A spit is an area of sand or shingle that has been transported by longshore drift and then deposited as the coastline has changed direction. It is attached to the land at one end. It is a depositional landformd. Hurst Castle Spit in Hampshire is a very famous example.

Where the coastline changes direction, sediment is deposited in the same direction as the original coastline (i.e. in line with the prevailing wind direction). Where there is a break in the coastline and a slight drop in energy, longshore drift will deposit material at a faster rate than it can be removed and gradually a ridge is built up. The material is deposited in the deeper water offshore until the ridge is built above the level of the sea. Drift continues along the seaward side of the spit extending it further down the coast while salt marsh develops in the slow-moving water on the landward side.

Spits can become a permanent feature. This happens when the prevailing wind picks up sand from the beach and blows it inland across the spit to form sand dunes. These dunes will then be colonised by vegetation, which stabilises them. It is common for a salt marsh to develop in the sheltered area of water behind the spit. Water is trapped behind the spit, creating a low energy zone. As the water begins to stagnate, mud and marsh begin to develop behind the spit.

A spit may grow out across a river estuary. Where the spit is crossing a river mouth, the river will be diverted so that it follows the coastline for some miles before reaching the sea.

Bars

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Bars can form in several ways: (a) a spit grows the whole way across a bay(b) a sandbank develops offshore, parallel to the shore, and is moved towards the coastline by the waves and wind until it joins the mainland

Slapton Sands is an example of a bar. The lagoon of water than has formed on the landward side of the bar is called Slapton Ley.

A tombolo is formed where a spit joins an island to the mainland. An example is the Isle of Portland which is joined to the mainland by a shingle ridge known as Chesil Beach. 

Blakeney Point in Norfolk is a coastal spit extending far along the coast. Longshore drift moves sand and shingle along the coastline in a zigzag pattern. Land is formed where the sand and shingle are finally deposited. The form of the spit is always changing, depending on weather conditions. The wind blows up sand to create dunes along the ridge of the spit. 

A sand spit is one of the most common coastal landforms. A sand spit is a linear accumulation of sediment that is attached to land at one end. Sand carried parallel to shore by longshore drift may eventually extend across a bay or between headlands especially where water is relatively calm. Spits are typically elongate, narrow features built to several dozen feet by wind and waves.

tombolo,  one or more sandbars or spits that connect an island to the mainland. A single tombolo may connect a tied island to the mainland, as at Marblehead, Mass. A double tombolo encloses a lagoon that eventually fills with sediment; fine examples of these occur off the coast of Italy. The shallower waters that occur between an island and the mainland are the loci of such features because sandbars form there.

A sandbar that connects an island to the mainland or to another island

WAVE REFRACTION

"True" tombolos are formed by wave refraction . As waves near an island they are slowed down by the shallow water surrounding it. These waves then refract or “bend” around the island to the opposite side as they approached. The wave pattern created by this water movement causes a convergence of longshore drifting on the opposite side of the island. The beach sediments that are moving by lateral transport on the lee side of the island will accumulate there conforming to the shape of the wave pattern. In other words, the waves sweep sediment together from both sides. Eventually, when enough sediment has built up the beach shoreline, known as a spit, will connect with an island and form a tombolo.[2]

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LATERAL LONGSHORE DRIFT driftIn the case of Chesil beach or spurn head , the flow of material is along the coast in a movement which is not determined by the now tied island, such as Portland, which it has reached. In this and similar cases, whilst the strip of beach material connected to the island may be technically called a tombolo because it links the island to the land, it is better thought of in terms of its formation- as a spit or otherwise.