Landforms of Deposition (Leaving Cert Geography): Revision Notes
Landforms of Deposition
Landforms of deposition are created when the sea drops sediment it has been carrying, building up new coastal features. These features form through the action of constructive waves and longshore drift processes.
Beaches
A beach is an accumulation of loose sediment deposited by constructive waves between the low and high tide marks.
How beaches form
Beaches develop through the action of constructive waves, which have more energy when they approach the shore (swash) than when they retreat (backwash). This process works in several stages:
- Constructive wave action: The powerful swash carries sediment up the beach, whilst the weaker backwash cannot drag all material back to sea
- Material sorting: The swash can transport larger materials like pebbles to the upper beach areas, whilst the backwash only has enough energy to carry smaller particles like sand and mud back towards the water
- Sediment accumulation: Over time, this process builds up layers of beach material, with larger stones remaining higher up the beach and finer materials settling near the water's edge
The key principle behind beach formation is the energy imbalance between swash and backwash. This fundamental difference in wave energy determines where and how sediment accumulates along the coastline.
Longshore drift contribution
Longshore drift plays an important role in beach development. When waves approach the coast at an angle, they create a zigzag movement of sediment along the coastline. This process continuously supplies new material to beaches and helps redistribute sediment along the shore.
Beach structure
A typical beach consists of two main sections:
- Backshore: The upper, steeper section made of coarse materials like shingle. Storm waves and very high tides are the only forces with enough energy to reach this area
- Foreshore: The lower section with a gentler slope, composed of finer sand and mud particles
The different slopes and materials in each section reflect the varying energy levels of waves that reach these areas. This natural sorting creates the characteristic profile of a beach.
Beach features
Several distinct features develop on beaches through ongoing wave action and sediment movement:
Berms are small ridges of slightly coarse gravel that run parallel to the coastline. They mark the furthest point reached by the swash during high tide and are constantly rebuilt by each tide cycle.
Runnels are gently sloping ridges of sand that form on the seaward edge of the foreshore. These features run parallel to the coast and develop near where waves break. When multiple ridges form, they create depressions between them, also called runnels.
Sand dunes form as hills of sand that develop inland from beaches. They require a good supply of fine beach sand and prevailing winds blowing from the sea. During low tide, sand dries out and gets blown inland by wind. When this sand meets obstacles like fences or vegetation, it accumulates to form dune systems.
Marram grass is crucial for dune development. This tough grass colonises the dunes and binds the sand particles together with its extensive root system. Without this natural stabilisation, dunes would quickly erode and disperse.
Irish examples
Notable beach examples in Ireland include:
- Kilkee Beach, County Clare
- Youghal, County Cork
- Ballybunion, County Kerry
Sandspits, tombolos, baymouth bars and lagoons
Sandspits
A sandspit is an elongated ridge of sand that extends from the coastline out into the sea.
Sandspits form where there is a sudden change in coastline shape, such as at a sheltered bay or inlet. This change disrupts the normal longshore drift process, causing sediment to be deposited in a concentrated area rather than spread along the coast.
Formation Process of a Sandspit:
Step 1: Sediment deposition occurs on the seabed in shallow waters where longshore drift is interrupted
Step 2: Material gradually builds up to sea level as more sediment accumulates
Step 3: Continued growth extends the spit outwards until reaching areas where tides and currents remove more material than longshore drift can supply
Marram grass helps stabilise spits by binding the sediment together. Constructive waves deposit sand on the seaward side, whilst some sand gets blown inland by wind to form small dune systems on top of the spit.
An example of a sandspit can be observed at Inch Beach in County Kerry.
Tombolos
When a sandspit continues growing and eventually connects with an offshore island, it creates a tombolo. Wave refraction or changes in wind direction can cause the spit's tip to bend inwards towards the land during this process.
Tombolos represent the final stage of spit development, where the feature has grown long enough to bridge the gap between the mainland and an offshore island.
An example of a tombolo can be seen at Howth, County Dublin.
Baymouth bars and lagoons
When a spit extends completely across the mouth of a bay, it forms a baymouth bar. These typically develop on gently sloping shores where waves break some distance from the shore, allowing the powerful swash to push sediment ahead of it.
The formation process demonstrates how coastal features can completely transform the local environment:
- Constructive waves add sediment to increase the bar's height
- Longshore drift extends the bar's length across the bay mouth
- Complete closure of the bay from the sea occurs
Once the bay becomes completely cut off, it transforms into a lagoon - essentially a lake separated from the sea by the baymouth bar. If no river flows into the lagoon to maintain water levels, it may eventually dry up and become a salt marsh.
The transformation from an open bay to a closed lagoon can dramatically alter the local ecosystem, creating unique habitats for both marine and freshwater species.
Our Lady's Island in County Wexford provides an excellent example of a baymouth bar formation.
Key Points to Remember:
- Beaches form through constructive wave action - stronger swash deposits material whilst weaker backwash cannot remove it all
- Beach material is naturally sorted - larger stones accumulate on the backshore whilst finer materials settle on the foreshore
- Longshore drift supplies sediment to beaches and creates spits when coastline direction changes suddenly
- Marram grass is essential for stabilising sand dunes and spits through its extensive root system
- Spits can evolve into tombolos (connecting to islands) or baymouth bars (closing off bays to create lagoons)