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The physical feature that you are describing is called a sandbar. It is a long, narrow, underwater embankment of sand, formed by the action of waves or currents. When a sandbar extends off the mainland, it can create a protected lagoon, which may be home to unique ecosystems and habitats. Barrier islands, on the other hand, are long, narrow islands that run parallel to the coastline and are separated from the mainland by a lagoon or bay. They are also formed by the action of waves and currents and can serve to protect the mainland from storms and erosion. Both sandbars and barrier islands play important roles in shaping coastal environments and influencing the distribution of plant and animal species.