Coastal bench formation at Hanauma Bay, Oahu, Hawaii
A coastal bench that developed from 1 to 6 m above sea level in basaltic tuff at Hanauma Bay conforms to the upper limit of wetting by wave wash at high tides associated with present sea level; it does not constitute evidence for a recent Holocene highstand on Oahu. Variations in bench width and elevation are related to differences both in exposure to waves and in exposure to daily heating and drying of the cliff behind the bench. Salt weathering of the sort usually invoked to explain weathering effects in deserts is a major factor in the retreat of the cliff and the consequent formation of the bench. The waves do not "cut" the bench but, instead, by daily wetting, protect it from desiccation. The bench forms as a result of the disintegration and retreat of the unprotected cliff. The same process can satisfactorily explain the formation of Koko Bench, presently submerged at -5 m along the north shore of Hanauma Bay. Use of similar benches as geological indicators of past sea levels requires a detailed understanding of the coastal setting and exposure to waves, and the different responses of specific rock types at and above the air-sea interface.