The western half of Quadra Island lies within the Insular belt and is underlain primarily by andesitic volcanic rocks of the Upper Triassic Karmutsen Formation, Vancouver Group. These are interbedded with, and overlain to the east by a northwest trending belt of Upper Triassic Quatsino Formation limestone, also of the Vancouver Group.

The eastern half of Quadra Island lies within the Coast Crystalline belt and is mainly underlain by Juro-Cretaceous intrusive rocks of the Coast Plutonic Complex. These rocks are in fault and/or intrusive contact with the Insular rocks along a northwest trending zone from Open Bay to Granite Bay.

The Open Bay Limestone consists of predominantly argillaceous limestone of the Upper Triassic Quatsino Formation interbedded with and overlying ellipsoidal and amygdaloidal andesite of the Upper Triassic Karmutsen Formation. The lowest limestone member is poorly exposed at the west end of Open Bay but well exposed about 1.6 kilometres inland where it exceeds 30 metres in thickness. At the shore it dips 30 degrees eastward and is overlain by 106 metres of ellipsoidal and massive andesite. This is overlain by 15 metres or less of limestone which in turn is overlain by 61 metres of andesite. Possibly as much as 152 metres of steeply dipping limestone and at least one andesite body are incompletely exposed in the next bay to the east. East of this bay, intensely folded argillaceous limestones and pillow lava are exposed in a belt about 609 metres wide bounded on the east by intrusive granitic rocks.

The prevailing dip of the rocks throughout Open Bay is northeast. The extreme folding of the sediments in the eastern half of the bay, however, obscures the general structure. Granitic dykes up to a metre wide and lenticular sills a few centimetres to a metre wide are common. The contact with the granitic rocks at the northeast edge of the belt is irregular but follows in general a relatively straight line northwest across the island, truncating the limestone belt at an acute angle.

The limestone is generally black and granular and emits a distinct odour or hydrogen sulphide when broken. Fine laminae of argillaceous impurities are distributed throughout the rock. Several samples were taken across the northeastern folded belt along the shore of Open Bay. One sample analyzed (in per cent): CaO 51.69, MgO 0.64, MnO 0.025, Fe2O3 0.29, R2O3 0.50, Insol. 5.08, P2O5 0.095, S 0.13, Ig. Loss 41.39 and H2O 0.06 (Bulletin 40, page 84).

The limestone in the upper part of the section is high in insoluble matter whereas the lowest belt of limestone is of a better grade.