COASTAL FEATURES OF EROSION

1) Capes and bays: On exposed coasts, the continual action of waves on rocks of varying resistance causes the coastline to be eroded irregularly. This is particularly pronounced where hard rocks, e.g. granites and limestones, occur in alternate bands with softer rocks, e.g. sand and clay. The softer rocks are worn back into inlets, coves or bays and the harder ones persist as headlands, promontorios or capes.

 

Along the Dorset coast of southern England, Swanage Bay and Durlston Head are examples. Even where the coast is of one rock type, irregularities will be caused by variation within the rock. Thus Penang Island, made of granite, has many bays and headlands.

Most of the capes and peninsulas found in West Africa are of igneous structure. The formation of Cape Verde, Senegal for instance, has been associated with past volcanic activities along the Senegalese coast. Capes of similar structure include Cape Mount, Cape Mesurado and Cape Palmas in Liberia, Cape Verga and the Conakry peninsula of Guinea and the Sierra Leone Peninsula. Their igneous structure is responsible for their resistance to marine erosion. Other capes include Cape Three Points of Ghana and Cape St. Ann, southern Sierra Leone. Rocky points and sandy bays are common along the coast of Ghana and Ivory Coast. Because of their structure, and the shelter they provide, some of the promontorios and bays in West Africa are sites for ports. Monrovia is sited on Cape Mesurado, Freetown on the Sierra Leone Peninsula, Sassandra on a rocky promontory facing a semi-circular bay, Conakry on the Conakry Peninsula, Dakar on Cape Verde, and Port Etienne (Mauritania) on the western side of the Levrier Bay, which is well sheltered from waves, storms and other dangers of the sea.

 

2) Cliffs and wave-cut platforms: Generally, any very steep rock face adjoining the coast forms a cliff. The rate of recession will depend on its geological structure, that is the stratification and jointing of the rock and its resistance to wave attack. If the beds dip seawards, large blocks of rock will be dislodged and fall into the sea. The cliff will rise in a series of ‘steps’ as shown in Fig 10.3.

On the contrary if the beds dip landwards as illustrated in Fig. 10.4, the cliff will be more resistant to wave erosion. Some of the best known cliffs are the chalk cliffs of the English Channel and include Beachy Head, which is 150 m (500 feet) high, the Seven Sisters near the mouth of the Cuckmere and the ‘White Cliffs’ of Dover.

At the base of the cliff the sea cuts a notch, which gradually undermines the cliff so that it collapses. As a cliff recedes landwards under the pounding of waves, an eroded base is left behind, called a wave-cut platform. The platform, the upper part of which is exposed at low tide, slopes gently seawards and its surface is strewn with rock debris from the receding cliff. Further abrasion continues until the pebbles are swept away into the sea. The eroded materials are deposited on the off-shore terrace.

When the platform attains a greater width, e.g. 48 km (30 miles) in the case of the wave-cut platform of Strandflat off Western Norway, it is entirely covered with water and further erosion of the cliffs is negligible. Examples of a wave-cut platform with a cliff on its landward end can be seen along the coast between Saltpond and Cape Three Points in Ghana. Cliffs are also found along Petite Cote between Dakar and the Saloum Estuary in Senegal, and at Cape St. Mary (The Gambia), where there are outcrops of sandstone. Some of the lagoons of Ivory Coast, Togo and Benin have cliffs.

 

3) Cave, arch, stack and stump: Prolonged wave attacks on the base of a cliff excavates holes in regions of local weakness called caves, e.g. at Flamborough Head, England, and at Senya Beraku near Winneba, Ghana. When two caves approach each other from either side of a headland and unite, they form an arch e.g. the Needle Eye near Wick, Scotland.

Further erosion by waves will ultimately lead to the total collapse of the arch. The seaward portion of the headland will remain as a pillar of rock known as a stack. One of the finest examples of a stack is the Old Man of Hoy in the Orkneys which is of Old Red Sandstone and is 135 metres high. Equally outstanding are the Needles, Isle of Wight, which are a group of stacks cut in chalk and diminishing in size seawards. In the course of time, these ‘stubborn’ stacks will gradually be removed. The vertical rock pillars are eroded, leaving behind only the stumps which are only just visible above the sea-level e.g. those of the St. Kilda group, off the Outer Hebrides, Scotland. Arches and stacks are common along erosional coastlines such as those of Dakar, Senegal, and Ghana.

 

4) Geos and gloups: The occasional splashing of the waves, against the roof of a cave may enlarge the joints when compressed air is trapped inside. A natural shaft is thus formed which may eventually pierce through to the surface. Waves breaking into the cave may force water or spray or just air out of this hole. Such a shaft is termed a gloup (from the noise made by the water gurgling inside) or blow-hole.

An example is at Holborn Head in Caithness, Scotland. The enlargement of blow-holes and the continued action of waves weakens the cave roof. When the roof collapses, a long, narrow inlet or creek develops. Such deep clefts, which may be 30 m (100 feet) deep and equally long, are called geos e.g. the Wife Geo, near Duncansby Head, Scotland.