A band of limestone extends north-northwest for 13.5 kilometres on the east flanks of Mount Spencer and Limestone Mountain, west of the Nitinat River and 20 kilometres southeast of Port Alberni.

The Mt Spencer limestone is part of the Upper Pennsylvanian to Lower Permian Mount Mark Formation (previously the Buttle Lake Forma- tion), Buttle Lake Group. Several east dipping thrust faults displace the limestone. The unit dips shallowly to near vertically to the west and southwest. The limestone is underlain by andesites of the Devonian Nitinat Formation (Sicker Group) and chert, argillite and limestone of the Mississippian to Pennsylvannian Fourth Lake Formation, Buttle Lake Group. Basalts of the Upper Triassic Vancouver Group, Karmutsen Formation unconformably overlie the limestone.