Controls of Shore Platform Width : the Role of Rock Resistance Factors at Selected Sites in Japan and Wales , UK
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Sea level has been relatively stable at its present level since completion of the Holocene transgression some 3000 years ago in the case of the N. Hemisphere and 6000 years ago in the case of the S. Hemisphere. Marine processes have acted upon exposed hard rock masses to develop the characteristic morphology of such coastal zones i.e. shore platforms and cliff faces; the former is the concern of this study. Platforms can be differentiated into two types: 'Type A', lacking a significant seaward step and 'Type B' with a step and consequent offshore increase in water depth. Shore platform development is a reflection of interactions between a large number of dependent and independent variables and, in general terms, variation in morphology is a reflection of wave assailing forces (F ) and rock resistance factors(F ). Platform development ceases when the resistance of the cliff mass equals the assailing forces of the waves. In storm wave environments such as the North Atlantic, mechanical wave erosion usually dominates platform development with water hammer, shock pressure of breaking waves and air compression in joints and crevasses being the most important processes, ( 1964). In addition, the effect of pebble abrasion on a cliff face, still needs to be resolved. (1987; 1997) demonstrated a strong positive correlation between platform gradient ( ) and spring tidal range (T ) for linear platforms extending below low tide level, suggesting that: