40 kilometres southwest of Port Alberni.
Limestone of the Upper Triassic Quatsino Formation (Vancouver Group), up to 700 metres wide, outcrops for 1.5 kilometres northward along the shore. The limestone strikes 150 to 178 degrees and dips 70 degrees southwest. The mass is bounded to the west by grano- diorite of the Jurassic Island Intrusions. Dykes are reported to intrude this limestone.