A simple mathematical model was used to test the hypothesis that the gradient and other aspects of the morphology of shore platforms in argillaceous rocks are determined by the number of wetting and drying cycles experienced at different elevations within the intertidal zone. Three model variants were considered: downwearing by wetting and drying on a pre-existing wave-cut surface; downwearing occurring simultaneously with backwearing by waves at the high tidal level; and downwearing combined with simultaneous backwearing by waves throughout the intertidal zone. Wetting and drying and tidal-duration distributions were obtained for four areas, representing tidal ranges from 1 m to 9.3 m. The amount of downwearing was made proportional to the wetting and drying frequency, and it therefore varied through space and time in response to changes in platform elevation. The model suggested that wetting and drying can produce very gently sloping shore platforms, especially in microtidal environments. Although low gradients were also produced by weathering under favourable conditions in macrotidal environments, they were significantly greater than those produced under the same conditions in low tidal range environments. Fairly high gradients, ranging up to about 2.5°, were produced in macrotidal runs with slow wave erosion and weathering. The model suggests that wetting and drying can provide an additional or alternate explanation to that of wave erosion for the relationship that has been reported between tidal range and platform gradient.
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