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The Lower Hawkesbury River Estuary is a drowned river valley that cut into Hawkesbury sandstone bedrock during the ice ages when ocean levels were much lower than they are now. It's part of the Sydney Basin, created in the Permian and Triassic periods (200-250 million years ago).
The sandstone settled in an ancient, braided river system and was uplifted at the end of the Triassic period. It’s hard to believe that these high dry ridges were once the bed of an aquatic ecosystem, however researchers occasionally uncover fossil remnants of crustaceans, shells and fish, including the rare Malayan Blue Flying Fish.