Description: This is a ridge of material that joins two headlands together cutting off a bay.
Formation: It can form in one of two ways:
1. A sand spit can extend across a bay, joining two headlands together, forming a baymouth bar. A shallow body of water separated from the sea called a lagoon forms behind the baymouth bar. This may develop into a salt marsh over time (Fig. 9.16).
2. A sand bar can develop offshore, parallel to the coastline. Known as an offshore bar, it can be moved towards the coastline by the waves and wind until it eventually joins the mainland.
Tombolo
Irish example: Sutton tombolo joins Howth to the mainland
Description: A ridge of material that joins an island to the mainland.
Formation: This is formed by longshore drift, which moves material along the beach. The sheltered area between the island and mainland encourages deposition at the end of the beach. Over time, the beach extends outward from the mainland and links with the off-shore island (Fig. 9.17).
Longshore drift
Prevailing wind
Island
Tombolo: sheltered area between mainland and island encourages deposition
Fig. 9.17(a) Formation of a tombolo. CHECK YOUR LEARNING
1. List three ways the load carried by a wave is sourced. 2. Explain two reasons why the sea will start to deposit its load. 3. List and explain two reasons why some waves deposit rather than erode. 4. What is the difference between a sand spit and a baymouth bar? 5. Explain the following terms: lagoon, salt marsh.
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Fig. 9.17(b) Tombolo: notice how the beach links the island to the mainland.
Fig. 9.17(c) Tombolo on an OS map. Tombolo Mainland Lagoon
Fig 9.16(b) Baymouth bar and lagoon.
Baymouth bar
Fig 9.16(c) Baymouth bar and lagoon on an OS map.
Longshore drift
Baymouth bar
Prevailing wind, waves approaching at an oblique angle Fig 9.16(a) Formation of a sand bar.
