The ability of waves to erode a large platform here is related to the rocks that outcrop at sea level.  They are part of the Tuggerah Formation and are made up of layers of sandstone, shale and claystone.

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The later is very reactive, meaning it expands when exposed to water and then contracts as it dries.

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The Patonga Claystone that sits above it is responsible for many landslides where the strata is at an angle.