Based on a 1930 Howchin diagram of the geological cross-section from Hallett’s Cove (SW) to the River Torrens (NE).
Typically, reefs initiate around dormant volcanic cones (Pacific Islands). As the apparent sea level rises, the coral grows to the light, forming a ring with gaps for tidal flows in and out of the lagoon. Darwin noted the various stages of these islands on his Beagle journey across the Pacific. The collapse of the lava vented volcanic cone leads to an apparent “rise” in sea level. The lagoon behind the reef slowly fills with material (Coral debris and sands) from the erosion of the reef. These sediments compact and sink as the volcanic cone collapses.