Final answer:
When deposited material creates a long ridge that isolates a bay from an ocean, it forms a baymouth bar. This feature is characterized by sediment extending across a bay's mouth, creating separation from the open sea and leading to the possible formation of a lagoon behind it.
Step-by-step explanation:
If the material deposited in a long ridge extending out from a coast grows to completely cut off the bay from an ocean, it becomes a baymouth bar.
A baymouth bar is a feature that occurs when an accumulation of sediment, such as sand, extends across the mouth of a bay, effectively isolating it from the ocean. It can form due to the process of longshore drift, where sediment is carried along the coast by the movement of waves. Once a bay is cut off, the water inside can become more stagnant, and over time, it may develop into a lagoon, which is a relatively shallow body of water separated from the ocean by the baymouth bar. However, the initial formation of the barrier cutting off the bay is the defining characteristic of a baymouth bar.