Helmet Peak on the northern end of Lake Island just off the southwestern side of the Don Peninsula is a steep-sided monogenetic cinder cone. It is composed of welded volcanic blocks and basaltic feeder dikes. With an elevation of , Helmet Peak is the highest volcano in the Milbanke Sound Group. During its eruption, basaltic tuff breccia was sent throughout the surrounding landscape that deposited on glaciated granitic rock and unconsolidated beach gravel near the volcano. This basaltic tuff breccia covers parts of Lake Island and nearby Lady Douglas Island. Blocks of basement granodiorite, some up to wide, are randomly suspended within the breccia. Basalt on Lake Island is characterized by abundant to labradorite and clinopyroxene phenocrysts.