spits A spit is a landform created by the extension of sediments from the shoreline into a water body (lake or sea). Spits commonly occur near the mouths of estuaries or embayments, where the line of the coast turns inland, the spit continuing the former coastal trend. As a result, many spits form a natural defence against coastal inundation.
Although spits are found in lacustrine settings, the majority are associated with coastal areas. They usually consist of sand or shingle that has been transported by wave action. The formation of a spit thus depends on a source of sediment and sufficient wave energy to transport the material towards and then across the mouth of the embayment. The direction of dominant wave attack in comparison to the available sediment load is crucial. Where sediment supply is lacking or wave energy greater, the resulting spit will tend to turn into the embayment rather than continue the line of the original coast.
The distal end of a spit commonly displays alterations in alignment, known as
recurves, which usually curve into the embayment. Recurves usually develop as a result of wave refraction in deeper water. The transport of sediment, and hence the development of a spit, reflect the changing direction of wave movement (e.g. Blakeney Point, Norfolk). In some circumstances, however, recurves reflect the complex wave environment at the site (e.g. Hurst Castle Spit, Hampshire) with the dominant direction of wave action altering along the spit.
Many instances spits are both prograding into the embayment and are being rolled onshore at the same time. As a consequence the modern shingle ridge truncates former recurves and such patterns can be used to reconstruct the evolution of the spit. Many spits also display cyclic phases of evolution, perhaps reflecting cyclic patterns of sediment supply. When coastal sediments abound, the spits tend to prograde, both in length and width. If the sediment supply diminishes, the wave processes continue to move material along the feature; as a result, progradation at the distal end results from cannibalization of the feature at its proximal end. This can result in breaching of the feature at its proximal end and ultimately detachment from the mainland. The subsequent input of a fresh supply of sediment results in the reattachment or regrowth of the spit. Many of the world's spits are currently showing signs of cannibalization. This may be due to the fact that they developed as a result of abundant sediment supplies derived from Quaternary sea-level changes. Now that this sediment has been fully utilized, the spits are decaying to a more stable form.
At certain sites two spits appear to be growing towards each other across the embayment. At some of these sites (e.g. Poole Harbour, Dorset) this is a product of the coastal configuration, the dominant wave action being in opposite directions along each side of the embayment. In other cases, a major spit has developed along the coast as a result of dominant wave activity (e.g. Orford Ness, Suffolk); the shelter the feature provides allows a second smaller spit to develop on the other side of the embayment. This second spit grows in the opposite direction.
Callum R. Firth
Bibliography
Bird, E. F. C. (1984) Coasts. MIT Press, Cambridge, Mass.
Carter, R. W. G. (1992) Coastal environments. Edward Arnold, London.
