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Neoproterozoic – 600 million years ago The Brachina Formation is deposited in the deep ocean as layers of silt and sand. Burial of the layers over time compacts and cements them into a sedimentary rock.
Cambrian – 500 million years ago The Brachina Formation is deformed into fold structures as part of a mountain building event called the Delamerian Orogeny. This event ultimately forms the Mt Lofty and Flinders Ranges.
Permian – 280 million years ago Southern Australia sits at the South Pole as part of the supercontinent Gondwana. It is covered by a glacier that carves scratches known as striations into the rocks beneath as it moves. As the glacier melts, the Cape Jervis Formation is deposited along with dropstones and erratics released from the ice.
Pliocene – 3 million years ago By this time, Australia has separated from Antarctica and drifted north to much warmer climates. The Hallett Cove Sandstone is deposited in a shallow sea full of shelly organisms.
Pleistocene – 2 million years ago The Mt Lofty Ranges are uplifted and a thick package of alluvial sediments are deposited as the mountain range is eroded. Australia then enters the last ice age, and by 30,000 years ago sea levels are much lower. Wind covers the landscape with dust from the exposed seafloor of Gulf St Vincent, eventually forming a crust of calcrete.
Holocene – 11,000 years ago to present day A thin brown layer of silty and sandy soil forms atop the Amphitheatre, and streams such as Waterfall Creek continue to erode the cliffs along with waves undercutting the shore platform. Hallett Cove is a constantly changing landscape, but we are lucky enough to see it today where it still preserves a cross-section through geological time.
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