Landforms of Wave Erosion

1An arch that is no longer there

For more than a century, visitors to the island of Gozo in Malta photographed the Azure Window, a limestone arch 28 m tall standing in the sea at Dwejra Bay. Slabs fell from it in the 1980s, the 2000s and again in 2012. Then, at about 9:40 in the morning on 8 March 2017, after days of storms, its pillar gave way and the whole arch fell into the sea, leaving nothing above the water. Coastal landforms of erosion are not fixed scenery: each is one stage in a sequence that the waves are still working through. This lesson explains that sequence, from the headland to the stump, and the cliffs and platforms that go with it.

2Headlands and bays

On a discordant coast, the sea erodes the weak rocks (clays, soft sandstones) faster than the resistant ones (granite, hard limestone, chalk), a process called differential erosion. The weak rock is cut back into bays and the resistant rock is left standing out as headlands. On a concordant coast, a band of resistant rock along the shore protects the weak rock behind it; if the sea breaks through at one point, it scoops out the weak rock behind to form a near-circular cove with a narrow entrance, as at Lulworth Cove in Dorset, England.

Once headlands exist, wave refraction keeps them under attack (Figure 5). Water shallows sooner off a headland, so the part of each crest heading for it slows first, and the crest bends round to face the headland on three sides. Wave energy is concentrated there, and erosional landforms develop. In the bays the same energy is spread along a longer shore: the waves are lower, and sand and shingle are deposited as a bay-head beach.

Figure 5: wave refraction at a headland and bays (plan view).

3Cliffs and wave-cut platforms

Waves attack a cliff mostly between the low- and high-water marks, where hydraulic action and abrasion cut a notch. As the notch deepens, the rock above becomes an overhang, which weathering and gravity eventually bring down. The fallen blocks protect the cliff foot for a while, until the waves break them up and carry them away; then a new notch is cut, and the cycle repeats. Each cycle moves the cliff inland and leaves behind a widening, gently sloping rock surface: the wave-cut platform (Figure 6). Platforms slope gently seawards, usually by only a few degrees, are cut across the rock structure and are scattered with rock pools and ridges where harder beds stand out.

Figure 6: the formation of a wave-cut platform as a cliff retreats.

The process slows itself down. As the platform widens, waves break further out and lose more of their energy crossing it, so less reaches the cliff foot. This is negative feedback: a wide platform protects the cliff that made it.

A cliff's shape depends on the balance between marine erosion at its base and sub-aerial processes (weathering and mass movement) on its face, and on the rock:

• Resistant rock with horizontal or landward-dipping beds: a steep or vertical cliff, because blocks are held into the cliff.
• Beds dipping towards the sea: a lower-angled cliff that follows the bedding planes, with slabs sliding seawards along them.
• Weak rock such as clay or glacial till: a lower, slumping cliff that retreats fast, because sub-aerial processes and the waves both remove material easily.

4Caves, arches, stacks and stumps

Headlands are rarely solid: they are cut by joints (cracks) and faults. Waves concentrated on the headland by refraction exploit these lines of weakness:

1. Hydraulic action forces water and air into a joint near the high-water mark, and abrasion scours it, widening it into a cave.
2. Caves on opposite sides of the headland meet, or one cave cuts right through, to form an arch.
3. The waves keep eroding the foot of the arch while weathering attacks its roof. In time the roof is too thin and wide to support itself and it collapses, leaving a stack separated from the headland.
4. The stack is undercut at its base by a notch, as a cliff is, until it topples, leaving a stump that is covered at high tide.

Figure 7: the sequence from cave to stump, seen from the sea.

Well-known examples are the arch of Durdle Door and the stack and stump of Old Harry Rocks, both in Dorset, England, and the stacks of the Twelve Apostles in Victoria, Australia. In Africa, the Hole in the Wall on the Wild Coast of South Africa is a detached rock pierced by an arch that the waves have cut right through it.

5Wave-cut features on lake shores

Waves on a large lake do the same work as sea waves, only with less energy, because the fetch is shorter and there is almost no tide. Erosion is concentrated at a single water level, so a lake that stays at one level for long enough can cut a notch and a narrow bench into its shore. The clearest example in the world is Lake Bonneville, the ice-age lake of Utah in the United States: its former shorelines still stand as benches, partly cut and partly built by waves, along the mountain fronts above Salt Lake City, hundreds of metres above the floor of the basin. On the great lakes of our region, such as Victoria and Kivu, wave action is real but weaker than on an open ocean coast, so wave landforms there are generally smaller.

6Erosional landforms and people

Where cliff retreat threatens property, people may defend the cliff foot with sea walls, rock armour or gabions, or decide that defence costs more than the land is worth and allow managed retreat. Any defence that stops a cliff eroding also stops it supplying sediment to the beaches down the coast.