A spit or sandspit is a deposition bar or beach landform found off coasts. It develops in places where re-entrance occurs, such as at cove's headlands, by the process of longshore drift and longshore currents. The drift occurs due to waves meeting the beach at an oblique angle, moving sediment down the beach in a zigzag pattern. This is complemented by longshore currents, which further transport sediment through the water alongside the beach. These currents are caused by the same waves that cause the drift.
Hydrology and geology
Where the direction of the shore inland re-enters, or changes direction, such as at a headland, the longshore current spreads out or dissipates. No longer able to carry the full load, much of the sediment is dropped, called deposition. This submerged bar of sediment allows longshore drift or littoral drift, to continue to transport in the direction the waves are breaking, forming an above-water spit. Without the complementary process of littoral drift, the bar would not build above the surface of the waves becoming a spit and would instead be leveled off underwater.
Spits occur when longshore drift reaches a section of headland where the turn is greater than 30 degrees. They will continue out into the sea until water pressure (such as from a river) becomes too much to allow the sand to deposit. The spit may then be grown upon and become stable and often fertile. A spit may be considered a specialized form of a shoal. As spits grow, the water behind them is sheltered from wind and waves, and a salt marsh is likely to develop.
Wave refraction can occur at the end of a spit, carrying sediment around the end to form a hook or recurved spit. Refraction in multiple directions may create a complex spit. Waves that arrive in a direction other than obliquely along the spit will halt the growth of the spit, shorten it or eventually destroy it entirely.
The sediments that make up spits come from a variety of sources including rivers and eroding bluffs, and changes there can have a large impact on spits and other coastal landforms. Activities such as logging and farming upstream can increase the sediment load of rivers, which may hurt the intertidal environments around spits by smothering delicate habitat. Roads or bulkheads built along bluffs can drastically reduce the volume of sediment eroded, so that not enough material is being pushed along to maintain a given spit.
If the supply of sediment is interrupted the sand at the neck (landward side) of the spit may be moved towards the head, eventually creating an island. If the supply isn't interrupted, and the spit isn't breached by the sea (or, if across an estuary, the river) the spit may become a bar, with both ends joined to land, and form a lagoon behind the bar. If an island lies offshore near where the coast changes direction, and the spit continues to grow until it connects the island to the mainland, it is then called a tombolo.
The end of a spit attached to land is called the proximal end, and the end jutting out into water is called the distal end.
Spits around the world
Farewell Spit in New Zealand, at 32 km (20 mi), in the north-west corner of South Island is believed to be caused by the strong prevailing winds and currents, bringing sand eroded from the Southern Alps of the South Island and depositing these into Golden Bay.
A well-known spit in the UK is Spurn Point at the mouth of the Humber, approximately 4.8 km (3.0 mi) long.
The Curonian Spit off the coast of Lithuania and Kaliningrad Oblast of Russia separates the Curonian Lagoon from the Baltic Sea and is 98 km (61 mi) long. In a similar fashion, the Vistula Spit separates the Vistula Lagoon from the Gdańsk Bay off the coast of Poland.
Human settlement patterns
Since prehistory humans have chosen certain spit formations as sites for human habitation. In some cases such as Chumash Native American prehistorical settlement, these sites have been chosen for proximity to marine resource exploitation; for example, the Chumash settlement on the Morro Bay sandspit is one such location.
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- Duane, D.B. and James, W.R., 1980, Littoral transport in the surf zone elucidated by an Eulerian sediment tracer experiment: Journal of Sedimentary Petrology, v. 50, p. 929-942
- Evans, O.F. 1942, The origin of spits, bars and related structures: Journal of Geology, v. 50, p. 846-863
- * C.Michael Hogan (2008) Morro Creek, The Megalithic Portal, ed. by A. Burnham [self-published source]