Final answer:
Barrier islands are offshore sediment-made islands running parallel to the shoreline, acting as protective barriers between the ocean and mainland and creating environments like bays and lagoons behind them.
Step-by-step explanation:
Barrier islands are offshore islands, made of sediment, usually parallel to the shoreline. These islands primarily consist of sand and silt and are located on the continental shelf.
They serve as natural barriers between the ocean and the mainland, often running parallel to the coastline, and can extend from several to hundreds of kilometers. Barrier islands are characterized by distinct environments on their windward and leeward sides, with the former typically being wetter and the latter drier due to differences in exposure to prevailing winds.
Behind these barrier islands, one may often find a bay or lagoon, which is a shallow body of water separated from the ocean by the barrier island. These lagoons are connected to the ocean through narrow passes and can have a high salinity and significant tidal activity. The environment within these lagoons is usually low energy, containing silt and clay, and may vary from being a marsh with heavy vegetation in coastal areas to more barren landscapes in others.