Spits are formed by the process of longshore drift depositing sediment along a coastline, typically where there's a change in the coastline's direction.
The Process of Spit Formation
Here's a breakdown of how spits develop:
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Longshore Drift: This is the primary force behind spit formation. Longshore drift occurs when waves approach the shore at an angle, carrying sediment (sand, shingle, etc.) along the beach face. The swash (water moving up the beach) carries material up the beach at an angle, and the backwash (water flowing back down) carries it straight back down due to gravity. This zig-zag movement results in a net lateral transport of sediment along the coast.
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Change in Coastline Direction: When the coastline changes direction, such as at a bay or river mouth, the energy of the waves decreases. This reduced energy leads to deposition of the sediment being transported by longshore drift.
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Sediment Deposition: As longshore drift continues to deposit material in the more sheltered water beyond the change in coastline, a ridge of sediment starts to build up.
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Emergence Above Sea Level: Over time, the accumulated sediment rises above sea level, forming a spit. The spit continues to grow in the direction of the dominant longshore drift.
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Formation of a Hook or Curve: The end of the spit often curves inland, forming a hook or recurve. This can be caused by:
- Changes in wind and wave direction.
- Secondary wave patterns.
- The influence of a river or estuary outflow.
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Sheltered Area Behind the Spit: The presence of a spit creates a sheltered area of water behind it. This area is often low-energy and conducive to the formation of salt marshes or mudflats, as finer sediments are deposited here.
Example
A classic example of a spit is Spurn Head in Yorkshire, UK. It's formed by the constant longshore drift along the Holderness coast, depositing sediment into the Humber Estuary.
Summary
In essence, spits are coastal landforms created by the continuous deposition of sediment through longshore drift at locations where the coastline changes direction, resulting in a long, narrow accumulation of sand or shingle extending into the sea or across an estuary.
