The coral deposits marked M are called

The coral deposits marked M are called barrier reefs. Barrier reefs are long, narrow reefs that parallel the coast and are separated from the mainland by a lagoon. They are formed by the accumulation and growth of coral and other organisms, and they play an important role in protecting the shoreline from erosion and storm damage. Barrier reefs are often found in tropical waters and are home to a diverse range of marine life. The Great Barrier Reef in Australia is one of the largest and most famous barrier reefs in the world, and it is a popular tourist destination and a UNESCO World Heritage site.