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A sand bar is formed when there is a gap (such as a bay) in the coastland with water in it. Longshore drift moves sand out across the bay. Where there is no river estuary, the sand can be deposited faster than it is removed.

Longshore drift continues to move sand across the bay. Shallow water allows deposited sand to build up.

The deposited material eventually joins up with the other side of the bay and a strip of deposited sand blocks off the water in the bay. This feature is called a sand bar and the area behind the newly formed bar is known as a lagoon.