Formation of Spits
Created | Updated Feb 21, 2006
A spit is a geographical feature found on coastlines. They are depositional features, that is to say they are formed from deposited matter - in this case, sand and other sediment from beaches.
Spits are essentially sand-bars, a long accumulation of matter attached at one end to a coastline, and stretching out at a narrow angle into the sea, sometimes with a hooked end. They are formed when sand and sediment is transported by the process of longshore drift - the movement of particles along a beach due to wave action. Longshore drift is responsible for those wooden groynes you see at the seaside sometimes - they are there to "trap" the beach, to prevent it from washing away.
Where the direction of longshore drift meets a river mouth, bay or other indentation in the coastline, matter is continually transported in the same direction. If the spit is formed over a river mouth, the sediment is not washed away, however, as the running water from the river is countered by the force from wave action - the matter is deposited here, and the spit is formed as longshore drift continues. The hooked end is a result of a temporary change in wind direction: wave action and the stronger force from the river in the centre of the channel often prevents the spit growing further from this point. Where the water is still behind the spit, a salty marsh can form and salt-tolerant plant life flourishes. If the spit is formed over a bay or similar non-river indentation, it can span the whole gap and cut off a section of the sea: this becomes a lagoon.
The world's longest spit is the Dungeness Spit in Washington, which extends over five miles into the Strait of Juan de Fuca, providing a natural shelter to the bay behind, and is the part of the National Wildlife Refuge System. Many claim it is still growing, and evidence shows that it has grown 15 feet per year for the last 15 years.
Spits are essentially sand-bars, a long accumulation of matter attached at one end to a coastline, and stretching out at a narrow angle into the sea, sometimes with a hooked end. They are formed when sand and sediment is transported by the process of longshore drift - the movement of particles along a beach due to wave action. Longshore drift is responsible for those wooden groynes you see at the seaside sometimes - they are there to "trap" the beach, to prevent it from washing away.
Where the direction of longshore drift meets a river mouth, bay or other indentation in the coastline, matter is continually transported in the same direction. If the spit is formed over a river mouth, the sediment is not washed away, however, as the running water from the river is countered by the force from wave action - the matter is deposited here, and the spit is formed as longshore drift continues. The hooked end is a result of a temporary change in wind direction: wave action and the stronger force from the river in the centre of the channel often prevents the spit growing further from this point. Where the water is still behind the spit, a salty marsh can form and salt-tolerant plant life flourishes. If the spit is formed over a bay or similar non-river indentation, it can span the whole gap and cut off a section of the sea: this becomes a lagoon.
The world's longest spit is the Dungeness Spit in Washington, which extends over five miles into the Strait of Juan de Fuca, providing a natural shelter to the bay behind, and is the part of the National Wildlife Refuge System. Many claim it is still growing, and evidence shows that it has grown 15 feet per year for the last 15 years.
