 Ripple marks in the oldest Hallett Cove rocks indicate that the sediments that formed them originated on a tidal plain during the upper Precambrian.The plain was part of a shallow sea that bordered an ancient stable landmass to the west while to the east lay ocean as the eastern half of Australia had yet to form.The shallow sea extended around , from the northern-most Flinders Ranges to Kangaroo Island and lay above an unstable depression called the Adelaide Geosyncline which slowly sank as sediments built up, keeping pace so that the water remained shallow from about 870 Ma (the middle Neoproterozoic) to ~500 Ma (the end of the Cambrian).