Everything you ought to know about Erosional features of the Coasts

There are certain features on the coasts, which are primarily due to erosion. These include (a) cliff, (b) wave-cut platform, (c) sea arch, (d) stack, (e) sea-caves, (f) crcnulate coast, etc.

Cliffs:

When a steep rocky coast rises directly above seawater or a wave-cut bench or platform in front of it, it is known as cliff. The true cliffs are very precipitous and are on hard durable rocks. If the sub-aerial weathering of the cliff face and crest is faster than marine erosion at base, the vertically of the cliff will degenerate and it will soon lose its cliff character. If, on the other hand, marine erosion is more marked than sub-aerial erosion the cliff will remain steep and may even overhang.

If the rock of the coast is resistant, there will be limited supply of debris to form or supplement beach. There will be concentrated attack at the base of cliff in the absence of wave-cut platform or beach. These circumstances will help in the persistence of cliffed coast. If the winds are orthogonal to the coast the wave attack at cliff base will be strong ensuring the persistence of cliff topography.

Submergence (local, regional or eustatic) in a rocky coast may immediately encourage cliff formation but suitable structure and layout of the coast with reference to powerful waves may create a topography, which may not be submergent but may resemble submergent topography. This might be the fact of the case on the western coast of India from near Bombay to near Mangalore.

Here cliffs may be seen rising directly above the sea. In such situation, it is difficult to set foot on the edge of water or shore. There are several points where the coastal plain of India becomes a myth and the cliffed coast rises abruptly above the seawater. In between the cliffed headlands and promontories there may be rias and inlets.

Some cliffs also occur on the Kutch, Kathiawar and Northern Circars (between the Mahanadi and Godavari) coast. If the rock is young, soft and poorly cemented, cliff recession will be relatively rapid provided the debris from cliff face or by marine erosion is taken away from cliff base. The soft rocks (Eocene to Pleistocene) of England, like brickearth, have caused coast recession of c. 1.75 meters per year. The volcanic ash of Indonesia receded at the rate of 30 meters per year. If the beach material, if any, is transported by long-shore drifting an active cliff is evidenced by slumping on the cliff face.

The height of the cliff will determine the rate of recession because the higher the cliff the greater will be the amount of rock to be dealt with by cliff recession under marine and sub-aerial action and greater the amount of debris to be removed before the wave can keep itself active on the cliff-base.

Numerous types of cliffs varying with structure can be identified. Fairbridge has illustrated several types of cliffs some of which are:

(i) cliffs marked by mudflows particularly in re-entrants, seen in Denmark (taken from Schou’s description),

(ii) cliffs marked by landslides in limestones underlain by softer and plastic marls, (iii) cliffs in chalk marked by talus at cliff-foot an streamless hanging valleys above,

(iv) cliffs marked by alternate beds of flagstones (i.e., sandstones which can be worked out in parallel slabs) and clay. Broken fragments of flagstone slabs are strewn over V clay. The cliff as a whole is of gentle slope,

(v) cliff haracterized by hard structures like sandstone or basalt or chalk are steep nearly vertical,

(vi) on sea-ward steeply dipping structures (e.g., sandstones, slates or limestones) the sea cliffs are gently sloping.

Columnar basalt, horizontal bedded chalk, sandstone, quartzite and slate generally forjj vertical cliffs. Glacial deposits and volcanic ash yield gentler slopes.

Wave-cut Platform or Bench:

Other details apart, it is a fact that wave is a persistent and universal force of erosion on the coasts. As the cliffs receded, they have in front of them flat surface cut in rock, with or without beach material.

The wave-cut bench or platform is slightly concave upwards. This is partly due to the accumulation of coarser debris at the upper edge of the platform near the cliff-base and finer sediments seawards. As to the abrasion of wave-cut platform, this is maximum at the upper end where the coarser shingle also forms the higher ground. The action of the up-rush or swash is the maximum not at the upper end of the platform but in position further down. Again, the backwash has also its action-maximized midway where the sediment load is still high and coarser than further seawards. These facts may also account for the slight concavity of the wave-cut platform.

Sea Arch:

When headlands are attacked by waves from two sides a marine passageway may be produced in the weak section between the promontory and the mainland with the roof of the passageway intact. This feature is known as sea arch.

The weakness in the rock producing such arch may be structural, e.g., joints, or chemical, e.g., limestone which is liable to solution.

The shape of the stack will depend on the lithology and structure of the rock. A high (137 m) rectangular pillar-like stack in Orkney Island (British Isles) of Old Red Standstone standing above lava platform has been illustrated by Holmes. It is called ‘the Old Man of Hoy’ because its silhouette strikingly looks so. Stack has been derived from stakkur, a word of a Scandinavian dialect spoken in Faeroe islands. Stacks are common there. They are also called needles, columns, pillars, skerries, etc.

Sea Caves

In a cliff made of strong rock there may be weaker elements, which will be eroded more rapidly than the harder elements. Thus, caves several meters deep will be formed in the cliff-face.

The caves are generally more common in limestones which are subject to solution. But weaker elements may occur in any rock. A very impressive cave is shown by Thornbury in columnar basalt. Photographs of sea caves (including one in Old Red Sandstone) have been given by Holmes.

The different erosional features of the coast, viz. cliff, wave-cut bench, sea arch, stack and sea cave can be seen on the western Indian coast particularly between Bombay and Mangalore. Arunachalam made field study of 25 km of basaltcoast line between Kurli village in the south and Kalbadevi in the north of Ratnagiri headland. The cliffs are steep and plunge steeply into the sea so that the base of the cliffs cannot be reached except during low tide and that too in non-monsoon season. The sea bottom is 2 to 6 meters at the cliff-base. During the South West monsoon the long fetchjpf the Arabian sea raises large waves so that it is not possible to reach the cliff-foot even at low tide. Arunachalam studied three headlands all of which are marked by overhanging cliffs in 45-90 meters high.

The forceful attack of the waves has produced cliff in columnar basalt by erosion along the joints. Stacks are found at places and the collapsed, roof of the arches are seen as boulders. There is a flat wave-cut bench (30 to 90 meters wide) on the southern, western and southeastern side of Ratnagiri headland. It is the narrowest (15 meters) on the western side of the headland.

The surface of the bench is pitted with pot-hold-like joint-guided rectangular and polygonal depressions (about 2 to 4 meters deep). A notch was noted about a meter above the bench on the landward side, probably at the normal high tide level.

The overhanging cliff face slightly above the notch line has several caves ‘about 2 to 3 meters high and 3 meters deep’, the biggest of the cave being 7 meters high, 5 meters broad and 5 meters deep.

Arunachalam noted two lines of caves in the region. One series lay about a meter above the wave-cut bench. A second line was higher up, 6-7 meters above the bench level. In his photo no. 2 wave-cut platform and photos, 3 and 4 sea caves have been illustrated. Blowholes are also an erosional feature of the coast. At the landward edge of a sea cave there may be a vertical shaft, which is very much like a chimney. It is known as blowhole (also gloup or throat) because during storms surging waves press spray up through the hole. The hole is formed by the air which is compressed against the coast by waves. This compressed air breaks jointed blocks whose fall causes the hole. Small bays or covers are also produced by differential erosion of a shore. The softer rock elements or those made weaker by joints will be eroded more quickly than the harder ones. This will result into a crenulate coast consisting of headlands and promontories and coves. Such coves are frequent on the lava coast of India between Bombay and Goa. Other such features will be inlets and creeks.

What we have described above are such erosional features, which are relatively durable and can exist as observable forms for some time. There are ephemeral features also such as the retrograding or receding coast of Kerala. Shrivastava on the basis of radio-carbon dating of sediments from the submarine floor round India concluded that the shoreline of India was 75-330 km., sea-ward during Holocene. This could be due to submergence but coasta recession might have contributed to this retrogradation in some parts as is now happening on Kerala coast, which receded at the annual rate of about 25 meters between 1957 and 1966.