Back of the islands are the quiet waters of lagoons, and at the mouths of rivers are several shallow bays indenting the mainland; these bays were formed by only a slight subsidence of the land and the rivers are filling them with deposits of silt.
After the subsidence came another period of uplift, possibly still in progress.
This cutting off was caused largely by subsidence, though partly by marine action.
The Cretaceous period was initiated by the subsidence of a large area to the south of the Gulf of Carpentaria, whereby a Lower Cretaceous sea spread southward, across western Queensland, western New South Wales and the north-eastern districts of South Australia.
A theory of a still continuing subsidence of the islands was formed by Kurz in 1866 and confirmed by Oldham in 1884.