The sea performs the function of erosion and deposition through sea waves, aided by currents, tides and storms in coastal areas.
The erosive work of the sea depends upon (i) size and strength of waves, (ii) seaward slope, (iii) height of the shore between low and high tides, (iv) composition of rocks (v) depth of water, etc. The wave exerts a pressure to the magnitude of 3000 to 30,000 kilograms per square kilometre. This wave pressure compresses the air trapped inside rock fissures, joints, faults, etc. forcing it to expand and rupture the rocks along weak points.
This is how rocks get worn down under wave action. Waves also use rock debris as instruments of erosion. These rock fragments carried by waves themselves get worn down by striking against the coast or against one another. The solvent or chemical action of waves is another mode of erosion, but it is pronounced only in case of soluble rocks like limestone and chalk.
The marine landforms can be studied under erosional and depositional categories.
These are narrow, deep indentations carved out through vertical planes of weakness in the rocks by wave action. With time, further headward erosion is hindered by the chasm mouth, which itself keeps widening till a bay is formed. (Fig. 1.66)
2. Wave-Cut Platform:
When the sea waves strike against a cliff, the cliff gets eroded gradually and retreats. The waves level out the shore region to carve out a horizontal plane or a wave-cut platform. The bottom of the cliff suffers the maximum intensive erosion by waves and, as a result, a notch appears at this position. (Fig. 1.65)
3. Sea Cliff:
It is the seaward limit of coast which is marked by a steep scarp.
4. Sea Caves:
Differential erosion by sea waves through a rock with varying resistance across its structure produces arched pockets in rocks. These are called sea caves.
5. Sea Arches:
When the waves attack a rock- form from two opposite sides, the differential erosion produces bridge-like structures or sea arches.
6. Stacks/Skarries/ Chimney Rock:
When a portion of the sea arch collapses, the remaining column-like structure is called a stack, skarry or chimney rock. (Fig. 1.67)
7. Hanging Valleys:
If the fluvial erosion by streams flowing down the coast is not able to keep pace with the retreat of the cliff, the rivers appear to be hanging over the sea. These river valleys are called hanging valleys. (Fig. 1.68)
8. Blow Holes or Spouting Horns:
A narrow fissure through the roof of a sea arch is called a blow hole or a spouting horn because the wave action compresses and squeezes out the air from the sea caves through blow holes making a peculiar noise.
9. Plane of Marine Erosion/Peneplain:
The eroded plain left behind by marine action is called a plain of marine erosion, and if the level difference between this plain and the sea level is not much, the agents of weathering convert it into a peneplain.
This is the temporary veneer of rock debris on or along a wave-cut platform. It is by the sea waves that the deposition of rock flour is carried out.
The long shore currents, tidal currents and the shore drift deposit rock debris and sand along the coast at a distance from the shoreline. The resultant landforms which remain submerged are called bars. The enclosed water body so created is called a lagoon.
It is the overwater counterpart of a bar.
4. Spit and Hook:
A spit is a projected deposition joined at one end to the headland, with the other end free in the sea. The mode of formation is similar to a bar or barrier. A shorter spit with one end curved towards the land is called a hook.
Sometimes, islands are connected to each other by a bar called tombolo. These islands are referred to as the tied islands.
The boundary between the coast and the shore is known as the coastline; it marks the seaward limit of the coast. The outline of the coast may be modified by sea waves.
According to D.W. Johnson, coastlines can be divided into the following classes:
1. Coastline of Emergence
2. Coastline of Submergence
3. Neutral coastline
4. Compound coastline
5. Fault coastline
This classification has two bases: (a) the change of levels, i.e., if sea level falls, the shores emerge, (b) the nature of shorelines, because it is necessary to know the nature of sea shore before emergence and submergence, whether the shore was upland or downland, for instance.
1. Coastlines of Emergence:
These are formed either by an uplift of the land or by the lowering of the sea level. This type of coast has bars, spits, lagoons, salt marshes, beaches, sea cliffs and arches. The east coast of India, especially its south-eastern part, appears to be a coast of emergence. The coast has, however, been invaded by the sea a number of times during the past. The west coast of India, on the other hand, is both emergent and submergent. The northern portion of the coast is submerged as a result of faulting and the southern portion, that is the Kerala coast, is an example of an emergent coast.
2. Coastlines of Submergence:
A submerged coast is produced either by subsidence of land or by a rise in sea level. The important types of such a coastline are ria, fiord, Dalmatian and drowned lowlands.
When a region is dissected by streams into a system of valleys and divides, submergence produces a highly irregular, embayed shoreline called ria coastline. The coast of south-west Ireland is a typical example of ria coastline. Some coastal regions have been heavily eroded by glacial action and the valley glacier troughs have been excavated below sea level. After the glaciers have disappeared, a fjord coastline emerges. These coasts have long and narrow inlets with very steep sides.
The fjord coasts of Norway are a typical example. The Dalmatian coasts result by submergence of mountain ridges which run parallel to the sea coast. Therefore, there is a series of fold mountain ranges with alternating crests and troughs running parallel to the coast.
The Dalmatian coast of Yugoslavia is a typical example. A drowned lowland coast is low and free from indentations, as it is formed by the submergence of a low-lying area. It is characterised by a series of bars running parallel to the coast, enclosing lagoons. The Baltic coast of eastern Germany is an example of this type of coastline.
3. Neutral Coastlines:
These are coastlines formed as a result of new materials being built out into the water. The word ‘neutral’ implies that there need be no relative change between the level of sea and the coastal region of the continent. Neutral coastlines include the alluvial fan shaped coastline, delta coastline, volcano coastline and the coral reef coastline.
4. Compound Coastlines:
Such coastlines show the forms of two of the previous classes combined, for example, submergence followed by emergence or vice versa. The coastlines of Norway and Sweden are examples of compound coastlines.
5. Fault Coastlines:
Such coastlines are unusual features and result from the submergence of a downthrown block along a fault, such that the uplifted block has its steep side (or the fault line) standing against the sea forming a fault coastline.
Marine Cycle of Erosion:
The waves are very active. Sea caves, arches and stacks begin to develop. Cliff undercutting is pronounced and wave cut platform begins to emerge due to wave erosion. The height of cliff increases in the middle youth and cliffs retreat a lot. By the end of youth, an irregular coastline remains.
The cliff and wave-cut platform are conspicuous. Stream deposition is taking place. These valleys may be normal or of the hanging type. Various landforms indicating continuous deposition are visible, such as bars, barriers and spits. A gradual straightening of the coastline heralds the arrival of old age.
Irregularities, such as caves and arches, have disappeared. The abrasion platform is so widened that all the wave energy is spent in friction and shifting of sand across the beach. Consequently, wave attack upon the cliff base is reduced. Finally, a straight, gently sloping coastline and broad beach result with little bedrock appearing at the surface.