The formation and characteristics of erosional coastal landforms

Erosional coastal landforms are created by the destructive action of waves wearing away the coastline over time. These processes, combined with the type and structure of coastal rocks, result in a variety of distinctive features that shape irregular shorelines.

Headlands and bays

These form along discordant coasts where rock layers run at right angles to the sea. Resistant rocks, like limestone, stand out as headlands, while softer materials, such as clay, are eroded to create bays. In contrast, concordant coasts, with rocks parallel to the sea, often have straighter shorelines.

Cliffs and wave-cut platforms

Cliffs are steep rock faces formed by the undercutting action of waves at their base, creating a wave-cut notch. Over time, the rock above collapses, causing the cliff to retreat while maintaining its steep profile. This process leaves behind a gently sloping wave-cut platform at the cliff's base, often visible at low tide.

Caves, arches, stacks, and stumps

These features develop on headlands where waves exploit weaknesses like faults or joints in the rock through hydraulic action and abrasion.

Formation sequence:

• Initially, a cave forms at a weak point.
• If it extends through the headland, an arch is created.
• Continued erosion weakens the arch's supports, leading to its collapse and leaving an isolated stack.
• Further wave action erodes the stack's base, eventually reducing it to a stump, often only visible at low tide.

Factors influencing erosional landforms

• Wave energy - Destructive waves, with a stronger backwash than swash, are key to eroding coastal rocks and shaping these features.
• Rock resistance - Harder rocks withstand erosion longer, forming headlands, while softer rocks erode faster, creating bays or other recessed features.

The formation and features of depositional coastal landforms

Depositional landforms occur where sediment, such as sand, shingle, or mud, accumulates faster than waves can remove it. These features are typically associated with constructive waves, where the swash is stronger than the backwash, leading to material being deposited along the coast.

Beaches

These are accumulations of sediment between the storm-beach area and low-tide marks, sourced from eroded coastal material, longshore drift, offshore deposits, or river sediment at estuaries.

Beach features:

• Storm beaches of large material at the top, formed during extreme weather.
• Berms, small ridges marking average high-tide levels.
• Beach cusps, semi-circular depressions caused by wave swash and backwash.
• Gentler slopes with finer materials like sand, compared to steeper profiles with coarser shingle.

Spits

Long, narrow ridges of sand or shingle attached to land at one end, extending across bays or estuaries. Formed by longshore drift in a dominant direction, spits deposit material where the coastline changes direction. Winds or waves may curve their tips, and salt marshes can develop on the sheltered landward side.

Bars

These form when a spit extends across a bay, connecting two headlands, often over a gently sloping beach with no significant river input. Bars trap water on the landward side, creating lagoons.

Tombolos

A type of spit that grows seawards to connect the mainland to an island, resulting from longshore drift and sometimes the movement of offshore sediment towards the shore.

Cuspate forelands

Triangular deposits of sand and shingle extending seawards, likely formed by longshore drift from two opposing directions colliding and depositing sediment.

Sand dunes

Though indirectly tied to coastal processes, these form when dry beach sand is blown inland by wind, creating ridges parallel to the coastline. Vegetation eventually stabilises older dunes over time.

Factors influencing depositional landforms

• Wave type - Constructive waves promote deposition by carrying and leaving sediment on the shore.
• Sediment supply - Sources like rivers, eroded cliffs, or offshore material provide the building blocks for these landforms.
• Longshore drift - This process moves sediment along the coast, contributing to the growth of features like spits and tombolos.

The role of wave processes and rock types in shaping coastal landforms

Coastal landforms are the result of a dynamic interplay between wave processes and geological characteristics. Understanding these factors helps explain the diversity of features along different coastlines.

Wave processes shaping the coast

• Destructive waves - These have a powerful backwash, eroding material from cliffs and headlands, crucial for forming erosional features like caves and wave-cut platforms.
• Constructive waves - With a stronger swash, these waves deposit sediment, building up features such as beaches and spits.
• Longshore drift - Waves approaching the coast at an angle move sediment along the shoreline in a zig-zag pattern, essential for creating spits, bars, and tombolos.

Influence of rock types and coastal alignment

• Rock resistance - Hard rocks like chalk resist erosion, forming prominent headlands, while softer rocks like sands erode easily, shaping bays.
• Coastal alignment - Discordant coasts, with rock bands perpendicular to the sea, develop alternating headlands and bays. Concordant coasts, with rocks parallel to the sea, tend to form smoother shorelines.
• Structural weaknesses - Faults and joints in rocks are exploited by wave action, leading to the development of caves, arches, and stacks.

Examples of specific coastal landforms in real-world locations

Real-world examples illustrate how the processes of erosion and deposition create unique coastal features, often becoming notable landmarks or areas of economic importance.

Erosional landform examples

• Purbeck Coast, Southern England - A classic discordant coastline with alternating hard chalk and limestone headlands (like The Foreland and Durlston Head) and softer clay and sand bays (such as Studland Bay and Swanage Bay).
• Old Harry Rocks, Southern England - Located between Swanage and Studland Bays, this chalk headland showcases a stack and a natural arch, demonstrating the progression of erosional features from cave to stump.

Depositional landform examples

• Sotavento Lagoon, Fuerteventura, Canary Islands - Features a bar of fine white sand enclosing a lagoon, formed by sediment deposition across a bay.
• Tombolo at Sharp Islands, Hong Kong - A 1.5 km strip of shingle and sand connects the mainland to a smaller island, formed by longshore drift and offshore sediment movement.

Human uses of coastal landforms

• Tourism and recreation - Sandy beaches and scenic cliffs, like those on the Jurassic Coast, attract visitors, boosting local economies.
• Infrastructure and industry - Spits often shelter estuary mouths, ideal for marina development, while exposed coasts host wind farms and power stations.
• Specialised activities - Sand dunes are used for world-class golf courses due to their unique terrain.