How can a sand spit develop into a bay-mouth barrier around a lagoon?

A spit is a narrow coastal landform composed of sand or shingle that is tied to the coast at one end. These features are created by the longshore movement of sediment, often forming where the coastline abruptly changes direction or across the mouths of estuaries.1

A spit can develop into a bay-mouth barrier through the continued transport and accumulation of material along the shore. As the spit extends across the mouth of a bay, it eventually encloses the water behind it, effectively creating a lagoon. These barriers can be composed of either sand or shingle, such as the feature known as Chesil Beach in southern England, which protects the brackish water body called The Fleet. Once the bay is cut off from the open sea by this barrier, the resulting lagoon becomes a distinct, sheltered environment where finer sediments can settle in the quieter water.23