Beyond the Cliffs: A Deeper Perspective of Nearshore Rock Coast Morphodynamics along the Central–Southern New South Wales Coast

Thom, B.G.; Short, A.D.; Hudson, J.; Cowell, P., and Kinsela, M., 2026. Beyond the cliffs: A deeper perspective of nearshore rock coast morphodynamics along the central–southern New South Wales coast. Journal of Coastal Research, 42(2), 211–235. Charlotte (North Carolina), ISSN 0749-0208. There has been limited investigation of the edge or drop-off of coastal rock platforms and what lies submerged seaward. This study and others have collected data in this area over the past few decades along the central and southern New South Wales coast through scuba dives, bathymetric surveys, and more recently inspection of marine terrain by LIDAR seabed mapping. Focus has been on examining the nature and variation in morphologies seaward of subaerial platforms in rocks of varying lithology, structure, and exposure to wave energy. Lithologies in folded metasediments, which include interbedded volcanics of early Palaeozoic age, show that enhanced subaqueous differential erosion produced pronounced linear ridges and channels, massive sandstones have a steep vertical drop-off and numerous large boulders, and shales have a more terraced drop-off and generated small debris. Tertiary basalts that form horizontal layers also offer a lithology in which platforms have been carved and have vertical drop-offs that slope seaward with scattered boulders. The level of wave exposure controls the depth of the drop-off and boulder movement, which appears to cease at depths of ∼14 m. Wave processes involving abrasion and block dislodgement lead to boulder rounding and transport. Platform and subaerial cliff formation also occurred during past interglacial periods of higher sea level. At exposed sites, subaerial cliff retreat appears to be occurring at a faster rate than retreat of the platform edge. This suggests that these platforms have not yet entered a stage of steady-state equilibrium after ca. 7000 years of relative sea-level stillstand. Resolving the polycyclic evolution of subaerial and subaqueous topographies of rocky coasts as time-dependent morphodynamic systems over a longer timescale remains a challenge.