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Cliffs and platforms

Cliffs and shore platforms are found where consolidated rocks outcrop at the shore. Australia is one of the flattest of the continents, with no extensive mountains, but where its denuded ranges run to the sea as in the Kimberley, or where the eroded sandstone tablelands of the Sydney Basin run to the coast, or where the lithified calcareous dunes (aeolianite) of the south and west outcrop, extensive cliff-dominated landscapes are found. A national survey of Australia's coastal landforms from aerial photography, carried out by the CSIRO, shows that cliffs and shore platforms makes up about a fifth of the length of the coastline. Tasmania has the largest proportion (51%) of hardrock coast, and Northern Territory (7%) and Queensland (9%) the least. In central and southern New South Wales the Hawkesbury Sandstone is an extensive feature along the coast. In South Australia and Western Australia many hundreds of kilometres of cliff, shore platform, and reef have been eroded in the Pleistocene dunerock, also known as aeolianite or calcarenite. The Tertiary limestones of the Nullarbor coast and the granites of south-west Western Australia are also extensively developed in cliffs.

Davies' generalised map of coastal types (see figure 2.8) shows those coasts with extensive continuous cliffed areas, but a continent-scale map cannot show the many visually and geomorphologically significant headlands within sand coast areas.

Shore platforms

A variety of denuding processes lead to the development of rock platforms sloping gently seaward and within the intertidal zone. These shore platforms are formed by cliff retreat and are best developed where wave energy is high and moderately erodible strata are exposed. Once formed, shore platforms are shaped by the processes listed in table 2.9.

The dolerites, sub-horizontally bedded sandstones and siltstones of southern New South Wales, Tasmania and western Victoria show magnificent examples of platforms sloping gently from just above the high tide mark to the low tide mark, beyond which they fall steeply away. These are known as high tide platforms. While these features are partly structurally developed in horizontal strata, it is clear that many such features have been eroded extremely slowly in resistant rock. This leads to the suggestion that they have not been formed entirely during the sea level stand of the last 6000 years, but can be attributed to a series of Pleistocene high sea levels.

In the following case study, cliff and platform forming processes at Robe South Australia are discussed in some detail, including some historic evidence of recession rates. This case study has been chosen because the rock type, aeolianite, is a common feature-forming lithology at the coast in southern and western Australia.

Table 2.9 Processes that form platforms

Process

Description

Visible signs

Wave quarrying

Air forced into crevices under great pressure by waves.

Enlargement of rock crevices to form the notch.

Angular rock breakage.

Abrasion

Waves move sand and rock fragments across platform.

Smoothing of shore platform. Drilling of potholes by pebbles.

Water-layer weathering

Alternate wetting and drying.

Pitting; honeycomb appearance; solution hollows with overhanging rims.

Dissolution by seawater and spray

Rate of lime dissolution in seawater falls with rise in temperature (diurnal). Seawater spray and aerosols dissolve lime-rich rocks.

Platform destruction. Caves, joint enlargement, pitting and hollows in the supratidal zone.

Biological erosion

Cyanobacteria (blue-green algae) secrete acids. Browsers (e.g. molluscs) break rock surface. Borers (e.g. molluscs, worms, some echinoderms) drill surfaces.

Indentations and holes in platform surfaces; racks pitted below algae.

 
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