LANDFORMS MADE BY WAVE ACTION

Wave erosion has three aspects. (i) Corrosive action involves the hurling of pebbles and sand against the base of a cliff by breaking waves; this causes undercutting and rock break-up. (ii) Hydraulic action involves water thrown against a cliff face by breaking waves causing air in cracks and crevices to become suddenly compressed; when the water retreats the air expands, often explosively, causing the rocks to shatter. (ill) Attrition involves the breaking up of the boulders and pebbles dashed against the shore into finer particles.
Cliffs are steep or vertical rock faces formed by waves undercutting the rock. The rocks of some cliffs are in layers which slope landwards. In other cliffs the rock layers slope sea wards and blocks of rock loosened by erosion easily fall into the sea. The cliffs are often very steep and overhang­ing. A cave develops along a line of weakness at the base of a cliff which has been subjected to prolonged wave action. If a joint extends from the end of the cave to the top of the cliff, this becomes enlarged in time and finally opens out on the cliff top to form a blow hole. Caves which develop on either side of a headland such that they ultimately join together, give rise to a natural arch. When the arch collapses, the end of the headland stands up as a stack.
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The depositional features of waves are beaches, spits and bars,.and mud flats. Beaches usually lie between high and low water levels. Material which is eroded from a coast may be carried along the coast as a spit; this is likely to happen along indented coasts or coasts broken by river mouths. A spit is a low, narrow ridge of pebbles or sand joined to the land at one end with the other end terminating in the sea.

A bar is very similar to a spit. A bay-bar grows right across a bay; such bay-bars are called nehrungs along the coast of Poland. When a bar links an island to the mainland it is called a tom bolo. Tides tend to deposit fine silts along gently shelved coasts, especially in bays and estuaries. The deposition of these silts together, perhaps with river alluvium, results in the building up of a platform of mud called a mud flat. Salt tolerant plants soon colonise the mud flat which in time becomes a swamp or marshland. In tropical lands these mud flats often become mangrove swamps.