The huge arc of Twofold Bay's southern and western parts is a sea carved landscape formed in soft sandstone and mudstone since the sea rose to its present level 6000 years ago. During the last ice age (that ended about 20,000 years ago), a great river flowed through a rocky gorge across what is now Twofold Bay. The drowned gorge is now filled with sand and it is clearly visible in sonar maps of the sandy bay floor. These maps are on display at the Sapphire Coast Marine Discovery Centre in Snug Cove.