As waves break at the base of the cliff, they can create a wave-cut notch, a small inlet into the cliff. Further erosion increases the stress on the cliff. The notch migrates inland, and eventually the stress on the overhanging rock becomes too much so it collapses, leaving a small stretch of platform. This process is repeated many times, so the platform continues to expand. However, a wave cut platform slope is always less than 5° and has a maximum width of 100m, as once it reaches this size, the waves lose energy by the time they reach the cliff, as the platform dissipates the energy, so less erosion can take place against the cliff.
Headlands and bays
These mainly form on discordant coastlines, where the bands of rock run perpendicular to the sea, so different lithologies are subject to the same waves. Their formation demonstrates the idea of differential erosion. Bands of softer rock will be eroded more quickly, as they are less resistant to the power of the waves. This means they retreat more quickly, creating bays, and leaving the more slowly eroded resistant rock protruding from the coastline as a headland. After this however, due to the changed shape of the coastline, the harder headlands begin to be more vulnerable to erosion. Wave refraction in order to line themselves up with the coastline means that most of their energy is concentrated against the headlands. Energy is dissipated in the bays, resulting in deposition, so they are built up more.
Arch, stack, stump etc
These features tend to form on a headland. Due to the refraction of the waves on a coastline with headlands, they are vulnerable to erosion. It begins as a crack in the inter-tidal zone (between high and low tide). As erosional processes, particularly hydraulic action and wave quarrying, work here, it develops into a geo, a steep-sided inlet in a headland. The geo continues to be enlarged by erosion, eventually forming a sea cave. From a cave, a blowhole can develop in the top of the headland. This happens because the cave gets bigger, pressure above increases so the water can start to blast through the top. If the cave breaks through the other side of the headland, it becomes a natural arch. As this is eroded more, it can collapse, leaving a stack. The base of the stack is exploited by erosional processes, so it collapses also, leaving a stump.
Landforms of deposition
These are the most common depositional landform at a coastline. Waves arrive at a beach at a certain angle, and this has influence over the movement of the sediment. Swash-aligned beaches from in low-energy environments where waves are mostly parallel to the shore. Sediment moves mainly just up and down the beach, and does not really show any lateral movement. Drift-aligned beaches form if the waves approach the coast at an angle. Sediment is transferred laterally along the beach by the longshore drift transportation process. The beach can be split into the upper beach (backshore zone) and the lower beach (foreshore, nearshore and offshore zones).
The upper beach is usually above the influence of waves and tends to be strongly characterised by a storm beach, which is a ridge of boulders thrown towards the back of the beach by the strongest waves. Nearer to the boundary with the lower beach, there are berms and cusps. Berms are relatively heavy collections of sand and shingle deposited just in front of the storm beach, the extent to which it can be pushed by other material. Cusps only form on beaches which have both sand and shingle, and are crescent-shaped indentations – swash is concentrated in these small regions due to the shape, so this makes backwash strength increase, removing material from this region.
The wave action forms numerous features in the lower beach. At the upper end of the region, there are ripples which form as a result of wave action and tidal currents moving some sediment. Ridges are raised areas of the foreshore, formed due to the deposition of a significant amount of sediment at the upper reach of wave movement. Runnels also form in the foreshore zone due to the wave energy at this point removing some sediment and leaving inlets on the surface which allow water to flow in and out.
Spits usually form at a point along the coastline where there is a change in direction. This may be at a headland or an estuary, where the shape of the land changes. The spit is basically a continuation of the beach sediment into the sea, and has a recurved lateral/hook on the end. Spits are often also characterised by having a salt marsh behind it, due to the build-up of sediment blocking water supply to that area. Spits tend to form as a result of the longshore drift process. This transports sediment along the coastline, in the direction of the prevailing wind usually, as this affects the angle at which a wave enters a beach. Material is transported along the coastline until it reaches a change in direction, usually at a headland or an estuary/river mouth. At this point, the water is quite shallow and sheltered. Material transportation continues to move material into this sheltered area of water, and over time, it accumulates, building up as a mass of land (in general, the material dropped first, so at the very base of the spit is the largest and heaviest because as energy reduces, it can no longer carry the heavier material. The recurved lateral can form on the end of the spit due to a change in wind direction. The spit length is restricted when forming across an estuary, because currents here are quite strong. Salt marshes can form behind the spit because with the spit reaching out in front of other water, the water supply is limited, so a mass of sand and water is created. The end attached to the land is the proximal end and the end away from the land is the distal end.
A bar is a ridge of sediment which joins two headlands together, whilst cutting off a bay behind it. For some time, a bar is forming as a spit, which eventually joins further land. Its features normally include two headlands, as between then are changes in the shape of the coastline, causing a spit to start forming. After the bar has formed, there is a lagoon behind it, which is the area of land cut off by the bar sediment. This can eventually become s salt marsh. Where there is a change in the shape of a coastline, in this case, usually where the rock type changes so there is a headland before the coastline dips into a bay, the longshore drift transportation process carries sediment out into the sea. As this continues to occur, a spit develops, until the sediment reaches the next headland, forming a bar. Because it has formed across a bay, there is an area cut off behind, which is referred to as a lagoon.
A tombolo is an extended spit, a ridge of land which joins the mainland to an island. Its features include the mainland, with a length of sediment reaching out, like a spit, with an island on the other end. This becomes known as a tied island. Similarly to a normal spit, the tombolo sometimes starts developing at a change in the shape of the coastline, for example, at a headland. Many tombolos are formed as a result of wave diffraction and refraction. Around the island, waves slow down when getting near to the land, which is due to the water becoming shallower. The waves begin to bend around the island, which causes longshore drift to start occurring. On certain parts of the coast, sediment starts building up away from the land, and can eventually reach the distant mainland. This is not always the case though. Some tombolos form from regular longshore drift on the mainland. At a change in shape of the coastline, longshore drift can keep working out into the sea, and eventually reach an island.
This is essentially a triangular extension of the coastline. There are usually two angular stretches on a cuspate foreland which face the water, and can be curved inwards – these are called marginal wings. There is also the central nose/apex which is the end of the foreland – the corner of the triangular shape. The forelands can reach up to 5km from the shore when exposed above the water, and there can be a further 15km underwater. They can also shelter areas behind them, creating landforms like salt marshes. It is not certain how cuspate forelands are formed, but the general theory predominantly involves longshore drift. On certain parts of the coastline, longshore drift can occur in both directions, largely dependent on the prevailing winds and origin of the waves (if two sets of waves coming from different directions merge together, longshore drift is able to occur in different directions). When this happens, two separate spits can start to develop, building up in opposite directions. The spits can then meet each other and join up, forming a cuspate foreland. Cuspate forelands can also form across lakeshores, such as Point Pelee in Lake Erie, Canada.