The Bridge River Cones consist of small trachybasaltic and basaltic eruptive centers. Sham Hill (), a high steep-sided volcanic plug, is the oldest volcano in the field with a potassium-argon date of one million years. The plug is approximately wide and its bare glaciated surface, strewn with glacial erratics, consists of large subhorizontal columns formed within the central conduit of an eroded stratovolcano.