A spit is a narrow strip of land that extends into a body of water from the coastline. It is a depositional landform, typically composed of sand or shingle, and remains connected to the mainland at one end. The formation of these features results from natural processes that shape coastal environments.
The Mechanics of Spit Formation
The primary geological process responsible for the formation of a spit is longshore drift. Waves typically approach the shore at an angle, driven by prevailing winds. As these waves break, they carry sediment up the beach in a movement called swash. The water then retreats directly back down the beach due to gravity, a process known as backwash. This continuous, angled movement of swash and perpendicular backwash results in a zigzag transportation of sediment along the coastline.
When the coastline abruptly changes direction, such as at a bay or estuary mouth, the longshore current spreads out. With the reduction in wave energy and current velocity, the water is no longer able to carry its full load of sediment. This causes the suspended sand and shingle to be deposited. Over time, this continuous deposition builds up a ridge of material extending outwards from the shore, forming a spit. The spit will continue to grow in the direction of the longshore drift.
Essential Conditions for Development
The formation and continued growth of a spit depend on specific environmental conditions. A consistent supply of sediment, such as sand or shingle, is necessary. This sediment often originates from eroding cliffs or is brought to the coast by rivers. This ensures enough material for longshore drift to transport and deposit.
Strong, consistent longshore currents and waves are also crucial. These waves, driven by prevailing winds, must approach the coastline at an oblique angle to create the longshore drift necessary for sediment transport.
A significant change in the direction of the coastline is another factor. Bends in the coast, bay mouths, or estuaries provide the necessary reduction in wave energy and current velocity, allowing sediment to accumulate. The spit will continue to extend into the water until factors such as deep water or strong opposing currents, like those from a river, prevent further deposition.
Evolution and Associated Landforms
Spits are dynamic landforms that can change shape over time. One common modification is the development of a recurved end or “hook.” This often occurs due to changes in wave direction, such as during storms, or from secondary currents and wave refraction around the spit’s end. These varying forces can cause sediment to be carried around the end of the spit and deposited in a curved pattern.
As a spit grows, the area behind it becomes sheltered from wind and waves, creating a low-energy environment. This sheltered area promotes the deposition of finer sediments, leading to salt marshes or lagoons. In some cases, a spit may extend completely across a bay, connecting two headlands and enclosing the bay to form a landform known as a bar. A spit that connects an offshore island to the mainland becomes a tombolo.