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Hard and Soft Rock Types
Generally speaking, sedimentary rock, which is formed by layers of deposited material being compressed, is softer than igneous or metamorphic rocks, which are formed respectively by the cooling of magma or by the exposure of rock to extremes of heat and pressure. There are exceptions to this rule, such as sandstone, which can form a hard sedimentary rock, or marble, which is a soft metamorphic rock. The waves hurl rocks and smaller stones against cliffs and shore line and force air into gaps in the rock. The action of impacting water and rocks and the expansion and contraction of air break down the rock and cause the coastline to gradually recede. Soft rock is much more susceptible to this kind of erosion than hard rock, and so a coastline composed of chalk or shale or other soft rock will recede much faster.
Chemical Breakdown
Seawater also carries in it dissolved carbonic acid, giving it a slightly lower pH than distilled water. This acid interacts with the rock, breaking down the chemical bonds which hold it together. However, certain types of rock such as limestone and chalk, which have been formed by the sedimentation of the shells of certain sea creatures, are particularly alkaline. This means that they react more powerfully with the acid in the seawater than other rock types and so break down more readily with exposure to acidic conditions.
Arrangement of Rock Strata
The appearance of a coastline will be determined to some extent by the arrangement of the different rock types and their relative hardness. If the coast is composed entirely of one rock type, then it will usually erode into a smooth curve. Conversely, if bands of hard and soft rock are arranged in alternating layers, then the differing rates of erosion will cause the coast to erode unevenly, creating bays, headlands, stacks and other features. If a layer of softer rock sits under harder rock at the base of the cliff, then sea caves may be created by the action of the waves.
Sand Types
Coastal rocks eroded by the sea will eventually be broken down into sand, which will be carried down the coast by a process known as long shore drift and may be deposited to form a beach. The type of sand which is produced is dependent on the rock type which was eroded. Dark grey sands are usually created by the breakdown of shales, while the bright white, picture postcard variety of sand is usually composed of crystals of feldspar and quartz.