Shore Platform Morphology and the Tidal Duration Factor

Abstract

A simple wave erosional model was used to simulate the profiles of shore platforms in eastern Canada, Australia, New Zealand, and Britain. The model assumes that the rate of intertidal erosion is determined by the time that still water level occupies each elevation within the tidal range (the tidal duration factor), by an erodibility factor related to wave energy and rock hardness, by platform gradient, and by the rate of submarine erosion. Simulated profiles were broadly similar to actual platforms in nearly all areas. All profiles eventually reached states of dynamic equilibrium, as the platforms assumed gradients which caused erosion rates to become equal at all points along their surfaces. Equilibrium was attained in periods which suggest that wave-cut platforms are essentially postglacial features. The model explains the effect of tidal range and erodibility on platform gradient and width, and the prominence of ramps and low tide cliffs in microand mesotidal environments, as well as identifying those factors which determine the elevation of platforms. The morphology characteristics of shore platforms are determined by a variety of morphogenic and geological factors. Bed dip, jointing, thickness, and other structural and lithological factors may be responsible for considerable variation in platform geometry within short distances along a coastline (Bird and Dent, 1966; Trenhaile, 1971). These local variations may be most marked along indented shorelines, and headland platforms are often quite different from those in the adjacent embayments; although few generalizations about the precise nature of these differences appear to be justified (Bartrum, 1935; So, 1965; Wright, 1970; Trenhaile, 1972). At the regional scale, however, differences in mean platform morphology between areas largely reflect differences in their morphogenic environments. The secondary role of geological factors at the regional scale is indicated by the relationship between aspects of platform geometry and a number of morphogenic factors (Wright, 1967, 1970; Trenhaile, 1972, 1974a, 1978, 1980). The geological contribution at the regional and larger scale is essentially limited to producing local deviations about morphological means which are determined by the morphogenic environments. Platform morphology also varies at the global scale. Davies (1964) has emphasized differences between the morphogenic environments of world shorelines, and Trenhaile (1974b) has applied this concept to shore platforms. The wave-cut platforms' in the storm wave, macrotidal environments of England and Wales, are generally wide, quasilinear surfaces, with gradients between 0-5 and 3'. They extend from a cliff-platform junction, generally close to high tide level, to low tide level or Trans. Inst. Br. Geogr. N.S. 6, 82-102 (1981) Printed in Great Britain This content downloaded from 207.46.13.146 on Tue, 23 Aug 2016 04:00:36 UTC All use subject to http://about.jstor.org/terms N .........iiiiiiiiiiiii!i!iii!!!i~~i~ii~~iiiii~iiiiii~~i . . . . . . . . . ...~iiiiiiiiiiiiiiiii~i~i~iiii!i~~ii~ i~iiiiiiiiiii!!iiiiiiiiiiiii~~ii~i!iiiiiii~ii~ ~~~~iliiiiiiiiii~iii~iii~ii~iii~iiiiiiiiiiiiilii~ i~i~i!i!ii~i~~iiiiii~iiiiiiii~iiiiiiiiiiiiiiiiiii~ i~ ~iiiiiii~iiiiiiiiiiiiiiii~iiiiiiii~i~i ii~i!ii!iiiiiii!i~ ~ii iii~ ~iiiiiiii~ ii~i~ iiii~iiiiiiiiiiiiii~ii~i~iiiii!iiii ii~ ~~~ii~i~ii~ii~iiiiiiiiiiiiiiiiiiiiiiiiiiiiii i i ii~ ~iiiiii~iiiiiiii~ii~iiiiiiiiii~iiii!!iii i iii~ii i~~iiiiiiiiiii~iiii~iiii!i!!!i~ ~iiiii~ iiiOW Nii~iii~i~i!~!iiiiiii~ii! i!!i! ii i iii% !i~!i~i!!!.!!.!%.l.i . ii.i.ii.i%.i.