Rock Platform Morphodynamics: Examples from the Southern NSW Coast

Short, A.D.; Thom, B.G., and Hudson, J.P., 2025. Rock platform morphodynamics: Examples from the southern NSW coast. Journal of Coastal Research, 41(3), 373–390. Charlotte (North Carolina), ISSN 0749-0208. Rock coasts occupy over half the world's shoreline. Many of them are eroding and cliffed with a rock platform at their base whose outer edge drops off into deeper water. Field investigations along the southern New South Wales coast, on ancient rocks of different lithology and structure, show how at a macroscale, such platforms are roughly horizontal, cutting across bedding in places. Lower platform elevations tend to occur in more sheltered locations and towards the rear of the platform, that is, at the base of a subaerial cliff. Platform elevations increase both longshore towards the more exposed section of the platform and in a shore-normal direction across the platform towards its seaward edge. This edge may contain a rampart that commonly rises 0.5 to 1 m above the rear of the platform, reaching more than 3 m in extreme cases. Seaward of the platform is the drop-off that descends to the subaqueous rock base. Rock structure influences the form of the drop-off or seaward edge, which may be vertical to sloping. Under present sea-level conditions, cliff retreat and platform surface formation are strongly influenced by the process of water layer weathering. This implies that the platform surface is saturated and resilient to weathering, whereas the cliff face undergoes accelerated disintegration of the rock fabric, with the weathered debris removed above the level of the saturated rock by wave action. Weathering also assists in weakening the cliff face, leading to mass movement of material onto the platform, from which it may be removed by wave action off or along the platform. However, the platform edge, drop-off, and subaqueous rock base are only exposed to physical wave attack and erosion. The process–response interaction, involving subaerial weathering and erosion of the cliff down to the saturated platform surface and wave erosion of the seaward edge of the platform, is an expression of rapid-response morphodynamics within the context of the slow response of a gradually retreating rocky coast to changing sea levels and tectonics during the Cenozoic era.