Sea Cliffs:
Sea cliffs are steep rock faces formed by the undercutting action of the sea waves. In the formation of cliffs, marine erosion is the main agent at work. The vertical or sloping rock faces are more prominent in storm wave environment.
On protected coastlines cliffs have only short vertical sections. Actually sea cliffs are formed where sea waves dash against steeply sloping land or highly resistant rock.
The erosive power of the waves gradually undercuts the land creating steep cliffs to the water. The nature of rocks, the force of the waves and the capacity of undertow and currents to remove the debris determines the rate of wave cutting.
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On more protected coasts the upper slopes of the cliffs are covered by vegetation which makes the slopes stable.
Cliffs are more common along convergent plate margins where areas of high coastal relief are formed by mountain ranges and hills, and where there is deep off shore water.
Coastal cliffs form as a result of the combined work of different processes. The beginning is made with the erosion of the basal rock at the foot of a slope when it is pounded by high-energy waves.
The impact of the breaking waves against rock is very great and it compresses the air between the wave front and the shore and in the rock joints. The result of this impact is that blocks and particles of rocks are loosened.
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Abrasion by wave wears away more rock. However, only the largest waves move away large boulders, so pebbles are more important than these strong waves in the case of cliff erosion.
Because of the combined impact of wave and abrasion by eroding tools such as pebbles, a notch is formed at the cliff base below high tide level. As more and more material is excavated from this notch, the upper part of the cliff becomes unstable ending in rock falls.
Steepness of the cliff is retained because of such rock falls, and cliff retreats. Remember that erosion by abrasion and wave impact is mainly controlled by the type of rock and the presence of joints and fractures.
Cliffs having steep rocky slopes are subject to the processes of weathering and mass movement. Certain type of cliffs are formed in unconsolidated materials and they are, therefore, subject to rapid removal of material by strong – waves as well as running water.
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Shore platforms:
They are nearly horizontal surfaces formed at the base of a cliff. Shore platforms resulting from the erosion of the – cliff are called wave-cut platforms. These are limited on the land side by a vertical cliff notched at the base.
According to Worcester, other terms are used for wave-cut platforms, which are bench, shelf, terrace and plain. Sometimes the eroded material derived from the cliff is taken out into the sea where it forms a wave-built terrace.
On the contrary, the eroded material is worn down to very fine particles and scattered over the sea floor.
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As the waves continue to attack the coast, the shoreline recedes and wave-cut plain widens.
Rock type determines the detailed shape of cliffs. If the rocks are resistant and recently uplifted, cliffs are generally higher. Where strong rocks are highly fractured and jointed, features such as caves, stacks and arches and blowholes form.
Stacks:
Stacks are isolated rock pillars rising steeply from the sea. Formerly stacks were part of the adjoining land, but have become isolated from it by wave erosion. Stacks are one of the erosional forms that evolve along cliff-dominated coastlines.
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As erosion continues, arches collapse leaving isolated stacks in the sea water. When parts of a headland are detached from the shoreline, they are called stacks, chimneys, skerries or islands. Examples of these forms are found around the sea coasts.
The Needles off the Isle of Wight, the Old Harry Rocks off the Isle of Purbeck and the Old Man of Hoy in the Orkneys are some of the fine examples of stacks.
Sea cave:
Sea caves are erosional features that develop along a sea coast by wave action. These caves are formed along the bases of cliffs by wave erosion, particularly when the lower portions of cliffs are composed of weak rocks with a lot of joints and fractures.
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It is to be borne in mind that where weaker and more resistant rocks exist alternately and where strong rocks are highly fractured, features such as caves, stacks, and arches form. Waves erode sea caves along their lines of weakness at the cliff-base.
This process is extended to undermine farther than a simple notch. A sea cave may resemble a passage or gallery in its shape which depends in part on the structure and joint pattern of the rock.
The type of process involved in its excavation plays important role in determining the shape of a sea-cave.
It be pointed out that by subsequent falling in of the roof and removal of the sediments or debris, long narrow inlets are developed. In Scotland and the Faroes, a tidal inlet of this kind is called a ‘geo’.
Arch:
When two caves on opposite sides of a headland unite, a natural arch is formed. After sometime arch falls in, and the seaward portion of the headland then remains as an isolated stack.
Blow-hole or group:
A blow-hole forms when the roof of a sea cave at the landward end of a geo may communicate with the surface by way of a vertical shaft at some distance from the edge of the cliff.
It is like a natural chimney. This opening is formed by wave erosion along a fault or a joint at the back of a sea-cave. When a wave enters into the cave the compressed air forces spray violently up the narrow opening.
In Scotland the term ‘group’ is used for a blowhole. The part of the cave-roof that still persists is called ‘the Devils Bridge’.