Coastal landscapes and landforms

Longshore drift is the movement of sediment (sand and shingle) along a coastline in the same general direction as the prevailing wind and waves.

It happens because the swash carries material up the beach at an angle (the angle of wave approach), while the backwash drags it straight back down the slope under gravity, producing a zig-zag net transport of sediment along the shore.

A headland is a section of resistant (hard) rock that juts out into the sea, standing higher and further seaward than the coast on either side.

A bay is a broad, curved indentation of the coast, usually cut back into a band of less resistant (soft) rock, and often containing a beach.

They occur together on a discordant coast because bands of hard and soft rock meet the sea at right angles: differential erosion wears the soft rock back into bays while leaving the hard rock protruding as headlands — so the two landforms alternate along the same coast.

The swash is the water that rushes up the beach after a wave breaks; the backwash is the water that drains back down the beach under gravity.

A constructive wave has a strong swash and a weak backwash, so it deposits more sediment than it removes, building the beach up (forming berms and ridges). These are typically low, gently spilling waves with a long wavelength.

A destructive wave has a weak swash and a strong backwash, so it drags more sediment offshore than it brings in, eroding and lowering the beach. These are typically tall, steep, high-frequency plunging waves.

The resource clearly shows differential erosion controlled by rock type. The resistant chalk projects 400 m seaward as a headland, whereas the weak clay has been cut back 600 m into a bay — a difference of about 1 km between the two rock outcrops.

This contrast arises because chalk is far more resistant to marine erosion (hydraulic action and abrasion) than clay. The clay is eroded rapidly, so the coast retreats there to form the bay, while the chalk resists erosion and is left standing out as the headland.

The pattern shown is therefore a discordant coastline: the alternating hard and soft rock bands meet the sea at right angles, and differential erosion produces the headland-and-bay shape. Over time wave energy will concentrate on the exposed chalk headland (through wave refraction), while the sheltered clay bay accumulates a beach.

A beach is a depositional landform made of sand and shingle that accumulates in the sheltered, lower-energy zone between the high and low tide lines, typically within a bay.

Beaches form where constructive waves dominate: their strong swash carries sediment up the beach and their weak backwash cannot remove it all, so material is deposited and builds up over time. Sediment is delivered by longshore drift along the coast and by rivers, then sorted by the waves.

The main features are:

Feature	Description
Berm	A ridge of coarser sediment built up at the high-tide line by constructive waves; there may be several relict berms up the beach.
Storm beach / ridge	The highest ridge, of the coarsest material, thrown up at the back of the beach by powerful storm waves.
Runnels and ripples	Small channels and ripple marks in the sand near the low-tide line, shaped by the backwash and tidal currents.
Cusps	Small, semi-circular hollows where swash and backwash are evenly matched.

Beach profiles change with the season: gentle, wide beaches build up in summer (constructive waves) and are cut back and steepened in winter (destructive storm waves).

All three are depositional landforms built by longshore drift, but they differ in what the deposited ridge connects to.

A spit is a ridge of sand/shingle attached to the land at one end and projecting out into the sea or across a river mouth, with a free (often recurved) end.

A bar forms when a spit grows all the way across a bay or between two headlands, so the ridge is joined to the land at both ends. This traps a body of water behind it as a lagoon (e.g. Slapton Ley, Devon). A bar can also form offshore where waves push sediment shorewards. The key difference from a spit is that a bar has no free end — it seals off the bay.

A tombolo forms when longshore drift (and wave refraction) builds a ridge of sediment that joins an offshore island to the mainland (e.g. Chesil Beach linking the Isle of Portland to Dorset). It differs from a spit because it connects the coast to an island rather than ending in open water.

A stack is an isolated pillar of resistant rock standing in the sea just off a headland, and it is the product of a long sequence of marine erosion working on lines of weakness.

1. Line of weakness attacked. A headland of resistant rock (e.g. chalk or limestone) contains joints, faults or cracks. Waves attack these weaknesses through hydraulic action (the force of water and compressed air in the cracks) and abrasion (sediment hurled at the rock), widening a crack into a cave.

2. Cave becomes an arch. Continued erosion cuts the cave right through the narrow headland — or two caves on opposite sides meet — leaving a natural arch (e.g. Durdle Door, Dorset).

3. Arch collapses to form a stack. The roof of the arch is unsupported and is further weakened by sub-aerial weathering from above. Eventually gravity causes the roof to collapse, leaving an isolated column of rock separated from the headland — a stack (e.g. Old Harry, Dorset).

4. Stack reduces to a stump. The stack is continually undercut at its base by wave erosion and weathered from above; in time it is worn down to a low stump, often only visible at low tide.

The full sequence is therefore crack/joint → cave → arch → stack → stump, driven throughout by marine erosion concentrating on lines of weakness in the headland.

A salt marsh is a vegetated area of intertidal mud that builds up in sheltered, low-energy coastal locations such as estuaries, or in the calm water behind a spit or bar.

1. Shelter and low energy. In sheltered water — behind a spit, in an estuary or in a bay — wave energy is very low, so the water is calm enough for fine sediment to settle rather than being washed away.

2. Deposition of fine sediment. Rivers and the incoming tide carry fine silt and mud, which is deposited on each slack (high) tide. The salt water also causes clay particles to clump together and sink (flocculation), speeding up deposition.

3. Mudflats build up. Repeated tidal deposition raises the surface into mudflats that are exposed at low tide and covered at high tide.

4. Pioneer colonisation. Once the mudflats are high enough, salt-tolerant pioneer plants (halophytes) such as glasswort and cord-grass (Spartina) colonise them. Their roots bind and stabilise the mud, and their stems slow the water further, trapping even more sediment.

5. Succession and marsh growth. As sediment accumulates, the surface rises so it is flooded less often; less salt-tolerant plants take over in a vegetation succession, and creeks drain the marsh. Over time an extensive, gently rising salt marsh develops (e.g. behind Spurn Head, or in Poole Harbour, Dorset).

The essential controls are therefore shelter (low energy), a supply of fine sediment, tidal deposition aided by flocculation, and stabilisation by vegetation.

Coastal landforms result from an interaction between the rock (geology and structure) and the processes acting on it (marine and sub-aerial). Deciding which 'controls' the landform depends on the landform in question, so this answer weighs both.

The case that geological structure is the dominant control. Structure clearly determines the large-scale plan of a coast. On a discordant coast (rock bands at right angles to the sea), differential erosion of alternating hard and soft rock produces headlands and bays — the resistant chalk of the Dorset coast stands out at Old Harry while the softer clay is cut back into Swanage Bay. On a concordant coast (bands parallel to the sea), a resistant outer band gives a straighter coast, broken only where the sea breaches it — as at Lulworth Cove. Structure also decides where erosion concentrates: waves exploit joints, faults and bedding planes, which is why the cave–arch–stack–stump sequence forms along a line of weakness, and why the dip of the strata controls cliff steepness and stability. Without these structural weaknesses the classic erosional landforms would not form where they do.

The case that marine processes are the dominant control. However, structure only provides the raw material — it is the processes that actually create the landform. Depositional landforms (beaches, spits, bars, tombolos, salt marshes) owe their existence to longshore drift, wave deposition and low wave energy, not to structure: a spit forms because sediment is transported and deposited where the coast changes direction, regardless of the underlying geology. Even erosional landforms need hydraulic action and abrasion to cut the notch, and the balance of constructive and destructive waves decides whether a beach builds up or is stripped away. Sub-aerial processes (weathering, mass movement) further modify cliff profiles.

Judgement. On balance, geological structure exerts the stronger control over erosional landforms and the overall shape of a coast — it explains why headlands, bays and stacks form where they do. But marine processes are essential for creating the landform and are the dominant control over depositional features. The most accurate view is that structure sets the template on which marine and sub-aerial processes then operate, so the two work together rather than one alone determining the landscape.