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Bars forms in exactly the same way as spits. In fact, a bar is just a spit that grows right across the entrance to a bay and spans the entire distance from one headland to the next one. Behind the bar a lagoon will form where the water that previously flowed into the bay has become trapped.
Lagoons are low energy enviroments and may develop into salt marsh as sediment is accumulated.
Slapton Sands, in Devon, has a brilliant example of a bar, cutting across the bay from Torcross to Slapton and creating the largest lagoon in SW England, Slapton Ley. Dating of the sediments in the lagoon suggest that it is around 3,000 years old.
The bar is formed from shingle, mostly flint transported from off shore deposits. The local rock, slate, is very easily eroded so very little is seen on the bar. The bar extends for 4km but is narrow, roughly 100m to 150m wide.