Characteristics and formation of coastal landforms

Waves concentrate their erosion at the base of a cliff between the high- and low-tide marks. Hydraulic action (the force of water and compressed air in cracks) and abrasion (sediment hurled at the rock) cut a wave-cut notch into the foot of the cliff.

As the notch is enlarged, the rock above it is left unsupported (undercut) and eventually collapses, so the cliff retreats inland while remaining steep.

This process is repeated, and the cliff retreats progressively landward. Each time the cliff retreats it leaves behind a gently sloping area of rock at the base.

The result is a wave-cut (shore) platform — a broad, gently sloping rock surface extending out from the foot of the cliff, exposed at low tide and often marked with rock pools.

A concordant coastline is one where the bands of rock run parallel to the coast, so a single resistant band fronts the sea and the coastline is relatively straight; if the sea breaches the outer band it can carve a cove in the softer rock behind (e.g. Lulworth Cove).

A discordant coastline is one where the bands of rock lie at right angles to the coast, so alternating resistant and weak bands meet the sea.

The key difference is the orientation of the rock structure: parallel bands (concordant) give a straight coast, whereas bands at right angles (discordant) are eroded differentially into headlands (resistant rock) and bays (weak rock).

A stack forms on a headland of resistant rock through a sequence of marine erosion that exploits lines of weakness such as joints and faults.

First, hydraulic action (compressed air and water forced into cracks) and abrasion (sediment thrown against the rock) widen a joint or fault in the headland into a small cave. Because erosion is selective, it picks out these weaknesses while the surrounding resistant rock is left intact.

Continued erosion deepens the cave, and where a cave is cut right through a narrow headland — or two caves meet back-to-back — an arch is formed.

The arch is widened and deepened at its base by wave erosion, while sub-aerial weathering attacks it from above. Eventually the unsupported roof of the arch can no longer support its own weight and collapses.

This leaves an isolated pillar of rock separated from the headland — a stack. Continued erosion and weathering may later undercut and reduce the stack to a low stump. A real example is Old Harry Rocks in the chalk of Dorset.

A coastal sand dune system develops where a wide sandy beach, a large tidal range and onshore winds supply sand that is blown inland, and vegetation is essential in trapping and stabilising that sand through a succession (a psammosere).

Nearest the sea, salt-tolerant pioneer plants such as sea couch and lyme grass colonise the dry sand and trap wind-blown grains, forming small, mobile embryo and fore dunes.

As sand accumulates, marram grass establishes and becomes dominant on the yellow dunes. Marram is the key dune-builder because it grows upward through accumulating sand and binds it with long, deep roots, so the dunes grow taller and more stable.

As the dunes stabilise, dead plant material adds humus and the conditions become less harsh, allowing mosses, lichens and herbs to colonise the more stable grey dunes, where there is less bare sand.

Over time the succession continues towards a climax community of shrubs and woodland, with damp dune slacks in the hollows. At each stage the vegetation traps more sediment and modifies the environment so that the next set of species can colonise.

Coastal landforms are produced by the interaction of marine processes (erosion, transport and deposition), sub-aerial weathering and vegetation acting on a coast of a given rock type, structure and wave energy. Rock structure — the resistance of the rock and the way its bands, joints and faults are arranged relative to the coast — is undeniably a powerful control, but this essay will argue that, while rock structure determines the large-scale plan of a coastline and its erosional landforms, wave energy, sediment supply and vegetation are equally or more important for the rate of change and for the depositional and biotic landforms, so rock structure is the most important control only at certain scales.

Rock structure is clearly fundamental to the plan of a coastline. On a discordant coast, where rock bands of differing resistance lie at right angles to the sea, differential erosion cuts the weak bands into bays and leaves the resistant bands as headlands, as at Swanage Bay on the Dorset coast. On a concordant coast, where the bands run parallel to the sea, the coastline is straight, and where the outer resistant band is breached the sea carves a cove in the softer rock behind, as at Lulworth Cove. Structure also controls the small-scale erosional landforms: erosion is selective, picking out joints and faults as lines of weakness, so caves, arches, stacks and stumps form where the rock is weakest — Old Harry Rocks being the classic example. The steepness and shape of cliffs likewise depend on rock resistance and the dip of the strata. In all these cases the rock structure determines where and what landform forms, which strongly supports the statement.

However, wave energy is arguably just as important, because it determines whether a coast erodes or deposits and how quickly landforms change. High-energy, exposed coasts with destructive waves and a long fetch erode rapidly, producing cliffs and shore platforms; low-energy, sheltered coasts with constructive waves accumulate sediment. Wave refraction concentrates energy on headlands and disperses it into bays, both accelerating headland erosion and allowing beaches to be deposited in the bays. The same rock structure under different wave energy would therefore produce very different landforms, so structure alone cannot be decisive.

Sediment supply is a third major control, and it dominates the depositional landforms that rock structure cannot explain. Where longshore drift delivers abundant sediment, spits form where the coast changes direction (Spurn Head), tombolos build out to join islands to the mainland (Chesil Beach linking Portland to the Dorset coast), and bars and barrier beaches close off lagoons. These landforms depend on the supply and transport of sediment, not on the resistance of the underlying rock; indeed they often lie across drowned valleys or soft coasts. Where the sediment budget is in deficit, no amount of favourable structure will build a beach.

Finally, vegetation is the leading control of the biotic coastal systems. Sand dunes form through a psammosere in which marram grass traps and binds wind-blown sand; saltmarshes develop where pioneer plants such as Spartina trap mud in sheltered water (a halosere); and in the tropics mangrove trees trap sediment and dissipate wave energy, building and protecting the coast. In all three, it is the vegetation, not the rock structure, that creates and stabilises the landform.

The most accurate assessment is that the importance of rock structure depends on the scale and the type of landform considered. At the scale of the whole coastline, rock structure is the most important control because it dictates the plan — headlands, bays, coves — and the location of erosional landforms. But for the rate of change, and for depositional and vegetated landforms, wave energy, sediment supply and vegetation are decisive, and on many coasts these operate where rock structure is almost irrelevant. The controls also interact: differential erosion (structure) provides the sediment that longshore drift (energy) then transports to build a spit (sediment supply) behind which saltmarsh (vegetation) develops.

In conclusion, rock structure is the single most important influence on the large-scale form and the erosional landforms of a coast, and on this basis the statement holds for cliffed, indented coastlines. But it is not the most important control overall, because the depositional landforms that occupy so much of the world's coastline are governed chiefly by wave energy and sediment supply, and the dune, saltmarsh and mangrove systems by vegetation. The view is therefore only partly correct: rock structure determines where and what erosional landforms form, but the formation of coastal landforms as a whole is best explained by the interaction of structure, energy, sediment and vegetation, with the dominant control shifting according to the landform and the scale in question.

Coastal depositional landforms — beaches, spits, tombolos, bars, barrier beaches and the vegetated dune, saltmarsh and mangrove systems — form where sediment is laid down faster than it is removed. Two controls are essential to this: the supply of sediment available to be deposited, and the wave energy that transports it and decides whether deposition or erosion dominates. This essay assesses their relative importance and argues that, while an abundant sediment supply is a necessary precondition for any depositional landform, it is the wave energy regime that determines where, and in what form, that sediment is deposited, so the two are interdependent rather than one simply mattering 'more'.

There is a strong case that sediment supply is the more important control, because without sediment no depositional landform can form regardless of the energy regime. Sediment is supplied by cliff erosion, by rivers delivering load to the coast, and from offshore. Where supply is abundant, large depositional landforms develop: longshore drift carries sediment along the coast to build spits where the coastline changes direction, such as Spurn Head; tombolos build out to join islands to the mainland, as Chesil Beach links Portland to the Dorset coast; and barrier beaches and bars enclose lagoons. Crucially, where the sediment budget is in deficit — for example where sea defences updrift starve a beach of material — depositional landforms shrink or disappear even though the wave conditions are unchanged. This dependence on supply supports the statement.

However, wave energy is arguably the decisive control, because it determines whether sediment is deposited at all and the form the landform takes. Deposition occurs only where energy is low: in sheltered bays, behind headlands and in estuaries, where constructive waves push sediment onshore and there is insufficient energy to remove it. The angle of wave approach controls the type of beach: waves arriving parallel to the shore build a swash-aligned bayhead beach, whereas oblique waves generate the longshore drift that builds drift-aligned beaches and feeds spits. Wave refraction disperses energy into bays, encouraging the very deposition that forms a beach. The same sediment supply under high wave energy would be eroded and removed rather than deposited, so energy governs the outcome.

The vegetated landforms further show the interdependence. Sand dunes require both a supply of sand (from a wide beach) and the low-energy, wind-dominated back-beach environment in which marram grass can trap it; saltmarshes and mangroves require both fine sediment AND the sheltered, low-energy water in which pioneer plants such as Spartina, or mangrove roots, can trap it. Neither sediment nor low energy alone is sufficient; both must coincide, and vegetation then stabilises the deposit.

The most accurate judgement is that sediment supply and wave energy are interdependent and operate at different points in the process. Sediment supply sets the quantity of material available — it is a necessary precondition, and a deficit prevents any landform forming. But wave energy is the active control that transports the sediment, decides whether it is deposited or removed, and shapes the resulting landform. On a coast with abundant sediment but high energy, little is deposited; on a sheltered coast starved of sediment, low energy cannot build a landform from nothing. They reinforce one another: the same drift that supplies sediment to a spit is itself driven by oblique wave energy.

In conclusion, I agree only partly with the statement. Sediment supply is indispensable, and a deficit is fatal to depositional landforms, which gives it genuine primacy as a precondition. But it does not matter 'more' than wave energy, because energy determines whether deposition occurs and dictates the type and location of the landform. Most accurately, coastal depositional landforms owe their existence to the coincidence of an adequate sediment supply with a low-energy wave environment, and assigning primacy to either in isolation misrepresents a system in which supply and energy are inseparable.