The underlying Permian (?) or older rocks consist of mainly blackish weathering, dark grey quartzite, interbedded with buff weathering, brown limestone, and light green-grey, well-laminated quartzite. These rocks are concordant with diorite gneiss of the Coast Plutonic Complex.

Most of the limestone beds are less than a metre thick, however a 30 metre bed occurs on the small island (Robert Island?) off the western corner of Gurd Island. Bands of white limestone, 1 to 3 metres thick, occur on the low land at the northwest end of Gurd Island.