The Tweed River, like others in northern New South Wales, turns north before entering the sea. This change in direction is customarily attributed to a northern drift of sand at the beach. The lowest reach of the river, however, is a coastal lagoon, which the Tweed enters at its southernmost point. The orientation of the lagoon decides the direction of this part of the river. Previous studies have stressed the importance of sediment deposition, littoral drifting, sea-level change, and the timing of particular events, but make no effective use of soil information, which can differentiate ground surfaces formed at different times by deposition and erosion. Moreover, they neglect the fine details of geomorphology and sea-level change. The landforms, sediments and soils that form the barrier at the outlet of the Tweed River confirm that sea level change is important. Sand carried shorewards by the rising sea at the end of each glacial age builds the barrier that encloses the lagoon. The deposits would have appeared first as a sand bar, and then emerge to form the barrier as the sand accumulated. Littoral drift would favour a northern outlet for the combined river and lagoon, and would maintain that position as the sea approached its highest level. The soils developed on the strandplain identify their parent sediments with stages of strandplain development recognized in southern Queensland.

For this locality stranded pumice deposits and emerged tidal channels and sandbanks give evidence of a significant postglacial fall in sea level.