Composite Spits are shingle deposited before the finer sand (made of both constructive and destructive waves).
Spits are formed by longshore drift, which carries material along the coast and continues in the same direction when the coastline retreats E.g. Estuary.
There are variations on some spits, which produce Tombolas, Bars, and Cuspate Forelands.
Tombolas are spits, which connect to an island e.g. Chesil Beach, Dorest. This may have been formed be easterly directed long shore drift. Alternatively, it may have been formed by rising sea level.
Tombolas can also be described as bars, which extend to and link with an Island.
Bars (Baymouth Bars) are something different. They are spits which seal off the estuaries of rivers. At Slapton Sands, Devon, it is the River Gara that has been “blocked”. It has been suggested that LSD occurs in 2 directions but this seems unlikely due to the interference of strong currents. Another possibility is that the material was deposited offshore and gradually brought onshore by waves: in either case, river discharge has to be weak.
Cuspate forelands represent triangular accretions/additions of shingle that project into the sea. They may have been formed where waves approach the shore and are of equal strength. Two spits emerge and meet so their formation is similar to Baymouth Bars.
Because they are formed by wind and wave action, spits are unstable and dynamic. Bulkheads, highways, or railroads along the base of nearby eroding bluffs will reduce the supply of sediments necessary to maintain a spit. Logging and farming in adjacent areas can increase river-borne sediments and smother life on the intertidal parts of spits. Increased sediments from streams may also fill in the bay on the leeward side of a spit.
It takes hundreds of years for spits to develop.
An relevant apparent condition which spits require to develop is shallow water. Around the coasts of Britain, those areas which contain spits all have small tidal ranges, usually below 3 metres.
The main process, which causes spits, is long-shore drift. This is the movement of material along the coast parallel to the shoreline. Its occurrence depends upon the oblique approach of a wave to the shoreline, for, the wave may carry material up a beach at an angle approximately perpendicular to the wave crest, but gravity will cause the material and the backwash to take the steepest gradient seawards which in an oblique wave will be a different course from that taken by the swash.
Consequently, material may be seen to drift along a coastline, the effect being best demonstrated where accumulation has taken place against Groynes, or any other man-made obstacle that juts out into the sea. It is often recognised that most British coasts experience this drift in one predominant direction, which is obviously determined by the predominant wind direction. Thus, north Norfolk is considered to have an east-west drift whilst east Norfolk and Suffolk have a north-south rift. However such, generalisations can easily obscure important detail, for drift may change direction several times a day.
The drift departs from the beach and drops its sediment material onto the shallow waterbed; this gradually builds up the sediment gradually forming the eventual spit. This develops further at the high tide where the inner stages of the spit can be developed by further sediment deposits.
Spurn Head, Holderness Coast
- A drift lined spit formed when long shore drift occurred to the south. The spit receives sediment drifting south as far north as Flambrough Head. It receives glacial deposits that are as easily eroded and a abundant sediment supply. The wind is from the north and northeast and that is why the drift is to the south. It is projecting into a Humber Estuary. In the slack water the finer sediment is stored. Due to the projection into the estuary the Humber is diverted south. The spit has moved west due to the eroding coastline.
Hooked or Re-curved Spits
- As spits build out into deep water they require increasing volumes of sediment to build above the high mark. The tip or distal turns towards the land where it's shallower. If it built out in deep water it would be eroded. Once formed hooks are sheltered from the dominant waves by the spit and become permanent curves also formed by the second dominant wind.
Under free transport conditions, the coast is likely to be much straighter, and the associated oblique approach of the waves will induce drift along it. The beaches are thus drift aligned and if the coast changes direction suddenly, for example at a river estuary, the beach may well continue parallel to the drift and detached from the coastline. This is a good formation of spits.
