A bar is created when there is a gap in the coastland with water in it. This could be a bay or a natural hollow in the coastland. The process of longshore drift occurs and this carries material across the front of the bay. Material is pushed up onto beaches at a 45-degree angle when the swash brings it onto the coastline. The backwash takes it back out towards the sea at a right angle to the coast. Through this process, the material is constantly moved along the coastline. The deposited material eventually joins up with the other side of the bay and a strip of deposited material blocks off the water in the bay. The area behind the newly formed bar is known as a lagoon.
Find out more about landforms of coastal deposition.