Depositional Landforms

Spec mapping (AQA 7037): Paper 1, §3.1.3 Coastal Systems and Landscapes — landforms of coastal deposition: beaches (and beach features — berms, cusps, ridges and runnels); spits; bars; tombolos; offshore bars; salt marshes; and the development of sand dunes (psammosere succession). These landforms are the stores of the systems model (Lesson 1), built by the transport-and-deposition transfers of Lesson 5. The lesson links synoptically to §3.1.1 (succession and biomass accumulation in the psammosere parallels ecosystem/carbon ideas; salt marshes and dunes are blue-carbon stores) and to §3.1.4 Glacial Systems (outwash and morainic deposition as comparable sediment stores). The dominant Assessment Objectives are AO1 (knowledge of each landform's formation and characteristics) and AO2 (explaining how landforms link to process, sediment supply and sea level). The transect exercise on dune succession exercises AO3 (interpreting and manipulating ecological/sediment data).

Why this lesson matters. Depositional landforms are where students most often lose marks by treating a spit or a dune ridge as a static object deposited in one go. The examiner wants process over time: a spit grows because longshore drift delivers sediment to a point where the coast changes direction; its recurved ends record shifts in wave approach; the salt marsh behind it exists because the spit creates an energy shadow. Every landform here is a story of accumulation, and the best answers tell that story with a named place and data.

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Beaches

Beaches are the most common and widespread depositional landform. They are accumulations of sediment deposited between the low water mark and the highest point reached by storm waves.

Types of Beach

Beach Type	Sediment	Gradient	Permeability	Key Characteristics
Sand beach	Fine to coarse sand (0.0625-2 mm)	Gentle (1-5°)	Moderate	Wide, flat profile; well-developed runnels and ridges; strong backwash due to low permeability
Shingle beach	Pebbles and cobbles (4-256 mm)	Steep (10-20°)	High	Narrow, steep profile; prominent berms; weak backwash (water percolates through gaps)
Mixed beach	Sand and shingle	Variable	Variable	Often zonated — shingle at top (storm berm), sand on lower beach

Beach Cusps

Beach cusps are crescent-shaped features consisting of alternating ridges (horns) of coarse material and shallow bays of finer sediment. They form at regular intervals along the beach face.

• Horn spacing is typically 1-50 m, related to wave conditions
• Two theories explain their formation: edge wave theory (Guza and Inman, 1975) attributes them to interactions between incoming waves and reflected edge waves; self-organisation theory (Werner and Fink, 1993) suggests they emerge spontaneously from feedback between swash flow and sediment transport
• They are best developed on steep, coarse-sediment beaches
• Classic examples can be found at Lulworth Cove, Dorset, and Chesil Beach

Berms, Ridges and Runnels

Beyond cusps, the upper and lower beach carries a suite of smaller depositional features that record the recent wave history:

• Berms are ridges of sediment built at the upper limit of the swash by constructive waves. A beach may carry several berms at different heights, each marking the run-up limit of a recent high tide or storm. The highest, coarsest ridge — the storm berm — marks the reach of the most powerful waves and is built of the largest material the waves could throw that high.
• Ridges and runnels are alternating low ridges and shallow troughs (runnels) running parallel to the shore across the lower, gently-sloping part of a sand beach. They form in the intertidal zone where the interaction of swash, backwash and the falling tide moulds the sediment; runnels often trap water at low tide, draining seaward through small channels. They are best developed on wide, macrotidal sand beaches.

These features are transient — a single storm can erase a berm or rebuild a ridge — which is precisely why they are useful evidence of the current energy regime rather than the long-term form.

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Offshore Bars

An offshore bar (breakpoint bar) is a submerged or partly-exposed ridge of sand lying parallel to the shore in the nearshore zone, beyond the normal low-water mark.

Formation

1. Destructive waves, with their strong backwash, drag sediment seaward from the beach face.
2. This sediment is deposited at the point where the waves break (the breakpoint), where the seaward-moving backwash meets the incoming wave and loses energy.
3. A ridge accumulates here, growing during stormy (winter) periods when destructive waves dominate.

Offshore bars are a crucial part of the beach's negative-feedback defence: by forcing waves to break further offshore, the bar dissipates wave energy before it reaches the beach, limiting further erosion. In calmer periods, constructive waves gradually drive the bar's sediment back onshore to rebuild the berm — the seasonal exchange described in Lesson 5. Where an offshore bar becomes large and stable and is then driven onshore by rising sea level, it can evolve into a barrier beach detached from the coast, enclosing a lagoon — the likely origin of Slapton's bar and a stage in the development of barrier-island coasts such as the Outer Banks.

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Spits

A spit is an elongated ridge of sand or shingle that extends from the coastline into open water, usually across a river mouth, estuary or bay.

Formation

1. Longshore drift transports sediment along the coast in the direction of the prevailing waves
2. Where the coastline changes direction (at an estuary mouth, bay or change in orientation), sediment continues to be transported into open water
3. The spit grows progressively longer, extending across the bay or estuary
4. Changes in wind direction or wave approach cause the tip of the spit to curve, forming a recurved tip (hook)
5. Multiple recurved tips may develop as the spit extends, recording past changes in wave direction
6. The sheltered area behind the spit experiences low-energy conditions, allowing mudflats and salt marshes to develop
7. The spit cannot grow completely across an estuary because the river current prevents deposition in the channel

graph TD
    A["Longshore drift transports sediment along coast"] --> B["Coastline changes direction"]
    B --> C["Sediment deposited into open water"]
    C --> D["Spit extends, tip curves in response to wave changes"]
    D --> E["Salt marsh develops in sheltered area behind spit"]
    E --> F["River current maintains open channel"]

Case Study: Spurn Point, East Yorkshire

Spurn Point (also called Spurn Head) is a narrow sand and shingle spit extending 5.5 km southward across the mouth of the Humber Estuary:

• Fed by sediment from the rapidly eroding Holderness coast (cliff retreat ~1.8 m/year)
• The spit is only 50 m wide in places and is extremely vulnerable to storm breaching
• Historical records suggest a 250-year cycle of spit growth, breaching and regrowth — identified by de Boer (1964). The current spit began forming around 1650
• Major breaches occurred in 1849, 1856 and most dramatically during the December 2013 storm surge, when the road was destroyed and the Spurn lifeboat station was cut off
• Following the 2013 breach, the Environment Agency decided to let the spit evolve naturally rather than repair defences — a form of managed retreat
• Supports important habitats including sand dunes, salt marsh and mudflats, and is managed by the Yorkshire Wildlife Trust

Case Study: Hurst Castle Spit, Hampshire

Hurst Castle Spit extends 2 km south-eastward from the Hampshire coast towards the Isle of Wight, narrowing the western entrance to the Solent to just 1.2 km:

• Composed primarily of flint shingle derived from erosion of the Hampshire cliffs to the west
• Longshore drift moves material eastward along the coast
• Henry VIII built Hurst Castle on the spit in 1544, recognising its strategic position guarding the Solent approach
• The spit has been reinforced with rock armour and beach nourishment to protect the castle (a Scheduled Ancient Monument) — raising questions about the sustainability of defending a naturally evolving landform

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Bars and Barrier Beaches

A bar is a ridge of sand or shingle that extends across a bay, connecting two headlands. It differs from a spit in that it completely closes off the bay from the open sea.

Formation

1. A spit grows across a bay through longshore drift
2. If conditions allow (no strong river current to prevent closure), the spit extends to reach the opposite headland
3. The enclosed body of water behind the bar becomes a lagoon, which may gradually fill with sediment and develop into freshwater or brackish wetland

Case Study: Slapton Ley, Devon

Slapton Ley is the largest natural freshwater lake in south-west England, enclosed behind a shingle bar:

• The bar is approximately 4.5 km long and composed of flint and quartzite shingle
• It was formed during the post-glacial sea level rise (approximately 3,000 years ago) as rising seas pushed beach material landward, eventually enclosing a freshwater lagoon
• The lagoon is a Site of Special Scientific Interest (SSSI) and National Nature Reserve, supporting rare aquatic plants and bird species
• The A379 road runs along the top of the bar, making it vulnerable to storm damage — major breaches occurred in 2001 and 2014
• The bar is not currently maintained by longshore drift (it is essentially a relict feature), making it increasingly vulnerable to future storm damage and sea level rise

Barrier Beaches and Islands

In some locations, bars form offshore as barrier beaches or barrier islands, separated from the mainland by a lagoon or marsh. These are common on low-gradient coastlines:

• Chesil Beach, Dorset — a 29 km tombolo/barrier beach connecting the Isle of Portland to the Dorset mainland, enclosing The Fleet lagoon
• Outer Banks, North Carolina, USA — a chain of barrier islands stretching over 300 km, subject to frequent hurricane damage and coastal retreat

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Tombolos

A tombolo is a bar of sand or shingle that connects an offshore island to the mainland.

Formation

1. Wave refraction around an offshore island creates a zone of low energy in the lee of the island
2. This reduces wave velocity, causing sediment to be deposited
3. Over time, the deposit grows until it links the island to the mainland

Examples

• Chesil Beach connecting the Isle of Portland to the mainland — the best-known tombolo in the UK, though it is sometimes classified as a barrier beach due to its enormous scale (29 km long, up to 15 m high, and 200 m wide)
• St Ninian's Isle, Shetland — connected to mainland Shetland by a beautiful sand tombolo approximately 500 m long
• Mont Saint-Michel, France — connected to the mainland by a natural tombolo that is submerged at high tide

Chesil Beach repays closer study because it illustrates how a single landform can resist neat classification. Strictly, it links the Isle of Portland to the mainland, which makes it a tombolo; but its sheer scale, its role enclosing The Fleet lagoon (the largest tidal lagoon in England), and the fact that it is largely swash-aligned and barely fed by present-day longshore drift mean it is also described as a barrier beach. Its remarkable west-to-east increase in pebble size (Lesson 5) and its origin during the Holocene transgression — when rising seas drove a mass of glacial and fluvial shingle landward and pinned it against the coast — mark it as partly a relict feature, built under conditions different from those of today. Chesil therefore exemplifies a recurring truth about depositional landforms: many are inherited from the rapid sea-level rise of the early Holocene and are now being slowly reworked or are quietly degrading, rather than actively building under present sediment supply. Recognising a landform as relict rather than contemporary is an important evaluative distinction — and a vulnerability, since a feature no longer maintained by active deposition cannot readily repair storm damage (the same point applies to the Slapton bar).

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Sand Dunes

Sand dunes are mounds of wind-blown sand that develop above the high tide mark on beaches with an abundant supply of sand and persistent onshore winds.

Dune Formation and Succession

Sand dune systems demonstrate a clear ecological succession (sere), moving from pioneer communities on young, mobile dunes to climax communities on stable, mature dunes. This succession is called a psammosere.

graph LR
    subgraph "Sand Dune Succession (Psammosere)"
    A["Embryo dunes"] --> B["Fore dunes (yellow dunes)"]
    B --> C["Grey dunes"]
    C --> D["Dune slacks"]
    D --> E["Mature dunes (climax woodland)"]
    end

Dune Stage	Distance from Sea	Height	Vegetation Cover	Key Species	Soil
Strand line	Beach	N/A	Minimal	Sea rocket, saltwort	No soil — bare sand
Embryo dunes	Just above high tide	< 1 m	< 20%	Sea couch grass, sand couch	Very thin; sand with little organic matter
Fore dunes (yellow)	20-50 m	2-5 m	20-60%	Marram grass (Ammophila arenaria) — the key coloniser	Sandy; low humus; pH 7-8 (alkaline from shells)
Grey dunes	50-200 m	5-10 m	60-80%	Mosses, lichens, red fescue, heather	Darker grey soil; increasing humus; pH 5-7
Dune slacks	Between ridges	At water table level	Variable	Creeping willow, orchids, sedges	Wet; organic-rich
Mature dunes	200+ m	Variable	80-100%	Bramble, hawthorn, birch, oak	Well-developed soil; high humus; pH 4-6 (acidic)

The Role of Marram Grass

Marram grass (Ammophila arenaria) is the critical species in dune development:

• It thrives in mobile, well-drained sand environments where few other species can survive
• Long roots (up to 3 m) stabilise the sand and access water deep beneath the surface
• Leaves roll inward to reduce water loss — an adaptation to the dry, windswept environment
• It traps wind-blown sand, causing the dune to grow vertically — marram grass responds to burial by growing upward, creating a positive feedback loop
• As the dune stabilises and humus accumulates, conditions change and other species outcompete marram grass (ecological succession)