shore platforms Shore platforms are prominent features of rock coasts. Two main types have been distinguished. Gently sloping platforms (1–5°), which are sometimes called ramps, usually extend from the cliff base to below the low tidal level, without any major break of slope (Fig. 1a). Subhorizontal platforms, which can occur above, within, or below the intertidal zone, generally terminate abruptly in a low-tide cliff or ramp (Fig. 1b). Much of the literature has emphasized the occurrence of subhorizontal platforms in Australasia and Hawaii, and of sloping platforms in Britain, the north-eastern USA, and elsewhere in the northern Atlantic.
Most workers accept that mechanical wave action is the main erosive mechanism operating on the sloping platforms in the North Atlantic and other vigorous-wave environments. It has been argued, however, that, because of variations in wave intensity, and in the level at which they operate, subaerial weathering is essential for the formation of horizontal surfaces. In Australasia and other warm temperate and tropical-swell wave environments, horizontal platforms have therefore been attributed to cliff weathering or the modification of wave-cut ramps by weathering processes. There is no consensus about the way in which weathering produces horizontal platforms. Critical examination of the various theories that have been put forward over the past 100 years shows that most attribute a surprisingly major role to mechanical wave erosion, although this is rarely explicitly stated.
The geographical distribution of platform types has been generally thought to reflect differences in climate and wave regime. Most of the century-old debate on their origin, however, has been conducted in almost total ignorance of the fundamental role of tidal range. Although geological and other local factors may produce sloping and horizontal platforms along a coast, the former are most common in areas with high tidal range and the latter in areas with low tidal range. Along the storm-wave coast of eastern Canada, for example, horizontal platforms occur where the spring tidal range is between 2 and 2.5 m, and sloping platforms, with gradients up to about 5°, where the tidal range is almost 15 m. This suggests that horizontal platforms can be cut by waves where there is a small tidal range, but it does not rule out the possibility that they can also be produced by chemical, salt, or frost weathering, or by coastal ice. Nevertheless, any attempt to attribute a primary role to these mechanisms must explain how they are controlled by the tidal range.
Geological factors are responsible for marked variations in the shape of individual platform profiles, although over large areas their effect is often limited to providing local variations in platform morphology about regional means which are determined by tidal range and other aspects of the morphogenic environment. Geology influences the development and morphology of shore platforms in many ways:1. The nature, intensity, and efficacy of the erosional mechanisms are governed by the structure, lithology, and mineralogy of the rocks.2. Geological factors help to determine whether the foot of the cliff and the back of the platform are abraded or protected by accumulating debris and beach sediment.3. The surface roughness of shore platforms in sedimentary rocks is strongly influenced by the dip and strike, bed thickness, joint density, and variations in the strength of the beds. Platform surfaces are usually coincident with the more resistant members of horizontal or gently dipping strata, but corrugated (washboard) relief often develops in steeply dipping rocks.4. The age of coastal features, and the possibility that platforms are at least partly inherited from periods when sea level was similar to that of today, increase with the strength of the rocks.Early models of platform development were descriptive and were usually structured within a cycle of erosion. More recently, mathematical modelling has suggested that platform width and gradient are maintained through a balance in the rates of erosion at the high and low tidal levels. This is consistent with the changes that occur in wave strength and erosion rates at the high tidal level in response to changes in platform width and gradient. Modelling has also supported the argument that the shape of platform profiles, and the close relationship that exists between platform gradient and tidal range, reflect the way in which wave energy is distributed between the high and low tidal levels. Simulated platforms attained equilibrium when erosion occurred at the same rate at all points along their profiles. This was accomplished by the platform gradient varying along a profile in such a way as to compensate for differences in the frequency of tidally controlled wave attack.
In many areas, erosional processes may be modifying platforms and other elements of rock coasts that were formed, at least in part, in the past. This hypothesis is consistent with the palaeo-sea-level record, which shows that interglacial sea levels were similar to those of today on a number of occasions during the Middle and Late Pleistocene. Inheritance is most likely to have occurred on tectonically stable coasts, and in resistant rocks where present rates of erosion are too low to account for the formation of wide shore platforms since the sea reached its present level. For example, on the southern coast of New South Wales, uranium–thorium (U/Th) dating of ferruginous and calcareous crusts and thermoluminescent dating of associated sediments show that a subhorizontal platform, which is awash at high tide, was formed during a period of higher sea level in the last interglacial stage. It was buried under soil during the last glacial stage, and then exhumed and partly modified by wave erosion in the Holocene.
There is less justification for assuming that inheritance has necessarily occurred in areas of weaker rocks. Even if the platforms were inherited from ancient surfaces in, or slightly above, the modern intertidal zone, till covers, raised beaches, structural remnants, and other evidence would probably have been removed by fairly rapid wave erosion at the present level of the sea. Relationships between platform morphology and aspects of the morphogenic environment are often lacking in resistant rocks. Their occurrence in weaker rocks, however, suggests that even if these platforms were partly inherited, they have been substantially modified by contemporary processes.
A. S. Trenhaile
Bibliography
Sunamura, T. (1992) The geomorphology of rocky coasts. John Wiley and Sons, Chichester.
Trenhaile, A. S. (1987) The geomorphology of rock coasts. Cavendish–Oxford University Press.
Trenhaile, A. S. (1997) Coastal dynamics and form. Cavendish–Oxford University Press.
