The area is underlain, from north to south, by Paleozoic to Mesozoic layered quartzites interbedded with minor layers, up to 1 metre wide, of limestone, buff-weathered layered limestone, dull grey dolomitic limestone, and calcareous chloritic schists with minor graphitic schists. The strata trends northeast and dips about 40 degrees east. A northwest trending shear zone, 0.1 to 10 metres wide, likely separates, by left lateral movement, the limestones on Randell Island (103J 037), 5.5 kilometres to the northwest. Norian (Late Triassic) ammonoids have also been noted within limestones on the east shore of Dunira Island.