In geography, a natural barrier is a naturally occurring obstruction that prevents or slows down movement, especially of people and forces of nature at technological levels. Sand dunes, reefs, mountain ranges, ice fields and barrier islands are considered natural barriers.
Natural barriers, such as reefs and barrier islands, protect the shorelines from big and destructive waves. In the absence of barrier reefs, sand dunes are the first form of protection of the beach against erosion and coastal storms. They absorb the impact of the huge waves and prevent damage to inland structures and areas. Mountain ranges along the coasts serve as barriers and help assuage strong winds from incoming hurricanes. The ice fields in Antarctica slow down the flow of fragmented ice sheets into the ocean, preventing the sea level from rising because of the melting ice shelves.